The Smith-Kettlewell Technical File

A Quarterly Publication of The Smith-Kettlewell Eye Research Institute’s Rehabilitation Engineering Research Center

William Gerrey, Editor

Issue: sktf-Winter-1981

Original support provided by: The Smith-Kettlewell Eye Research Institute and the National Institute on Disability and Rehabilitation Research

Note: This archive is provided as a historical resource. Details regarding products, suppliers, and other contact information are original and may be outdated.

Questions about this archive can be sent to


Soldering (Part II)

Solderless Breadboards

Auditory Volume Level Indicators Old and New (Part 1)

Smith-Kettlewell Universal NiCad Battery Charger


Editor's Corner


By Bill Gerrey

In the previous discussion, we covered the principles of forming a solder bond. These principles are summarized below:

  1. Molten solder acts as a solvent--it dissolves metal from the surface of the pieces being joined, forming a bridge of alloys from one metal to the other.
  2. All the metals being joined must reach the "alloying temperature" in order to be soluble in molten solder. The metals must be in firm contact with each other so that they all reach this alloying temperature.
    1. Heat transfer from the hot iron to the joint of the metals must be extremely efficient to afford good soldering and to prevent damage to connected components of the work. If the transfer of heat is done inefficiently, a long time will elapse before the adjoining metals reach the alloying temperature; during this time, considerable energy will be absorbed by the components and damage may result.
    2. In order for heat to be efficiently transferred to the metals from a hot iron, a complete metallic path must be established between the iron and the work. In fact, the iron itself must be involved in the continuity of alloys. The iron and the metals being joined must all have their surfaces in solution with the molten solder; this is known as "wetting".
  3. Oxides on the metallic surfaces prevent wetting by the molten solder; the solder cannot reach clean surface metal to alloy with it. Furthermore, oxides build up very rapidly at high temperatures, insulating the hot iron from the work. A chemical "reducing agent" known as flux, must be applied to all the surfaces during soldering to strip these surfaces bare of oxides, allowing the formation of pure metallic alloys.
  4. Rosin Flux (used in electrical soldering) is a fairly active reducing agent when heated, but remains inert at temperatures below soldering temperatures. Its resistance to chemical interaction after the solder has cooled makes it ideal for electrical work; its residue is non-corrosive.
  5. Given ideal conditions, soldering is done in the following manner: The tip of the hot iron is put in contact with all the adjoining metals. Flux-core solder is put in contact with the work and the tip of the iron which causes a column of molten solder to flow between the iron and the work. After this initial melting of solder, the application of additional solder should primarily be fed to the work pieces and not to the tip of the iron. Solder which is applied to the iron alone and which subsequently runs down into the work will generally not involve itself in bonding. Its flux will be used up cleaning the iron and not the work; it will not be able to get through the oxides onto the bare metal surfaces. Feeding solder to the work alone maximizes the effectiveness of the flux, and provides a good indication as to whether or not the metals being joined have reached soldering temperature.

(Editor's note -- Gee, that was good. There will be a pop quiz on Friday.)

Continuous - Heat Soldering Irons

Much of what follows is a clean break from theory; it is a highly subjective discussion of the techniques I use when soldering with a continuously hot iron. Many of the statements to follow are refutable. Take each stated technique as a seed from which you can "grow your own". Please submit your suggested additions and improvements to this Editor, so that they can be shared with the rest of us.

There are two basic classes of continuous-heat soldering irons, simple garden variety constant-power irons, and "temperature-controlled" irons.

Constant-power units are comparatively inexpensive. Their heating element is a simple power resistor which is energized continuously. Their main disadvantage is that while they are not in contact with a work piece, they must dissipate all their heat energy in free air, which means that they reach fairly high temperatures between soldering operations. Their tip temperature may approach 1,000 dg F, (about 450 dg C) while the iron is in the rest stand. To protect the "tinned" surface of the tip from oxidizing, at these high temperatures, it is crucial that fresh solder be always present on the tip.

Temperature-controlled irons are those which have "thermostatically-controlled" mechanisms for sensing and maintaining the temperature of the tip. With these units, the temperature is held to a specific value, whether the iron is in use or at rest. Typically, these irons are designed to maintain a tip temperature of 650 dg F (about 350 dg C).

The tip of a temperature-controlled iron is not subjected to heat cycling over a wide range of temperatures, and the overall tip temperature is lower (more than 300 dg lower). These features give rise to a longer tip life. In addition, the likelihood of a solder joint reaching a very high temperature, which can damage the flux, is minimized. Finally, the 30 percent reduction in tip temperature may be slightly less injurious to the fingers, if the job is such that the tip must be frequently touched. (In all fairness, it should be stated here that Dennis Bernier, Vice-President of Research and Development at Kester Solder Company, prefers a well-designed constant-power iron over temperature-controlled ones. He argues that a well-designed constant-power iron is able to maintain a fairly constant temperature, and that temperature-controlled irons can often subject the work to electrical transience as they switch on and off. He also recommends that when choosing a temperature-controlled iron, obtaining one with a tip temperature no lower than 700 dg F is preferable for efficient heating of the work.)

When choosing a soldering iron, pick one with a high power rating, so that it can supply heat energy to the localized area of interest at a much faster rate than energy can be conducted away from the connection by components of the work. Small-sized, lower-power irons are made for specialized applications, such as miniaturized assemblies on which larger irons cannot be maneuvered into position. (1 used a 25 watt iron for years, only to find out that components were absorbing enough heat energy to be damaging, while I shifted from one foot to the other waiting for the solder to melt). The iron should be as large as can practically be used, given its physical size and configuration.

Unless they are a very small size, temperature-controlled irons are designed to supply high levels of heat energy when they are called upon to do so. When the tip temperature drops below their specified value, a high amount of energy will be supplied by the iron until the tip temperature is restored to the rated value.

Constant-power irons, on the other hand, only have their given power rating available for transferring heat energy to the work. Ideally, given the size of electronic components used in most assemblies, an iron of 50 watts or more enables the user to make connections quickly and efficiently. However, for the blind technician, irons of this power rating may have unfavorable physical characteristics; they are often very long, and their heating element may be comparatively large in diameter. The blind technician may wish to compromise on the power rating to obtain an iron of smaller physical size. I recommend not using irons of less than about 35 watts.

Key Physical Parameters

By picking an iron with certain key physical characteristics, you can optimize your accuracy and stability in performing the task of soldering.

The length of the hot portion of the iron (from the handle to the tip) should be as short as possible, so that your sense of the tip position can be accurately predicted. The iron becomes an extension of your hand; the position of your hand and the angle at which you hold the iron are valuable pieces of information which you must use to predict the tip's location. The shorter the iron is, the more meaningful will be this information.

By the same logic, the handle of the iron should be well enough insulated from the heat to afford holding the handle at its extreme forward end.

The diameter of the "barrel", which is the heating element, should be as small as possible to minimize the likelihood of its contacting nearby wiring or your other hand.

It is my experience that a great advantage in stability can be gained from choosing a tip which has flat contact faces. In contrast to this, the tip style which has gained overwhelming popular acceptance is conical in shape; it has no flat contact face. When holding a conical tip against the work, the contact force must be exactly perpendicular to the surface of the cone, otherwise the iron will tend to slip or "glance" off the connection. In other words, holding a round tapered tip against a work piece is difficult, and is subject to instability. I have found that by using a tip with flat contact surfaces, "glancing" of the iron off the work is less of a problem.

Replacement tips which have flat contact faces are available for almost any make of iron. In general, three such tip styles are available from soldering iron manufacturers - pyramid-shaped, chisel-shaped and screwdriver-shaped tips. (Incidentally, all these styles have far better heat conductivity than does a tip of conical shape). Screwdriver tips are the most suitable for electronic assembly, since they are usually thin enough to fit between closely spaced terminals. Whichever style is chosen, the handle of the iron should be marked adjacent to each flat surface, so that the tip can be properly oriented with respect to the work. The handle can be marked by filing notches into it, or by attaching narrow strips of Dymo tape along it.

Care and Feeding of the Soldering Iron

A soldering iron is a very vulnerable instrument. Operating at extremely high temperatures, the iron can supply enough heat energy to promote a variety of "endothermic" chemical reactions, all of which are detrimental to its effectiveness.

Since the surface metal of a soldering iron tip is chemically active, it is prone to oxidation; and this tendency is greatly accelerated at high temperatures.

During idle periods, the heat of the iron can carbonize any flux residue present on the tip.

The iron may accidentally come in contact with foreign materials which melt when subjected to its intense heat. Some notable examples are plastic building materials, insulation on the wiring, painted surfaces, and insufficiently damp cleaning sponges. If deposits of such foreign matter melt off on to the tip of the iron, they quickly carbonize and cling stubbornly to the tip.

Any soil on the tip acts as a heat insulator, and it renders the iron incapable of effectively heating the work. The only protection the iron can have against this soil is to be kept wet with solder. Since the surface metal of the tip will not be exposed to the atmosphere under these conditions, oxidation of the tip metal itself will not occur. Any carbonized residues will tend to "float" on top of the molten solder, thus protecting the tip itself from contamination.

Therefore, a solution of molten solder must always be kept present on the tip of a well cared for iron. The solder-wet tip can easily be wiped clean on a damp cleaning sponge, but this procedure should be followed soon after with the application of fresh solder to the tip.

In the normal sequence of making solder connections one after another, the tip of the iron is kept reasonably wet automatically. However, an occasional "slap-dash" application of solder to the tip assures that voids in the surface solution do not go untreated; a good bath in fresh flux will strip oxides and other contamination off the surface metal. Initially, of course, new tips should be bathed in fresh solder before being used.

Special mention should be made here regarding the simplest of tips, those which are made of bare copper. Copper is extremely soluble in solder. Copper from these tips is actually dissolved into each solder connection; these tips eventually become pitted and worn away. They can be redressed by filing or sanding them down to a smooth new surface, at which point they must be treated as a new tip. Never attempt this redressing procedure on steel or iron-clad tips (see "Soldering", Part I).

"Tinning" the iron specifically refers to applying a coat of fresh solder to the tip. This can be done in two ways. If the iron is cold, wrap about three inches of solder around the tip and turn the iron on. (To some people, these three inches may seem like an excessive amount of solder, but the point is to assure that the entire surface of the tip is bathed in fresh solder. Overdoing the amount harms nothing.) If the iron is to be "tinned" while hot, solder must be "brushed" along the tip. To make sure that the entire tip is being bathed in fresh solder, turn the iron slowly while "brushing" the solder over the tip.

Finding the tip of a hot iron with a piece of solder is no easy task.

To aid in doing so, rest both hands against some familiar object, such as your vise or the rest stand, so that you have some idea as to where the iron and the piece of solder will intersect. I often extend the piece of solder an inch or so beyond my projected point of intersection, thus allowing the iron to "cut" the solder to the exact length.

Do the "tinning" procedure over an unimportant surface which is not flammable. It is a good idea to "tin" the iron over a wet cleaning sponge, but remember to retrieve the solder droplets from the sponge; they will be larger than those left in the sponge after normal wiping, and you do not want the iron to pick them up later.

The iron can be cleaned by wiping it on a wet cloth or a wet cellulose sponge. Whichever you use, the item must contain nothing which will contaminate the iron. For example, no cloth having components of polyester are acceptable. Also, many general purpose cleaning sponges contain chemicals which can soil the tip. Above all, the item used must be wet (dripping wet is acceptable). This is essential, since while the iron is being wiped free of its protective excess solder, it is very vulnerable to charring foreign matter.

Sponges designed for this purpose are available from various soldering equipment manufacturers. The best cleaning sponges are those which envelope or surround the tip when it is being wiped. These sponges can either be convoluted (being made up of lobes), or they can be comprised of a "sandwich" of separate sponges standing on edge. With simple flat sponges, several passes must be made (rotating the iron between each pass) to assure that the tip has been wiped on all sides. This tends to cool the iron more than is done by making a single pass through a sponge of complex configuration. (For the blind technician, it is vital that all droplets of excess solder be removed, since they can cause serious burns. Dennis Bernier of Kester Solder Company pointed out to me that wiping the tip on a simple flat sponge tends to transfer droplets to the unwiped side of the tip, and does not assure that they will be removed.) Wipe the iron immediately before soldering. After soldering has been accomplished, return the iron to its rest stand; do not wipe the tip free of excess solder at this time.

Many accidents can happen to damage the iron. All of these accidents are preventable if reasonable steps are taken.

The power cord of the iron should be kept off to one side at all times to prevent the iron from coming in contact with it. In fact, all cables should be kept well out of the way, even if they do not pose an immediate electrical hazard. If the tip of the iron comes in contact with any such cable, it will become contaminated with very stubborn soil.

Plastic handle tools should be kept clear of the rest stand to avoid being grazed by the tip, resulting in its contamination.

Finally, the tip should not be banged into things which can mar its surface. To avoid striking the tip on sharp corners and edges of the rest stand, carefully note the position of the rest stand so that you can approach it slowly and gracefully with the iron.

Handling the Iron

This section covers a lot of territory - practice reaching, establishing well-defined reference points and rest positions ("land-marking"), and touching and holding the iron. The limitation of writing is that these ideas cannot all be conveyed simultaneously. As with swimming, which can be described as the integration of kicking, paddling, breathing, and adopting a good posture, the components of this discussion must be taken together when performing the task of soldering.

It goes without saying that the techniques to follow should be practiced while the iron is cold. In addition, a practice iron which is always cold may be worth having. A dummy iron can be made by thrusting a pencil through a couple of bottle corks of appropriate size; the pencil should protrude the same distance as does the hot portion of your iron, and the corks should be of a size that roughly simulates the handle of your iron. With this tool, you can take a practice run any time you want to. Even for veterans like myself, an occasional need arises for a practice shot. For example, when working in a nest of wiring or when working in a small space, a practice run can lead the way to taking the best approach, and may spare you an unpleasant surprise.

Holding Instruments

Surgeons, jewelers, and people working in micro-assembly all know how to hold their tools and to support their hands to maximize control and stability. We will take the following lessons from them. (My thanks to Dr. Irene Gilbert of University of California Medical Center and to Dr. Brian Brown of Smith-Kettlewell Institute for these lessons in physiology.)

We possess one set of muscles capable of precise manipulation, the thumb and fingers. The muscles have a large number of nerve fibres dedicated to them, and a large part of the brain is devoted to controlling them. In Dr. Gilbert's words, "The area of cortical neurons overseeing the thumb alone is almost as large as the area over-seeing the entire leg and foot. Therefore, precise manipulation of instruments can best be done with the thumb and fingers.

Coarse muscle systems which are not designed for fine work, namely muscles controlling motion of the arm, should be taken out of play. Free motion of the arm has a profoundly detrimental effect on the precision of finger motion. The main muscles which operate the fingers are not actually in the fingers, they are in the forearm. The fingers are controlled through a complicated pulley system of tendons and ligaments in the wrist. Precise control of the thumb and fingers cannot be attained through an unstable pulley system.

Much of what is seen as hand and finger tremor is caused by failure to stabilize the arm and wrist. People who do fine work learn quickly to stabilize their wrists and forearms on solid objects. Dr. Gilbert remarked, "I once made a plaster half shell of my forearm from elbow to wrist and mounted it through a ball and socket onto a base. The shell could pivot and rock, and yet hold my wrist and forearm steady, leaving my fine hand and finger muscles to manipulate freely and precisely." At the very least, your elbow should be braced against your body, and your wrist or hand should be resting comfortably against some solid object. Bracing your hand against the work piece is often sufficient; however, books, blocks, or the spool of solder should be used as needed.

Holding the iron like a pencil is usually advocated in discussions on soldering. With the side of your hand resting on the stabilizing object, the iron is held between your thumb and first finger, with the middle finger curled under the handle to provide a supporting cross-bar. I modify this grip by uncurling the middle finger and placing it along the handle under the index finger; this arrangement gives me better vertical position information. I hold the solder between my thumb and middle finger of the other hand, leaving the first finger free to touch and guide the iron where necessary. To maximize control of it, the solder should be held about three-quarters of an inch from the end.


In the performance of all tasks involving movement, the sense of joint and muscle position (kinesthetic sense) is relied upon heavily. When pianists and typists have reached perfection, they no longer look at the keys. A skill is really learned when one can say, "I can do it with my eyes closed."

In soldering without visual feedback, much about the position of the iron can be predicted using the kinesthetic sense, but the limits of this bio-physical system must be understood. "Can the sense of the position of your hand ever achieve a resolution of one-tenth of an inch, which is necessary for soldering integrated circuits?" This question must be answered with another question, "Where is your starting point with respect to the target?" The accuracy with which a movement can reliably be made is a fixed proportion of the distance to be moved. Generally, it is about 5 percent of the distance to be moved, and so to achieve the accuracy necessary for soldering integrated circuit pins, the last movement should be from a land-mark about two inches away. I propose that a few reference points and rest positions along the way to the target be identified, a procedure I shall call "land-marking".

Note the position of main items associated with the project on which you are working. While holding the iron in your hand, these items can be located with the heel of your hand or with a couple of straying fingers. By bringing the iron over to one of these cross reference points, the distance to the target can be reduced to at least one-fifth the original distance (from the rest stand). By staying in contact with these items (using them as rest points to stabilize your hand), the iron can be very precisely controlled with the thumb and fingers.

For the next stage, the iron can be used to do some of the land-marking. There are usually a number of relatively inert items which will not be harmed by bringing the barrel or the tip of the iron into brief contact with them. The edge of the chassis, the handle of your vise, or strategically placed C clamp, alligator clip, or clip-on heat sink are examples worth noting. If you are as lazy as your Editor, you may use the edge of the circuit board, a nearby terminal strip, or the body of a Bakelite component for land-marking with the iron (naughty-naughty). Make sure that the tip of the iron is wiped free of solder before you do this, or you will spill droplets of solder into the project. Remember, be gentle with the tip at this time, since it will be vulnerable to damage without its protective coating of excess solder.

The final land-mark should be chosen so that you have a short hop to the target. Regarding the choice of this landmark's position, Dr. Gilbert suggests that the following information on current research be noted:

  1. Movements made horizontally are more precise than those made vertically.
  2. One is more accurate in knowing a movement's direction than the length of the movement.

During the final reach, the iron becomes your "cane". In this way, it can indicate to you when it has come to rest on the target. Unlike the "cane" traveler, the person wielding the soldering iron has control over the features of his terrain. He can arrange for the target to have unique features. Some examples of this are listed below:

  • If a component is being soldered into a printed circuit board (PC board), leave the leads long, so that the target is "the only tree in the forest". These long leads will be easy to find with the tip of the iron, especially if you remember to cut the excess wire off components you have previously soldered.
  • A wire being soldered to a terminal lug may be arranged so that its end protrudes beyond the terminal, making this terminal easy to find with the iron. After it has been soldered, cut off the excess wire.
  • When wiring integrated circuits on perforated board (point-to-point wiring), lay a component lead across its intended IC pin; cut it off so that it extends conspicuously beyond the center line of the IC (between the two rows of pins). When approaching this connection with the soldering iron, reach not for the IC pin but for the point at which the component lead crosses the center line of the IC. In other words, fish for the extended wire with the tip of the iron; when you have found it, follow it over to the pin. After this connection has been soldered, cut the excess wire off close to the pin and proceed to the next pin. Arrange the order of your tasks so that the pins are soldered in consecutive sequence (it is harder to hit the pin if it is between two previously soldered ones). (You will have more space between pins and have less trouble with bridging them if you use only one component lead per pin. Other components going to this pin can be soldered to the lead of the first component.)
  • When soldering multi-pin components on which the pins are very long, such as PC-mount potentiometers, and wire-wrap sockets, solder these pins in sequence, and cut them off near the solder joint as you go.

Touching the Iron

With some jobs, such as soldering components with short leads into a PC board or removing defective components from a PC board, no convenient system exists to give the target unique features by which it can be easily found. In such cases, the iron can be guided into position by a free finger of your other hand. Believe it or not, the hot iron can be touched.

In touching the iron, two overriding principles must be adhered to: the tip must be wiped clean, and you must not make any quick, uncontrolled movements.

Any fisherman knows not to make fast jerky movements around his tackle, otherwise he may impale himself on a hook. The same philosophy should prevail when handling a hot soldering iron. The more relaxed and even your motions are, the less chance you have of coming into unexpected firm contact with the barrel or tip of the iron. If the job is such that you must touch the iron, do so with smooth, light, brushing motions. (Your Editor considers that fear of such things is relative. I would much rather solder than light a cigarette, which scares the Devil out of me.)

Tactile Feedback

When you have hit the target, several indications can be used to affirm that it is the desired one. At this point you have a "cane" in each hand the iron is in one hand, and the solder (which awaits your arrival at the target) serves as a "cane" in the other hand.

Using the iron as your "cane", the contour of the target should make sense as you gently scan it with the iron.

The solder, which is being held against the connection by your other hand, will vibrate when the iron touches the connection. At this point, move the solder over to where you suspect the tip of the iron is resting on the connection. If it melts, the iron is on target and you were right about its position. Bring the solder back to a point which is not in direct contact with the tip of the iron and feed the desired amount of solder to the joint (from one-eighth to one-half inch depending on the size of the connection and of the solder).

The solder may melt immediately when the iron touches the connection; this is your indication that the iron and the solder touched the target in the same place. Often, you will lose the connection with the solder. When this happens, feel around with the solder until you apply it to the connection which causes it to melt.

Another indication of hitting the desired target is that the wiring and components associated with the connection will heat up. A spare finger of the solder-feeding hand can be used to monitor this condition.

Indications that you have missed the target are:

  • Components associated with a different connection may heat up, indicating that you have found their connection instead of the desired one.
  • The solder will refuse to melt when it is scanned over the connection. Scanning adjacent connections with the solder will reveal which one is being mistakenly heated.
  • If you suspect that you have gotten on to the wrong connection, there is a clue which may indicate to you whether or not this connection has been previously soldered. If small motions of the iron feel "squeaky", the surfaces against which the iron is in contact are wet with solder, which probably signifies that the connection has been soldered before (if the work materials have been previously "tinned", the iron may be squeaking against individual items rather than a fully completed solder connection). If small motions of the iron do not feel "squeaky", the items you have contacted have never been soldered.

Tools and Accessories

General Comments

When people refer to the "balance" of instruments, they are actually referring to the ease and control with which these instruments can be manipulated. Used in this sense, the term "balance" implies that a favorable compromise in weight and distribution of mass was made in the design. In general, the tools of the highest utility are those which are light in weight, and which present a minimum of inertia.

It follows that smaller tools are easier to control than larger ones, because characteristics unfavorable to manipulation become less significant.

For many years I used a large fancy jack knife whose handle contained a pair of pliers. However, for many small jobs (such as stripping wire) it was clumsy to hold and would frequently topple out of position. Since then, I have come to prefer very small pen knives with no extraneous gadgets in the handle.

The above principles can be expanded to include all hand tools used in soldering. Short light-weight tools which can be held near their center of gravity afford precision control and promote stability when holding them in place.

With regard to vises and holding clamps, "low-profile" systems are preferable. Having the work close to the work bench puts you in a comfortable position and greatly increases the number of objects against which your wrist can be stabilized.

(NOTE - - an asterisk appearing after the name of a supplier refers the reader to a list of addresses at the conclusion of this Article. Note also that many of these items are available from other than the supplier mentioned. Whenever possible, we chose to mention the supplier whose current price listing was lowest.)

Soldering Irons

Two grades of constant-power irons are listed, as well as detailed information on my favorite temperature controlled iron made by Weller. On the advice of David Plumlee of Independence, Missouri, the Wahl "Isotip Cordless Solder Gun" appears in this list even though it is an instant-heat fast-cooling iron as discussed in "Soldering, Part 1".


This iron is almost universally available in parts stores. The price is always less than $13.00.

  • 776 - - handle with cork grip
  • 64-2082 - - 33W heating element (screw-on tip is used)
  • 64-2083 - - 45W heating element (screw-on tip is used)
  • PL151 - - plated screw-on screwdriver tip.

Avoid the 64-2080 "Cool Grip" handle, which gets very warm near its forward end.


Although I have never used irons of this brand, they come very highly recommended by Dennis Bernier of Kester Solder Company. He specifically suggests trying the "hatchet-shaped" model.

Hexacon claims that their irons are much more efficient than those of their competitors. A 1-1/4 inch insertion of the tip into the heating element provides very tight coupling.

These irons cost about $16.00, with the exception of the little H14 Hornet Iron for $8.50.

  • 22H-35 - - 35W hatchet-shaped
  • 22A-35 - - 35W straight iron, with about 3 inches from handle to tip 23A-50 - - 50w straight iron, with 3-1/2 inches from handle to tip
  • H14 - - 20W (Hornet series), 2 inches from handle to tip.

Available from Marshall Industries.*


A permanent magnet plunger operates a set of switch contacts which turns the heater on and off. The magnet in this plunger is attracted to a ferromagnetic "temperature sensor" in the shank of the tip. The ferromagnetic sensor in the tip loses its magnetic properties when it reaches its "Curie temperature"" at which point the magnetized plunger is no longer attracted to it. Thus, when the tip is at its Curie temperature, the spring loaded plunger moves back and disconnects power to the iron. When the tip's sensor drops below its Curie temperature, the magnet jumps forward, causing power to the heater to be restored.

Tips for three different temperatures are available. Units of 600, 700, and 800 degrees F. are denoted by suffixes 6, 7 and 8 respectively.

  • STCPM - Weller "Soldering Station" comes with conical tip
  • PTA - - 1/16 inch screwdriver
  • PTB - - 3/32 inch screwdriver
  • PTC - - 3/8 inch screwdriver
  • PTD - - 3/16 inch screwdriver
  • PTH - - 1/32 inch screwdriver
  • PTL - - 5/64 inch screwdriver (long)
  • PTM - - 1/8 inch screwdriver (long)

PTB, PTD and PTL tips are only available as 700 degree units, PTB-7, PTD-7, PTL-7. All others are available at three temperatures.)


tips for this iron are "iron-clad" with a surface plating of nickel to accept tinning by the manufacturer. If this tinned surface becomes flawed, the tip must be replaced).

This WTCPM Station is available from Fordham Radio at $60.00. Tips are five for $12.00.*


Not only is this an instant-heat device, but it has a hands-free solder-feeding system that enables the user to deal out solder from the same hand as holds the gun. I have always had poor success with these solder-feeders, since monitoring the melting of solder cannot be done directly. However, David Plumlee advises me that by attending to the temperature of leads going to the connection (with the other hand), the time at which solder should be fed from the gun can be accurately guessed. A very sharp rise in the temperature of these leads after the initial feeding of solder will indicate that the solder has melted. Perhaps the squeaky feel of solder-wet metals can serve as an additional indicator.

  • 7900 - - Wahl "Isotip Cordless Solder Gun"

Available from Fordham for $40.00 and from Allied at $45.00.*

Cleaning Sponges

The cleaning sponges which come with the rest stand of most soldering irons are of the simplest configuration, flat. While these are better than using your apron, several passes must be made over them while the iron is being rotated in your hand. This cools the iron, weakens the power cord, and does not guarantee that the tip will be wiped free of dangerous droplets of solder. Take the trouble to procure the following item:


contains four 3-inch sponges standing on edge in a holder. These sponges have beveled edges to permit the tip of the iron to "dive" down between them.

  • 480 - - American Beauty "One-Pass Tip Cleaner" with holder
  • 480S - - replacement set of four sponges.

Available from Marshall Industries and Allied Radio at about $5.00.* However, the 480 and 480S are also marketed by Mouser* as Catalog Numbers:

  • Mouser 543-63040 - - complete set with holder (480)
  • Mouser 543-63041 - - replacement sponges (480S).

Soldering Aids

Kits from two companies are listed here - Radio Shack and X-acto. The four tools in the Radio Shack "Solder Ease Kit" are double-ended tools, with different shaped probes extending from either end of plastic handles. However, by sawing through the plastic about 3/4 inch from either end, these tools become manageable. The probes included with the X-acto kit fit into a pin vise type handle, which allows them to be used with or without their heavy handle.

  • Radio Shack 64-2227 - - "4-piece Solder Ease Kit", $3.00
  • Mouser 590-6348 - - X-acto "Solder Aid Kit", $6.00.

Tweezers and Heat Sinks

Forceps which can lock or clamp on to wire leads are very useful tools. These devices can be clamped on to the leads of heat sensitive components in order to absorb the heat and prevent damage to the components. These forceps can be used as "handles" by which wire leads are held in place while soldering them.

My favorite tools of this kind are surgeons' forceps. Shaped like scissors, they can be used as very small pliers to aid in forming wires around connection terminals. They have mechanisms allowing them to be locked in to place. Being made of stainless steel, they cannot accidentally be soldered into the project.

Spring-loaded heat sinks can serve many of the same purposes. Radio Shack sells a very nice kit of aluminum heat sinks which contains one such clamp mounted on a strong permanent magnet.

"Seizers" (two position locking forceps):

  • 42H - - 6-inch serrated straight nose, $10.25
  • 43H - - 6-inch serrated curved nose, $10.50.
  • Available from Fordham. *

  • Radio Shack 276-001 - - "Heat Sink Kit"" $2.00.

Lab Jack

If supporting the wrist is so important, perhaps there are those of you who would prefer to return the dictionary to the bookshelf and use a sophisticated adjustable platform instead. A "Lab Jack" is made up of two platforms separated by a sturdy scissor-jack mechanism. By turning a thumb-screw, the height of the upper platform can be adjusted as desired.

  • "Jiffy-Jack" - - 6-1/4 by 7-1/2 inches (platform size),
  • Cole-Parmer* No. 8056-20, about $60.

Vises and Holding Devices

A good strong vise is the best tool for rigidly supporting a work piece. A vise takes on a new measure of convenience when it can be swiveled to orient the work in a favorable position for soldering. It is for this reason that products of the Panavise Company are surveyed in this list. The bases listed provide great flexibility in positioning the vise heads.

Besides traditional vises, fixtures specifically designed for holding circuit boards have become very popular. Many such holders grip one edge of the board between two strips or blocks. In order for this type to be used, a section along one edge of the board must be left free of circuit elements. Holders made by Panavise on the other hand, hold opposing edges of the board between two spring-loaded V-shaped channels. This arrangement permits the greatest flexibility of board layout.

All board holders have provision to flip the board over, so that work can be done from either side.

Panavise Products:

Any combination of the items below can be assembled. My favorite items are the low-profile head (304), circuit board holder (315), and the vacuum base (380). I rarely take the time to clean off the bench and secure the vacuum base as intended, but the rubber pad on the bottom of this base minimizes slipping while allowing me to position it anywhere.


The jaws of these vise heads are nylon, and readily melt when touched by the iron. This must be avoided or the tip of the iron will be contaminated.

  • 300 - - original screw-down base
  • 305 - - low-profile screw-down base
  • 380 - - vacuum base
  • 308 - - weighted base mount for screw-down units
  • 311 - - bench clamp base
  • 303 - - original regular vise head (jaws are 2-1/2 inches across)
  • 304 - - low-profile vise head
  • 366 - - wide opening head, up to 6-1/2 inches}
  • 315 - - circuit board holder.

Available from Fordham and Jameco.*

All items are from $15.00 to $17.00 with the exception of the circuit board holder and the vacuum base which are $20.00.


This is the simplest of board holders. It consists of a spring-loaded slot into which one edge of the board is fitted, and this whole assembly is hinged to a bench clamp. Its one disadvantage is that it puts the work out beyond the edge of the bench, making bracing of the wrists very difficult. However, an "Extension Bench Clamp" can be purchased which puts the unit 6 inches back from the edge of the bench.

  • PCB-3 - - "Third Hand", $5.00
  • PCB-X - - "Extension Bench Clamp", $13.00.

Available from Jameco.*


Although companies like Panavise make outfits which have everything on them, including the soldering iron rest stand, few are worth the money. Here's one that is. (I have been unable to find out just who the manufacturer is.)

This device has a low-profile cast iron base on which an upright bar is mounted. At the top of this bar a sandwich-type board holder is assembled with provisions for tilting and rotating (flipping) the board. Two sockets with set screws are located at the rear corners of the base, and these are intended to accept cold -roll steel rods supporting the spool of solder and the iron rest stand.

My first recommendation is that the upright member supporting the board holder be shortened. It is secured to the base by a stove bolt up through the bottom, and you can drill and tap this upright bar to be mounted again after it has been shortened.

The sockets in the rear corners can be used to mount any number of ingenious gadgets having a 1/4 inch peg to fit into them. Whether you wish to attach a home made hand rest, or fly your school colors from the work bench, this stand gives you the versatility to do so.

  • ST10 - - "Deluxe Solder Station", $25.00.

Available from Fordham.*


The basic differences between a sighted person soldering and a blind person soldering can be described in terms of feedback. There is no argument that the blind person is operating "open loop" (without direct feedback) part of the time. In reaching from one place to the next and in ascertaining what is actually happening during the soldering process, the blind person is forced to use discontinuous bits of information, whereas his sighted counterpart has information which is continuous. The gaps - the discontinuities in information can be made less significant. By shortening the reach, a target can be hit fairly accurately without vision. By attending to alternative or indirect cues, deductive reasoning can verify facts which are not seen. These principles are not new, but they have tragically remained unsaid.

Blind people have been soldering throughout this entire century. As for myself, I was employed as a technician for three years building very complex electronic equipment. Since then, I have wired the prototypes of my own engineering.

Topics yet to be covered include: tinning stranded wire, soldering of various connectors, de-soldering, and "resistance soldering".

* Address List of Suppliers

  • Allied Electronics, 1355 McClean Blvd., Elgin, IL 60120; Phone: (312) 697-8200.
  • Cole-Parmer Instrument Company, 7025 N. Oak Park Avenue, Chicago, IL 60648;Phone: (800) 323-4340.
  • Fordham Radio, 855 Conklin Street, Farmingdale, NY 11735; Phone:(800) 645-9518.
  • Jameco Electronics, 1355 Shoreway Road, Belmont, CA 94002; Phone: (415) 592-8097.
  • Marshall Industries, 788 Palomar Avenue, Sunnyvale, CA 94086; Phone: (408) 732-1100.
  • Mouser Electronics, 11433 Woodside Avenue, Lakeside, CA 92040; Phone: (714) 449-2222.

Solderless Breadboards

Breadboards have always been a part of experimenting in electronics. The original "breadboards" were exactly that - wooden boards such as those used in the kitchen. (In fact, kitchens in the homes of budding scientists were plagued with a chronic shortage of this item.) Components were screwed down to the board, and additional wood screws served as binding posts by which the circuit connections were made.

Fortunately for the culinary artist, the configuration of modern electronic components has driven little Maxwell out of the kitchen and into his favorite parts store to get a breadboard more suited to his needs.

The majority of modern components are printed circuit (PC) compatible; their terminals are either comparatively short wire leads or pins intended for soldering into printed circuit boards. There is no practical way of connecting these components together with screw terminals.

The breadboards to be described are arranged so that these components can be "plugged in". They have sequences of interconnected holes (spaced at 1/10 inch centers) which permit multiple connections to be made at each component terminal. Major components such as IC's, transistors, and PC-mount potentiometers are plugged in at convenient positions on the board, where they can be connected together using resistors, capacitors, diodes, and jumpers of hook-up wire. The purpose of this article is to survey the breadboards of three manufacturers.

General Description

All of these boards have the same basic design configuration. A large rectangular central block is in the configuration of a gigantic IC socket. A groove along its length denotes the "center line" along the two rows of pins commonly found on IC's. IC's are plugged in so as to straddle this groove.

This discussion will be carried out with the center line groove being oriented horizontally. Each column of five holes (on either side of the groove, leading from the groove to long edge of the rectangular block) represents a continuous electrical path. In other words, all five holes in each column are electrically connected together. If a dual in-line IC were plugged in as mentioned above, each pin would have four electrical connections associated with it, running in a straight line between the pin and the edge of the socket block.

Mounted above and below the socket block are "bus strips", which are narrow strips containing long strings of continuously connected holes. These can be used for distribution of power to the circuit or wherever the need arises for very long bus bars having dozens of electrical connection points. Although these long strings of holes are connected electrically, they are physically arranged in groups of five, with every sixth hole being omitted.

Bus strips and socket blocks can be purchased in modular form, permitting the experimenter to place them as he wishes. In my case, I like to mount the modules with a separation in between them, allowing me to mark the edges of the socket block with notches every five pins. (You can easily become lost when confronted with these large assemblies, and you don't have to be very lost to make a wiring error in a field of holes with 1/10 inch spacing. Marking the socket block can save you the trouble of repeatedly scanning the columns of holes with a Braille stylus.)

Tongue-and-groove arrangements on the edges of certain of these modules permit them to be fastened together to provide the ambitious experimenter with acres of breadboard (fastening them together leaves no space between the modules)?

All the breadboard products listed here have provision for mounting them to a sub-panel; even those with tongue-and-groove interlocking systems. All of them contain screw holes for mounting. Many of them come with a covered adhesive backing by which they can be secured to a sub-panel. Those which do not have an adhesive backing can be secured using double-sided Scotch tape, a standard item at stationery stores.

A generous supply of plastic-covered 24-gauge hook-up wire should be laid in store by the experimenter as well as a large spool of spaghetti tubing which will comfortably fit over the component leads of the largest size anticipated (about 20-gauge wire). The bare ends of wire to be plugged in to the board should be exposed for a distance of at least 3/16 inch beyond the insulation. Wire leads will hit bottom at a penetration of about 3/8 inch, so no more than this length should be exposed if shorting between adjacent columns is to be avoided.

My favorite breadboard arrangement is one which I prepare on a piece of copper-clad PC board stock (used with the foil side down, and with this foil grounded to provide a "ground plane" under the project). My arrangement has a socket block mounted slightly below the center of the sub-panel, with two bus strips placed 1/2 inch away from the socket, one above and one below. In addition to these items, I mount at least one six-terminal barrier strip along the top edge of the sub-panel. (Barrier strips are available at Radio Shack, Cat. No. 274-659). This barrier strip (which consists of six pairs of screw terminals) can be used to connect old-fashioned components whose leads are too large to fit into the breadboard. The offending components can be secured to the barrier strip, and jumpers of hook-up wire can then be run from the barrier strip to the breadboard.

Occasionally, the need arises for soldering to be done on components associated with a breadboard project. Stranded wire leads must be "tinned" before they can be fitted into the board. Components with terminal lugs need to have hook-up wire leads attached to them. Do this soldering elsewhere on the work bench; the plastic from which these breadboards are made will melt readily when subjected to splattering solder droplets or direct heat from the iron. The breadboard can easily become marred, and you run the risk of contaminating the tip of the iron if it touches the board accidentally.

Incidentally, I know well-respected engineers who recognize that their time is too valuable to be spent committing each design to a formalized solder project. They actually use this breadboard technology in building the finished device. (Don't drop it!!)


All manufacturers make separate modules and completed breadboard assemblies in a wide variety of sizes some even include power supplies.

The prices of these three manufacturers' products are competitive. It can be said of the socket blocks that they are two bucks an inch, give or take 20 percent. A 6-1/2 inch socket will be about $13. The bus strips (even of differing lengths) stay in the range of $2 to $2.75 each.

The number of available holes for a given length item varies. Continental Specialties and H. H. Smith both sacrifice about a half inch on either end to accommodate the mounting holes, while A. P. Products' units continue the pattern of holes much closer to the ends.

Both continental Specialties and H. H. Smith boards can accommodate wire sizes between 22 and 30 gauge. (1/4 watt resistors typically have leads of 22-gauge). A.P. Products' boards will accept 20-gauge wire leads, permitting the use of 1/2 watt resistors.

Bus strips from Continental Specialties and H. H. Smith have two rows of continuously-connected holes along the entire length (discounting the room taken by the mounting holes). A.P. Products' bus strips are electrically divided in the middle, giving a total of four half-length bus strips. (Remember this feature - more than one bright experimenter has supplied power to only half his project by forgetting to jumper two lengths together).

A.P. Products' modules do not interlock. They must be individually mounted on a sub-panel.

Boards made by Continental Specialties are by far the most readily available. Outlets in your area which handle the other two brands must be discovered through consultation of the manufacturers' brochures.

The survey to follow is by no means complete. Continental Specialties and H.H. Smith make modules as small as individual sockets. A.P. Products and Continental Specialties make assemblies of their modules on sub-panels containing a few binding posts for connection of power supplies and other accessories. In general, the prices of the various arrays follow the rules outlined above. The moral of the story is: "Beware of windy Editors who decide what you shall hear."

A.P. Products

(A.P. Products, Inc., 1359 West Jackson Street, Painesville, Ohio 44077

Phone: (216) 354-2102.)

"Terminal Strips" and "Distribution Strips":

The term "Terminal Strips" refers to the socket block. "Distribution Strips" are the bus strips. Each "Distribution Strip" is made up of four half-length bus bars. Also worthy of note is that the pattern of holes on this item is uniquely different, even differing from other A.P. Products bus strips. The "Distribution Strips" have their holes arranged in groups of four instead of five.

  • 264L - 6-1/2 inch socket
  • 212R - 6-1/2 inch distribution
  • 248L - 4.9 inch socket
  • 209R - 4.9 inch strip
  • 234L - 3-1/2 inch socket
  • 206R - 3-1/2 inch strip.
"Super Strip" (Model SS2)

This unit has a single row of bus holes along each edge of the block. The hole pattern is in groups of five, and these buses are electrically cut in half. The price of this 6-1/2 inch long unit is about $17.00.

"Power Ace" (Model No. 103):

This is the largest of all A.P.'s systems. It contains three power supplies (plus 5, plus 15, and minus 15V) , along with two SS2 Super Strips. Price: $125.00.

Continental Specialties

(Continental Specialties, Corp., 70 Fulton Terrace, Box 1942, New Haven, Connecticut 06509.

Phone: (203) 624-3103.)

* "Modular Sockets" and "Bus Strips"

These units can be interlocked by means of small tongues and matching grooves near either end. The bus strips contain a double row, the two rows being electrically insulated. They bare the prefix QT, followed by the number of pins or columns the socket has along its length. Thus, a QT59 unit refers to a block about 6-1/2 inches long, with 59 columns of pins comprising the socket. A B suffix denotes a bus strip, and an S suffix denotes a socket. Note that a QT59B bus strip only contains 50 holes along its length because of every 6th hole being omitted.

  • QT59S - 6-1/2 inch socket
  • QT59B - 6-1/2 inch strip
  • QT47S - 5.3 inch socket
  • QT47B - 5.3 inch strip
  • QT35S - 4.1 inch socket
  • QT35B - 4.1 inch strip.
"Experimental Sockets":

This is Continental's version of the "Super Strip", containing a single bus bar along either edge. They have interlocking channels along all four edges to permit assembly of blocks in all orientations.

  • Exp600 - 6 inch Experimental Socket for small-size IC's.
  • Exp650 - 6 inch Experimental Socket for wide 40-pin IC's.
  • Exp4B - 6 inch bus strip containing four bus bars.
"Protoboard" (Model PB103A)

This has three power supplies (plus 5, plus 15, and minus 15V). It contains three sockets separated by double bus bars and surrounded on all sides by double bus bars. Its price seems to vary widely, depending upon the distributor - from $125.00 to $160.00.

Radio Shack:

(These units look suspiciously like "Experimental Sockets" from Continental Specialties.)

"Modular Breadboard Sockets":

If you wander in asking for breadboards, you will be directed to perforated boards and pre-etched PC boards. Have the proper name and number in hand.

  • 276-174 - 6 inch unit block (Exp 600), $10.95.
  • 274-659 - 6 terminal barrier strip.

H. H. Smith

(Herman H. Smith, 812 Snediker Avenue, Brooklyn, NY 11207. Phone: (212) 272-9400.)

"Breadblox" and "Breadstix":

Said to be "Modular Prototype Aids", these units are socket blocks and double bus strips, respectively. For a given length, they are equivalent to Continental's QT series; a 6-1/2 inch "breadblox" has 59 socket terminals along its length (equivalent to QT59S). The bus contains 50 terminals in groups of five. Small interlocking devices are located near either end.

  • 9416 - 6-1/2 inch "breadblox"
  • 9406 - 6-1/2 inch "breadstix"
  • 9415 - 5-5/16 inch "breadblox"
  • 9405 - 5-5/16 inch "breadstix"
  • 9414 - 4-5/16 inch "breadblox"
  • 9404 - 4-5/16 inch "breadstix".

(Editor's note - many thanks to both Al Alden and Bill Loughborough for doing the leg work on this Article. Without them, I would still be in the kitchen looking for the breadboard.)

Auditory Volume Level Indicators Old and New (Part I.)

This discussion will be of considerable interest to the audiophile. If you are employed in the field of sound recording or radio broadcasting, or if you do high quality recording as an amateur, a consistent system of setting and monitoring the audio levels is essential for realizing the maximum benefit from the equipment. (For use with home recorders, the unit can be connected to the monitor jack.)

Much background work in developing volume level indicators was published in the BTP. In every case, the indicator signals the user when the audio level exceeds a preset threshold. This threshold may be set to 0 VU which is not to be exceeded, or a calibrated input network may be included into the instrument so as to permit measurement of the program level.

By far, the finest indicator to be developed was designed by Jim Swail, an engineer at the National Research Council of Canada. His auditory level indicator has become the standard of the trade for blind people employed in the recording and broadcasting industry.

We at Smith-Kettlewell Institute have attempted to duplicate the performance of Mr Swail's circuit with a design using IC's. In function, we have done so, but not without paying a price. If the user forgets to turn off Mr. Swail's indicator, the battery will still give good service the following week; it's idling current is only 1/2mA. 'As much as we want to hail our instrument as being in the forefront of technology, my integrated-circuit design will bust your pockets overnight if the unit is accidentally left on. Hats off to Jim Swail.

Both the aforementioned circuits have an audio oscillator which has a "triggering" threshold under which the oscillator is shut off. When the control voltage threshold is exceeded, the oscillator "triggers", and its pitch rises as a relative indication of the control signal above the threshold. Both circuits have a rectifier "peak-detector" system which converts the program material into a d.c. control signal at the input of the oscillator. Finally, both circuits have an input audio amplifier to isolate the rectifier oscillator system from the program line, and which boosts the program material to a usable level.

The instruments are calibrated by adjusting the pot at the input of the audio amplifier so that the oscillator noticeably sounds at the level to be indicated.

The Swail Indicator Circuit:

This circuit uses four transistors. Q1 and Q3 are Motorola S0015's, Radio Shack 276-2009's, or 2N2222's. Q2 must be a germanium transistor, Motorola GO 011, or Radio Shack 276-2002. Q4 is a PNP Silicon transistor, Motorola S0019, or Radio Shack 276-2023, or a 2N2907.

The cold side of the audio input is grounded and goes to the negative side of the battery (from 6 to 9 volts). The hot side of the audio input goes to the top of a 50K pot, with the bottom of this pot going through 27K to ground. The arm of the pot goes through 4uF (negative side toward the arm), then through 10K to the base of transistor Q1. The Q1 emitter goes through 1.2K to ground. The Q1 collector goes through 4.7K to the plus V line, and this collector also goes directly to the base of Q2, the germanium transistor. The Q2 emitter goes through 2.2K in parallel with 50uF to ground (negative side of the capacitor at ground). The Q1 base is biased by going through 470K to the emitter of Q2. The Q2 collector goes through 2.2K to the plus V line, with this collector also going through 4 7K in series with 2 uF to the emitter of Q1 (negative of the capacitor at the emitter). (This complicated input amplifier circuit has a relatively constant gain over a wide range of battery voltages.)

The collector of Q2, the output of the audio amplifier, goes through 4uF (positive toward the collector) to the cathode of a germanium diode (1N270), with the anode of this diode being grounded. This diode is shunted by 150K. The cathode of the diode also goes through 10K, then through 4uF to ground (negative toward ground). The junction of 10 K and the 4 uF goes through 1 meg to the base of Q3. The Q3 emitter is grounded. The Q3 collector goes directly to the base of Q4. The base of Q4 also goes through 0.1uF to the bottom of the output transformer primary winding (500 ohm CT to 8 ohm speaker voice coil). The emitter of Q4 goes to the center tap of the transformer primary. The entire primary is shunted by 0.11uF. The top of the primary goes through a JD 0 ohm decoupling resistor to the plus V line, with the top of the primary also being by-passed to the ground through 50uF (negative toward ground). The transformer secondary goes to the voice coil of a loud speaker.

Parts List

Resistors (1/4 watt):

  • 1--100 ohms
  • 1--1.2K
  • 2--2.2K
  • 2--10K
  • 1--27K
  • 2--47K
  • 1--150K
  • 1--470K
  • 1--1 meg.


  • 2--0.1uF
  • 1--2.0 uF
  • 2--4.0 uF
  • 2--5 0uF.

Transistors and Diodes:

  • 1--1N2 70 or 1N914 Germanium Diode
  • 2--NPN Silicon Transistors (HEP 80015 or equivalent)
  • 1--PNP Silicon Transistor (HEP SOO 19 or equivalent)
  • 1--NPN Germanium Transistor (HEP G00 11)


  • 1--Output Transformer (500 ohm CT primary, 8 ohm Secondary).

Our Smith-Kettlewell indicator uses a Signetics NE555 timer IC as the voltage-controlled oscillator. A dual op-amp, national semi-conductor LM358, is used, one half for the input audio amplifier and the other to sense the threshold voltage with the drive oscillator. Both chips are available at Radio Shack. A stereo unit could be built using a dual timer (NE556) and a quad op-amp (LM324). This circuit will operate on a supply voltage ranging from 7 to 30 volts.

NE5 55 :
  • pin 1--Ground
  • Pin 2 --Trigger
  • Pin 3--Output
  • Pin 4 --Enable
  • Pin 5--Control Voltage
  • Pin 6--Threshold (not entirely synonymous with "threshold" as we have been referring to it in this Article).
  • Pin 7--Discharge Terminal
  • Pin 8--Plus V.

(The first timer is confined to pins 1 through 6, with the second having its connections at Pin 8 through 13.)

  • Pin 7--Ground
  • Pin 14--Plus V
  • Pin 1--Discharge Terminal
  • Pin 2--Threshold
  • pin 3-- Control Voltage
  • Pin 4 --Enable
  • Pin 5--Output
  • Pin 6 --Trigger
  • Pin 13--Discharge
  • Pin 12--Threshold
  • Pin 11--Control Voltage
  • Pin 10 --Enable
  • Pin 9--Output
  • Pin 8--Trigger.
LM35 8:
  • Pin 4--Ground
  • Pin 8--Plus V
  • Pin 1--Output A1
  • Pin 2 --Inverting Input A1
  • Pin 3--Non-Inverting Input of A1
  • Pin 7--Output of A2
  • Pin 6--Inverting Input of A2
  • Pin 5--Non-Inverting Input of A2.

(Note that the outputs are at the four corners of the package, and that the inverting inputs are next to the outputs.)

  • Pin 11--Ground
  • Pin 4--Plus V
  • Pin 1--Output A1
  • Pin 2--Inverting Input A1
  • Pin 3--Non-Inverting Input A1
  • Pin 7--Output A2
  • Pin 6--Inverting Input A2
  • Pin 5--Non-Inverting Input A2
  • Pin 8--Output A3
  • Pin 9--Inverting Input A3
  • Pin 10--Non-Inverting Input A3
  • Pin 14--Output A4
  • Pin 13--Inverting Input A4
  • pin 12--Non-Inverting Input A4

The NE555 is a very fancy relaxation oscillator. In the free-running connection, pins 2 and 6 (which are input each of two separate comparators) are tied together and go through an external charging capacitor to ground. Inside the chip, a series string of three 5K resistors is connected across the power supply to 'obtain voltages of 1/3V and 2/3V. The other end of the pin 2 comparator goes to 1/3V, while the other input of the Pin 6 comparator goes to 2/3V. -The outputs of the two comparators operate a bi-stable flip-flop which controls the chip as follows: as the capacitor is charging and Pins 2 and 6 are below 2/3V, the output (Pin 3) is high. When the voltage on the capacitor crosses 2/3V, two things happen; the output goes low and pin 7 (the collector of a transistor) shorts to ground, causing the capacitor to discharge through the resistor between Pins 6 and 7. pin 7 remains shorted until the voltage on the capacitor drops below 1/3V. At this time, the initial conditions are re-established; the output (Pin 3) goes high and pin 7 opens, allowing the capacitor to charge again.

In our level indicator circuit the timer will not oscillate unless the far end of the 47K charging resistor is pulled above 2/3V (the resistor off Pins 2 and 6); the charge on the capacitor must be pulled up above 2/3V before the discharge cycle is initiated. Furthermore, the higher the voltage on the far end of the charging resistor, the faster the charge rate, and the higher the frequency of oscillation.

Very handy for us is that the junction of the top two resistors on the internal voltage divider is available at pin 5, whose connection was provided for "FM-ing" the oscillator. We are not using pin 5 for its intended purpose, we are just sampling the voltage above which the timer chip will oscillate. The second op-amp "compares" the rectified program level with the 2/3V point, and supplies charging current through the 47K resistor when the program level exceeds the voltage on pin 5.

The Smith-Kettlewell Volume Level Indicator Circuit:

The cold side of the audio input goes to the negative side of the battery and to ground. The hot input goes to the top of the calibration pot, with the bottom of this pot being grounded. (This pot is 100K.) The arm of the pot goes through 0.47uF, then through 47K to the inverted input of the first op-amp. This inverting input also goes through 910K to the output of the op-amp. This output goes to the anode of a diode (1N914), with the cathode going through 10K, then through the parallel combination of 820K and the 1uF to ground (negative side grounded). The top of this parallel RC combination goes to the non-inverting input of the second op-amp, with the inverting input going through 100K to pin 5 of the NES 55. This second op-amp also has a 100K feedback resistor from its output back to the inverting input.

The output of this second op-amp goes through a 47K charging resistor to pin 7, the discharge terminal, with pin 7 going through 10K to both pins 2 and 6 (2 and 6 are tied together). Pins 2 and 6 also go through the charging capacitor (0.022uF) to ground.

Because all the reference voltages are taken directly from the power supply, a Fairchild 7805 is used to power the NE555. The positive side of the battery goes to the input terminal of the 7805, while its common terminal is grounded. The output terminal of the regulator (plus 5V) is by-passed to ground through 0.01uF. Pins 4 and 8 of the NE555 are tied together and go to the plus 5V terminal. Pin 1 of the timer is grounded. The 5V output terminal also goes through two 470K resistors to ground, with the junction of these resistors going to the non-inverting input of the first op-amp. This non-inverting input is by-passed by 5uF in parallel with 0.05uF to ground (negative side of the electrolytic grounded). The op-amp IC is powered directly from the battery (Pin 8 goes to plus V, Pin 4 is grounded).

The output of the timer (Pin 3) goes through 47 ohms (1/2 watt) to one side of the speaker, while the other side of the speaker goes to Pin 8 and plus 5V.

(The far end of the speaker can be grounded if it is capacitively coupled to the 47 ohm resistor through 10uF.)

Parts List

Resistors: (1/4 watt)
  • 2--10K
  • 2--47K
  • 3--100K
  • 2--470K
  • 1--820K
  • 1--910K.
Resistors: (1/2 watt)
  • 1--47 ohm.
  • 1-0.01uF
  • 1--0.022uF
  • 1--0.05uF
  • 1--0.47uF
  • 1--1uf
  • 1--5uF
  • 1--10uF (output coupling optional).
Integrated Circuits:
  • 1--LM358
  • 1--NE555
  • 1--UA7805C

Smith-Kettlewell Universal NiCad Battery Charqer

In our development work at Smith-Kettlewell Institute, we use a wide variety of rechargeable battery supply systems to power our prototype devices. Most lab-bench battery chargers of conventional design require the technician to set the charging current using an ammeter, and the setting is usually different for each particular battery string. I have always considered this procedure to be a nuisance and it was my ambition to design a charger which could be easily pre-set to any desired charging current, and which would deliver this pre-set charging current without regard to the number of cells in the battery string.

The battery charger described here uses an active current source which is controlled by switches on the front panel. A three-position range switch sets the scale of a main voltage divider; this range switch can be set to multiply the range of the subsequent controls by 1, 10, and 100mA. The active current source is voltage controlled; an 11-position switch steps through a voltage divider so that the current source delivers 0 through 10 times the setting of the range switch. A toggle switch shifts the main voltage divider up 1/2 position, so that the current can be set to such values as 45mA or 250mA.

As an example of how these controls are used, if a charge current of 75mA is desired, the range switch should be set to the times 10mA position so that the main voltage divider changes in steps of 0mA, 10mA, 20mA, etc. The main voltage divider is, for this example, set to 70mA. Opening the toggle switch will move the main voltage divider half way between 70 and 80mA, or 75mA as desired.

Once the switches have been set, the charger will deliver the desired current to strings of up to 25 NiCad cells in series. (In fact, calibration of the charger can be done under short-circuit conditions.)

An added embellishment was incorporated into our charger, a timer cuts the charge rate down to a 5 percent trickle charge after 16 hours. This permits the unit to be left on and forgotten about until the batteries are needed. This timer is optional, and is so noted in the circuit description.

Our charger was built into a Radio Shack cabinet (No. 270-269) measuring about 3-1/2 by 8 by 6 inches. Except for the power transformer and power transistor, the entire circuit was wired point-to-point on perforated board having holes spaced on 1/10 inch centers. A large heat sink is needed for the power transistor which was mounted outside on the rear panel of the cabinet. The transistor must be mounted with its appropriate insulating hardware, and heat sink compound should be applied to all the contacting surfaces to assure good heat transfer.

In operation, an RCA CA3130 op-amp, together with a Darlington connection of transistors Q1 and Q2, constitute a voltage-controlled active current source. The 11-position main voltage divider and its associated offset toggle switch provide a selectable calibrated voltage to the input of the voltage-controlled current source. When this voltage is applied to the non-inverting input of the op-amp (pin 3), the output of the op- amp (pin 6) adjusts the base bias of the Darlington circuit so that the IR drop across Q2's emitter resistor equals the voltage on the non-inverting input. In other words, this emitter resistor is a "sampling" resistor which is monitored by the inverting input of the op-amp; for any given input voltage on pin 3, a corresponding current will be developed through this resistor to assure that the inverting input voltage follows that of the non-inverting input. A change in the collector load causes the op-amp to reestablish this "sampling" resistor voltage at its inverting input (pin 2), keeping the emitter current constant. Since the collector current approximately equals the emitter current, the collectors of the Darlington circuit can be seen as a constant current source whose current is set by the input voltage.

The three-position range switch changes the "gain" of the current source by selecting different "sampling" resistors, 180 ohms for times 1mA, 18 ohms for times 10mA, and 1.8 ohms for times 100mA. The full-scale range of the main voltage divider is set to 1.8 volts. If the main voltage divider is set to apply the full-scale voltage of 1.8 volts at the non-inverting input, and if the 1.8 ohm resistor is switched into the circuit, the current source will assure that 1 amp flows through this 1.8 ohms, thereby establishing 1.8 volts on the inverting input of the op-amp. With the 18 ohm "sampling" resistor switched into the circuit, the current source will assure that 0.1 amps or 100mA flows through 18 ohms, thereby establishing 1.8 volts on the inverting input. In this way the three selectable emitter resistors of Q2 act as multipliers.

Basic Charger Circuit:

A 28V center-tapped power transformer capable of delivering 1 amp or more is used. One side of the primary winding goes through the on-off switch to one side of the a.c. line while the other end of the primary goes through a 1 amp slow-blow fuse to the other side of the a.c. line. The full secondary winding feeds into a bridge rectifier to obtain about 40V. The center tap of the secondary is used to supply about 20V to a 12V voltage regulator which runs the current source circuit and the timer.

Specifically, each end of the secondary goes through the cathode of a rectifier diode capable of handling an amp (1N4005). The anodes of these two diodes are connected together and go to the circuit common ground (this is not common to either side of the battery; for good safety practice, do not ground the metal cabinet). Each end of the secondary also goes to the anode of another identical diode, with the cathodes of these two diodes being connected together and going through 500uF to ground and to the anodes of the first pair of diodes. The junction of the two cathodes of this latter pair of diodes goes through a 1.5 amp fuse to the positive terminal of the battery. The center tap of the secondary is by-passed to ground through 250uF. (Both the 250uF and 500uF electrolytic capacitors have their negative ends at ground.) The center tap also goes to the input terminal of a Fairchild 7812 voltage regulator. The common terminal of the regulator is grounded, while its output terminal is by-passed by 0.01uF. The output of this regulator is the plus 12V line.

The calibrated voltage divider is supplied from the regulated 12V. This 12V line goes through 47K (this resistor is split up into two resistors if the timer is to be included) then through a 20K calibration rheostat to Position 11 of the voltage divider switch (11-Position single-pole). Ten 1K 1 percent resistors are connected between adjacent poles of the switch; Position 11 goes through 1K to position 10 which goes through 1K to position 9 which goes through 1K to Position 8, etc. position 1 goes through the offset toggle switch to ground. This toggle switch is shunted by 510 ohms. with the toggle switch closed, the 20K calibration rheostat can be adjusted so that 1.8V appears on Position 11. The arm of the 11-Position switch goes to the non-inverting input, pin 3, of the RCA CA3130.

Pin 4 of the RCA CA3130, the negative supply terminal, is grounded, while pin 7, the positive supply terminal, goes to the plus 12V line. Pin 1 goes through a 30pF compensation cap to pin 8. Pin 6, the output of the op-amp, goes through 1K to the base of Q1, with this base also being by-passed to ground through 0.1uF. (Q1 is a 2N2219.) The collector of Q1 goes to the collector of Q2 (2N3055), and these collectors also go to the negative side of the battery. The emitter of Q1 goes directly to the base of Q2. The Q2 emitter goes to one end of all three emitter resistors, 180 ohm 1 percent, 18 ohm 1 percent (1/2 watt), and 1.8 ohm 1 per- cent (5 watt?. The Q2 emitter also goes to the inverting input of the op-amp pin 2. The arm of the 3-Position single-pole range switch is grounded. The far end of the 180 ohm resistor goes to Position 1. The far end of the 18 ohm resistor goes to Position 2. The far end of the 1.8 ohm resistor goes to Position 3. This completes the circuit of the basic charger.

The timer was designed using two Exar integrated circuits (Type XR2242). The XR2242 has an internal time-base oscillator or clock, and an 8-bit counter (8 dividers or flip-flops in cascade). Two XR2242's were connected in cascade to form a chain of 16 dividers. The output of the last divider resets both counters to their initial state. (The circuit which was chosen from the Exar application notes resets and triggers both counters simultaneously, which wastes one cycle of the first 8-bit counter. This circuit stops counting after 32,640 clock pulses.)

The time-base oscillator section of the XR2242 is a very fancy version of a "relaxation oscillator". (Don't despair, the resultant hookup is very simple on the outside of the chip.) The charge state of a capacitor is compared with two points on a voltage divider inside the IC; the capacitor is allowed to charge up to 73 percent of the supply voltage, at which time it is forced to discharge rapidly down to 27 percent of the supply. During the discharge part of the cycle, a pulse appears on the time-base output, pin 8. (These levels of 27 and 73 percent are apparently chosen so that the frequency of the clock comes out even to equal 1/RC.) Only the clock of the first chip is used, and a 1K resistor from pin 7 to ground on the second IC disables its oscillator.

The outputs of the first and the last of the eight dividers are available on pins 2 and 3 respectively. Thus, a square wave at half the clock frequency is available at pin 2, and a square wave of 1/256 the clock frequency is available at pin 3. (All outputs are "open collectors"; they require pull-up resistors as external components in order for these outputs to be made available.)

All outputs are high until the timing circuit is triggered. At this point the output of the last divider remains low until the clock has generated 32,640 pulses. The 32,641st pulse causes the output of the last flip-flop to go high which activates the reset terminal of both counters (pin 5). Both trigger terminals (pin 6) are connected to a single-pole single-throw push-button switch for starting the timer. Once the circuit has been triggered, further pushing of the "start" button will have no effect, because the trigger input does not reset the timer. No "reset" switch was thought to be necessary on our charger, since resetting can be accomplished by turning the power off and on. The output of the last divider drives a transistor which heavily loads the main voltage divider, thus drastically cutting the voltage input to the current source. The timer must be triggered and running in order for the current source to deliver normal charging current.

Timer Circuit:

Pin 4 of each XR2242, the negative supply terminal, is grounded. Pin 1 of each timer IC, the positive supply terminal, goes to plus 12V. Pin 6 of both IC's, the trigger terminals, are tied together and go through a SPST push-button switch to plus 12V. Pin 7 of IC1 goes through C to ground, and also through R to plus 12V (C is 1uF, and R consists of 1 meq in series with a 1 megohm calibration rheostat). Pin 8 of IC1, the clock output, goes through a 22K pull-up resistor to plus 12V. Pin 3 of IC1, the output of the eighth divider, goes through a 47K pull-up resistor to plus 12V, and this pin 3 also goes directly to pin 8 of IC2. Pin 7 of IC2 goes through 1K to ground. Pin 3 of IC2, the output of the last divider, goes through a 10K pull-up resistor to plus 12V, and pin 3 also goes through 47K to pins 5 of both IC's, the reset terminals. Pin 3 of IC2 also goes through 100K to the base of control transistor Q3 (2N2222). This base also goes through 47K to ground. The emitter of Q3 is grounded. The collector of Q3 goes through 470 ohms to the junction of two series resistors, a 27K resistor from this junction up to 12V and a 22K resistor from this junction down to the 20K calibration rheostat in the charger circuit. Of course, the 47K resistor in the basic charger circuit from plus 12V down to the 20K rheostat is omitted.

This timer can be calibrated by connecting a test amplifier (through a 1 meg resistor) to pin 8 of the first timer chip, and counting the clock pulses by ear over a three minute period. For a 16 hour charging time, the clock should be running 0.566 pulses per second. This amounts to having 102 beats in three minutes. By the same arithmetic 117 beats in three minutes will set the timer for 14 hours.

In general, one should charge NiCad batteries at one-tenth their amp-hour rating. Often, AA cells are rated at 450mAH (milliAmpere hours), so these should be charged at 45mA. C cells are often rated at 1Ah, so that these should be charged at 100mA. Radio Shack sells a NiCad 9 volt battery which is rated at 70mAh, and it specifies a charging current of 7mA. (With this charger, you can charge four of these 9V batteries in series.)

There are times when the "poor-man's" decade voltage divider cannot be adjusted to give you exactly the current specified. (A true decade voltage divider does not use a simple offset switch, and its switching network is prohibitively complicated.) One such case arises when charging the G.E. battery pack used in talking book machines (G.E. No. 402988). These are 1.2Ah batteries designed to be charged at 120mA. The closest you can get with our charger is 105mA and 150mA. Since it is never advisable to charge NiCad batteries at higher than their rating (unless you are monitoring their temperature), use 105mA.

This circuit has served us well and we sincerely hope you get a charge out of it.

Parts list (Basic Charger)

Resistors: (1/4W 5%)

  • 1--510 ohm
  • 1--1K
  • 1--47K (omitted if timer is included)
  • 1--20K PC-mount pot

Precision Resistors: (1%)

  • 10--1K
  • 1--180 ohm
  • 1--18 ohm 1/2W (1 selected this value by using a bridge-type ohm meter.)
  • 1--1.8 ohm 5W (made up from a parallel combination of junk box values. )


  • 1--30pF Disc
  • 1--0.1uF Disc
  • 1--250uF 25V
  • 1--500uF 50V


  • 4--1N4005
  • 1--2N2219
  • 1--2N3055
  • 1--uA7812C
  • 1--RCA CA3130



  • 1--Single-pole 11-position
  • 1--Single-pole 3-position (good for 1 amp)
  • 2--SPST toggle


  • 1--117V to 28V center-tapped (good for 1 amp)

Parts List (Timer)

Resistors: (1/4W 5 %)

  • 1--470 ohm
  • 1--1K
  • 1--10K
  • 22--22K
  • 1--27K
  • 3--47K
  • 1--100K
  • 1--1meg
  • 1--1meg PC mount pot


  • 1--1uF 15V


  • 1--2N2222
  • 2--XR2242
  • (Available from Jameco Electronics 1355 Shoreway Road Belmont, CA 94002 (415) 592-8097)


  • 1--SPST Momentary push-button


Unfortunately, this column is a regular feature in electronics journals. I had hoped to avoid its appearance in our magazine, "so soon?" I advise that you take down the following information on a separate piece of paper and staple it to the appropriate page of the Fall, 1980 issue.

Roger Stewart of Pasadena, California points out that three terminal voltage regulators for negative voltages (7900 series and LM320) have different pin connections than do their positive voltage counterparts. Furthermore, the listing of pin connections for the T03 style case was incorrect. The revised section should read:

Pin Connections

(Page 50 for Braille, Page 34 for Large Print).

T0220 Package:

hold the package with the leads pointing upward and with the mounting surface towards you.

For the 7800 series (LM340), the three leads from left to right are input, common, output.

For the 7900 series (LM320), the three leads from left to right are common, input, output. Caution! The case is connected to the input, and not to common, as with the 7800's.

T03 Package:

hold the package with the mounting holes at the extreme left and the extreme right, and with the terminal pins closer to the lefthand mounting hole (bottom view, pins facing you).

For the 7800 series, the top pin is output, the bottom pin is input, and the case is common.

For the 7900 series, the top pin is output, the bottom pin is common, and the case is input.

Also, on the sound advice of Bob Gunderson, I shall describe the connection of a volume control to the LM386.

The bottom of a 10K audio-taper pot goes to Pins 2 and 4 of the LM386 and to ground. The arm of the pot can go directly to pin 3, with the top of the pot going through a coupling capacitor (0.1uF or larger) to the signal source. (Sorry, Professor Gunderson, for this oversight. Can I at least have a B in the course?)

Editor's Corner

We are indeed off and running. I have never done anything like this before, and already the project is blessed with success. (As yet, the number of subscribers is pitifully small, so please continue to spread the word.)

My true reward has come from you. Almost everyone has taken time to comment constructively on my initial effort. All these suggestions have been duly noted, and some are being implemented. All who have written me have offered me genuine encouragement, and my spirit is vibrant with the energy of your enthusiasm.

Several good articles have already been contributed by readers. Bravo, folks! They will begin to appear in the next issue.

Many comments addressed our reprinting of TSI's engineering notes on the Mini Speech Boards, which we ran without sufficient supplemental explanation. I apologize this time, and offer the following statement of policy regarding reprints of manufacturer's data:

Manufacturers make available generalized engineering information which is intended to promote use of their product by purchasers with specific applications in mind. Manufacturers have the burdensome task of presenting this information in such a way as to promote use of their products in a wide variety of specific applications, thus maximizing their market. Depending on the nature of the product and the budget constraints of the company (good technical writers are expensive), the utility and the quality of the information varies widely. All the same, sighted hobbyists and professionals gain much of their inspiration from these data sheets.

We as blind people must preserve the right and the facility of independent access to such materials if their inspirational effect is to be part of our lives. When a manufacturer's brochure is reprinted in the Technical File, this will be done to put our readers on an equal footing with sighted counterparts having similar interests. Those of us who have relevant experience in using such a product (in this case I barely qualify) will have access to the manufacturer's data.

I do, however, recognize that sighted people have access to a larger pool of background information. To those of you who became discouraged while looking at TSI's Mini Speech Board material, I make the following promise. I will never print information on a product of overwhelming interest to the blind reader without following up this information with an exemplary Article on its use (within two or three issues subsequent to the reprint).

The Mini Speech Boards are important components which have various applications of interest to all of us. It would be irresponsible of this Editor to leave the use of them exclusively up to readers who already know how. Articles on digital electronics and the use of these Boards are coming. On the other hand, it would be equally irresponsible of this Editor to neglect data on products for which Smith-Kettlewell Institute has no immediate need, and this Editor may never get around to writing exemplary Articles on these devices. For example, there are IC's of complete dual-conversion radio receivers capable of receiving AM, FM, and SSB. If information is reprinted on these items, this will be done with the intent of inspiring someone who is familiar enough with the terminology of receiver design to build a receiver to suit his own purpose. Any follow-up Article would have to come from the ambitious reader.

I would like to close with a statement of my favorite old Chinese curse, "May you live in interesting times". The power of this curse rests in the implication that one can easily lose control of the forces influencing him. This curse reverts to opportunity as one refines the control of his own destiny.

Your enthusiasm over the project of this magazine has assured me that there are many blind people who are eager to assert and to refine control over our position in this technological age. Well then, bring on the "interesting times" we will transform them from a curse into a blessing. May the impact of these times bless you all with opportunity throughout the coming year.