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-1987

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 Details regarding products, suppliers, and other contact information are original and may be outdated.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


Inside the 555 Integrated Circuit

Bernie's Little Blue Box: A Multi-Function Qualitative Circuit Analyzer

The Vinther Fingertip Soldering Iron

A Maverick Passes Through



Although this material has been flirted with before, it is buried within other articles--ones using the 555 chip. While spotty discussions were sufficient for those papers, a comprehensive look at what this chip contains should exist somewhere in these pages. Furthermore, a specific reason warrants doing so in this issue; running the "Blue Box" of the article to follow will be much easier if you know the nuts and bolts of its heart--the ever-present 555.

Originally a Signetics integrated circuit (IC)--bearing the formal number NE555--this IC has seen more than a decade of common use, and the plain-ol' 555 is made by countless manufacturers. Its intended uses were:

  1. With two resistors and a capacitor, a clock oscillator for digital logic circuits could be simply made.
  2. By making pins 2 and 6 available separately, the IC could become a timer (a "one-shot"); a negative-going pulse on pin 2 would set in motion a time cycle, the length of which is set by a resistor and a capacitor.

The inventiveness of applications engineers soon took over, however, and this has become one of the most widely used chips in the industry. Virtually all the Smith-Kettlewell meter readers and ham-operators' aids use it; in addition, it is the heart of our light probe design, and it is the beeper in some of our most sophisticated aids for blind machinists, etc. Its wide usage is understandable, given its main features which are listed below: First of all, it is able to supply fairly high driving currents in either direction--200mA, returned either to ground or to VCC. (VCC is a fancy engineer's term for the plus battery line.) This 200mA is enough to operate relays or to drive the socks off efficient loudspeakers. (A key difference between the standard 555 and its CMOS equivalent is the capability of its output to drive heavy loads. A single "current limitation" for the CMOS version is not easy to state. At high currents, the FET's in the output of the CMOS version look like resistors; this means that the driving voltage to a low-impedance load may suffer, or that the power dissipation in the "resistance" of the output system may have to be considered in the design.)

Second, there are three good ways of turning an oscillator on and off: cutting the power (as in this project), interrupting the charging of the capacitor, or by operating a control pin called "the enable terminal." Finally, there are three good ways of changing the frequency (or timing of a one-shot)--changing the capacitor, varying the charging current via a resistor or transistor (as is done in this project), or by impressing a voltage on a pin called "the control terminal."


CMOS: Standing for "Complementary Metal-Oxide Semiconductor," this technology affords making integrated circuits out of FET's of complementary type. Devices made with CMOS technology require very little power to operate; high junction currents of standard "bi-polar transistor" circuits are not necessary. (The term "complementary" means that opposite types of FET's are used, exchanging, as you might say, their states, and cooperating as necessary within the circuit. An understanding of complementary arrangements is beyond the scope of this discussion, and would not significantly add to the reader's understanding of the CMOS 555 chip.)

Comparator: This is a device which has one output and two inputs (plus and minus inputs). When the voltage on the plus input is even slightly more positive than that on the negative input, the output of the comparator is "high"; the output will usually try to reach VCC. On the other hand, if the minus input is brought even slightly above the plus input, the output will drop to zero. (Comparators are just op-amps without any negative feed-back.)

FET: Standing for the term "field-effect transistor," it is used here when referring to the transistors in the CMOS version of the 555. In an FET, the field of a voltage applied to its control element (its gate) affects the current flow through a "channel" of semiconductor.

Flip-Flop: The most basic flip-flop is the so-called "RS flip-flop" (RS standing for "reset" and "set"). It has two outputs and two inputs. It "flips" (the first output high and the second low) or "flops" (the first output low and the second high) as one or other of its inputs is operated. It is content to remain at rest in either state when the inputs are at rest. In other words, if the "set" input is operated, the first output goes high (jumps to VCC) and remains there; further triggering of the set terminal causes no further change. If the "reset" input is operated, the first output goes low (goes to zero volts) and remains there until activity again occurs at the set input.

"High": Often called the "1" logic state, this is a voltage above ground, usually VCC (the positive battery voltage).

"Low": Often called the "0" logic state, this voltage is usually at or near ground.

Negative-Edge Triggered: A device is "edge-triggered" if the transition of its input causes subsequent events. As opposed to this, a comparator changes with the absolute state on its input; the comparator would only be considered edge-triggered if its input were capacitively coupled to an input pulse. As you might expect, negative-edge triggering would refer to triggering the device as the input signal went from a high to a low voltage.

One-Shot: This is a device which, when triggered, changes state (its output might go high, for example) for a predetermined length of time. The 555 can be set up as a negative-edge triggered one-shot; bringing pin 2 from high to low will cause the output to go high for a time that is determined by an external resistor and capacitor. The 555 is not a legitimate one-shot, in that if the input is brought low and held there, the "timing" of the one-shot will be overridden; its input must be capacitively coupled to a signal before it operates in the way of standard one-shots. Furthermore, the 555-type one-shot is not "retriggerable."

Retriggerable One-Shot: This refers to a type of one-shot, usually made up of interconnected flip-flops, whose timing is always refreshed by the triggering signal. If, for example, the timing were set to one second, a second triggering pulse in 0.8 seconds would immediately reset this time for one more second. The 555, in its one-shot connection, is not retriggerable; it must be left to run its course.

RS Flip-Flop: See "Flip-Flop."

State: The condition of a circuit--a flip-flop, a comparator, etc. This usually refers to its output state.

VCC: More properly V, followed by the subscript CC, this is an engineer's term for the collector supply voltage of transistor circuits. In FET circuits, the drain supply voltage is often referred to as VDD. What these actually mean is the battery terminal which is not the one being grounded.

Voltage Divider: This is a network whose output represents a percentage of its input voltage. A resistive voltage divider consists of a "series-connected" set of resistors which are driven by the source. For example, within the 555 IC, there is a group of three 5K resistors in series between VCC and ground. The junctions in this string provide voltages of 1/3 VCC and 2/3 VCC, since one-third the supply voltage is "dropped" across each of them.

Theoretical Operation of the 555 Timer IC

There are two comparators in the 555; these are set up to "sense" triggering and threshold voltages. Also within the chip is a voltage divider, made up of three 5K resistors in series between VCC and ground, this divider being what determines the trigger and threshold voltages. (One principal difference between the standard 555 and its CMOS equivalent is that the voltage divider is made up of three 100K resistors instead of 5K ones; this affords much lower current drain.)

The plus input of one comparator is looking at the 1/3 VCC point on the divider; this comparator's output "triggers" a flip-flop within the chip whenever its minus input (available at pin 2) goes below 1/3 VCC. The minus input of another comparator is looking at the 2/3 VCC point on the voltage divider; whenever this comparator's plus input (available at pin 6) is brought to the "threshold" of 2/3 VCC, it acts on the other input of the flip-flop.

The flip-flop performs two functions: The first is to turn on an NPN transistor, the emitter of which is grounded, and the collector of which goes to pin 7 (the discharge terminal). This occurs when the voltage on the threshold (pin 6) is brought above 2/3 VCC to "reset" the flip-flop. The other function is to control the output of the chip (pin 3), which goes high when the trigger (pin 2) is brought below 1/3 VCC to "set" the flip-flop. The flip-flop drives an output system which is a "complementary pair" of transistors; in other words, this flip-flop output is highly buffered and is the output of the chip (pin 3).

First, let us consider the one-shot connection, wherein the threshold and discharge terminals are connected together. These pins (pins 6 and 7) also go through a "timing capacitor" to ground, as well as going through a "charging resistor" to VCC. For the moment, let us tie the trigger input (pin 2) to VCC.

When power is applied, the capacitor will begin to charge by way of current supplied through the resistor. This process will continue until a "threshold" is reached, at which point the threshold comparator trips and "resets" the flip-flop. The flip-flop directs the output to ground--but it also turns on the discharge transistor which shorts the capacitor to ground. The flip-flop--and the whole chip--is perfectly content to stay in this state from now on, so long as no activity occurs on the trigger terminal.

Now suppose we dislodge pin 2 (trigger) from VCC and bring it toward ground. As soon as pin 2 drops below 1/3 VCC, its comparator causes the flip-flop to "set"; the output of the chip goes high, and the discharge terminal relinquishes its hold on the capacitor. The charging cycle now recurs.

If the trigger terminal is gotten out of the way in time (brought back above 1/3 VCC before the charge on the capacitor reaches the threshold voltage), the charging cycle will take just over one time constant, 1.1*R*C, and the output will stay high for this amount of time. (If the trigger terminal is kept below 1/3 VCC, the action of the flip-flop will be blocked, and the so-called one-shot will be locked in the triggered position.)

It is worth mentioning that because of the flip-flop being content to stay in either state, further triggering of pin 2 while charging is taking place will not "retrigger" the one-shot; this is in contrast to most other one-shots, which are "retriggerable."

Now let us consider the free-running oscillator connection of the 555. In this case, pins 2 and 6 (trigger and threshold) will be tied together. Oscillation will occur, since the trigger terminal is now forced to ride up and down with the charge on the capacitor. Pins 2 and 6 go through the capacitor to ground. Pins 2 and 6 then go through a "discharge resistor" to pin 7 (this just "buys some time" in the discharge process and gives us more than an infinitely short pulse during the discharge phase). In turn, pin 7 then goes through a "charging resistor" to VCC. (Actually, the "charging resistor" is now the sum of the two resistors, they being effectively in series.)

When power is applied, the capacitor will charge from current supplied via the two resistors in series. Charging will continue until the voltage on the capacitor (and on pins 2 and 6) reaches 2/3 VCC, at which point the discharge terminal (Pin 7) shorts the junction of the two resistors to ground. When this happens, the capacitor will discharge through the one discharge resistor until its voltage gets down to 1/3 VCC, whereupon pin 7 opens and allows charging to resume.

Notice that the charge time and discharge time are not equal. This is a common criticism of the 555 as an oscillator, and if a squarewave is absolutely necessary for a given application, the 555 may not be the IC of choice. (There is a way to obtain a squarewave by driving the charge and discharge cycles with the 555's output. While this works well for singlefrequency oscillators, flexibility in controlling the frequency by way of the charge current is lost.)

The operating frequency is a bit hard to calculate because of this. The author prefers to calculate the two times, charge and discharge, then add them together and take their reciprocal. For example:
T1 (charging time) = 0.7*(R1+R2)*C
T2 (discharge time) = 0.7*R1*C
f = 1/(T1+T2)

In these expressions, C is the timing capacitor. R1 goes from pins 2 and 6 over to pin 7. R2 goes from pin 7 to VCC. The figure .7 comes from the fact that a voltage swing of one-third of the supply can be accomplished in less than one time constant, 0.693 time constants, to be exact.

Pin Connections

  • Pin 1--Ground (VSS)
  • Pin 8--VCC
  • Pin 2--Trigger
  • Pin 6--Threshold
  • Pin 7--Discharge
  • Pin 4--Enable (high for enable)
  • Pin 5--Control (This is the 2/3 VCC point on the internal voltage divider.)


by Bernie Vinther

[Editor's note: An enhancement to this project was published in a later issue. Please click here to read the enhancement article.]


Using only a single chip (a CMOS version of the 555, such as the Intersil ICM7555), this pocket-sized "Blue Box" is basically a voltage- and/or current-controlled oscillator with selectable ranges and circuit variations. Once the user learns to interpret the indications of its audible tone, it can be used for tests such as those stated below:

It can be used as a continuity tester whose "range" is selectable; a change in pitch can occur for as little resistance as 10 ohms, or it can be set up to make sounds for leakage paths of 200 megohms or more. It can be used as a voltmeter which will respond to as little as 100 millivolts, and whose tone output is still audible at 50 volts. Its dynamic output makes it extremely useful as an attachment to talking meters; having it look at the input terminals allows the user to hear dynamic information as well as the accuracy of speech.

Selectable resistances on its input (primarily intended for selecting ranges of the continuity testing function) can present mild loads to voltages, making it ideal for testing small batteries. If low-value input loads are selected, the "Blue Box" becomes a sensitive current meter--indicating as little as 10 microamps with an impedance of 10K, and a usable range of frequencies can be selected for currents as high as 1 milliamp (with an input impedance of 1K). It can be used as a "relative" meter reader, making such instruments as S-meters accessible.

With it, you can compare the forward voltage drops of various types of diodes. You can detect and compare leakage currents of small capacitors and reverse-biased diodes. With any type of light sensor, it can become a "light probe" of selectable sensitivity (with its variable load, it can be set to detect subtle differences at intensities of sunlight, and with a cadmium-sulphide photoresistor, starlight is detectable).

Proper use and interpretation of the "Blue Box" will be much easier if the internal workings of the 555 (essentially the same as for the CMOS 7555) are known by heart. For this reason, review of the previous article, "Inside the 555," is recommended. Though this article looks horrendous, the instrument is extremely easy to build.

* * *

Description and Circuit Operation

I called this device the "Little Blue Box" because of the small all-plastic cabinet I used in building the original instrument. Since then, however, I have even reduced its physical size--fitting the next model into a box of 1 by 2 by 3 inches (black, unfortunately). For those who want "elbow room" in their first unit, the original cabinet is readily available from Radio Shack as No. 270-222. It measures 4-3/4 by 2-1/2 by 1-9/16 inches.

The "Blue Box" runs off a standard 9-volt battery. Because a CMOS IC is used, the battery will last nearly its shelf life. Its idling current can be as low as 50 microamps; however, this will increase to slightly more than 100 microamps when set for a "low threshold" (the reasons for this will become apparent as you study the circuit). Measuring the continuity of low resistances can draw 10 milliamps, but this is not often done for long durations.

Much space was saved by using "DIP" switches (switches in dual in-line packages) as the selector switches, and by using a piezoelectric transducer for the speaker (Radio Shack No. 276-076). Besides these items, three additional miniature toggle switches and two mini phone jacks complete the control panel--all of these items being mounted on the lid of the box. The IC and its associated components can easily fit on a small square of perforated board (a square 2.2 inches on a side will lie flat on the bottom of the box). Because the battery needs changing so infrequently, it need only be taped in place inside the box.

Because they are so close together, DIP switches are hard to manipulate; however, by nipping off every other one with diagonal cutters, the remaining ones are spaced far enough apart to make DIP switches convenient to use. They were first mounted on perforated board (clipping off unused terminals as well, to make them easier to solder), after which the board carrying them was fastened to the underside of the lid. Bill Gerrey, at Smith-Kettlewell, used rotary switches instead of DIP units; these work equally well (but if rotary switches are used, the designer must remember to include an unused or "open" position for the times where an infinite resistance--no resistor--is desired).

Perhaps the easiest way to understand this circuit is to watch it evolve--first describing the bare-bones 555 oscillator, then adding features of this instrument as modifications to that. The complete circuit description for building the Blue Box will be given later, so don't wire anything up on the basis of the description that immediately follows.

Basic Free-Running 555 Oscillator

A 9-volt power source is used. The power connections are assumed, and the "Enable" of the IC, pin 4, is tied high. The output, pin 3, goes through two resistors in series to the speaker. One of these resistors is shunted by a "volume switch"; when the switch is closed, the speaker is driven through only one of the resistors, thus making it louder.

Pins 2 and 6 are tied together and go through a capacitor to one of the battery lines. (This capacitor is usually returned to ground; however, returning it to VCC means that its leakage will not load the continuity tester in its most sensitive position, so I have made my second unit with this cap going to VCC.) Pins 2 and 6 also go through 100K to pin 7, the "Discharge Pin." Pin 7 now goes through a selectable charging resistor to a generalized input point--we shall call it point "IN." (This selectable "charging" resistor has a range of from 1 megohm to 10 megohms.)

Pins 2 and 6 are inputs to comparators whose other inputs are sensing voltage levels established elsewhere in the chip. (It will be seen that, given a supply of 9 volts, these voltage levels should be at 3 and 6 volts--1/3 and 2/3 of 9 volts.) If an attempt is made to bring them above 6 volts, pin 7 (the drain of an FET whose source is grounded) pulls down on them through the 100K resistor; pin 7 remains shorted to ground until pins 2 and 6 drop to 3 volts, whereupon pin 7 opens and allows pins 2 and 6 to be manipulated again. If, through the charging resistor, point "IN" is tied to a voltage that exceeds 6 volts, the circuit will oscillate; the charge on the capacitor going to pins 2 and 6 will first be pulled up to 6 volts by current from point "IN," then brought down to 3 volts by the discharge FET on pin 7. If point "IN" does not exceed 6 volts, the oscillator will be dormant; the first half of the cycle will not occur.

Evolving the Voltmeter

As you can see, we already have a 6-volt meter (assuming a 9-volt supply). As soon as point "IN" reaches 6 volts, a clicking sound will be heard from the speaker. Furthermore, the higher you bring point "IN," the faster will be the charging cycle; the clicking will become a tone whose frequency is a function of the input voltage. The degree by which the frequency rises is also dependent on the value of the charging resistor; changing it from 10 megs to 1 meg will mean that the frequency will rise much farther for a given upward swing of the voltage on point "IN." Thus, we have a voltage-controlled oscillator which works above a "threshold" of 6 volts, and whose frequency "sensitivity" can be selected by choosing a desired charging resistor.

Within the 7555 is a voltage divider comprised of three 100K resistors between VCC and ground. (These resistors are 5K elements in the standard 555.) The junctions at the ends of the middle resistor are the points looked at by the aforementioned comparators. Left alone, these junctions are at 1/3 and 2/3 of the supply voltage. If we could just change the voltage on this resistor string, we could alter the "threshold" at which the oscillator would trigger from a voltage on point "IN." Well, pin 5 gives us a way to do that.

Pin 5 of the IC has been made available so that the threshold voltage can be manipulated. It goes directly to the upper junction of the middle resistor--the 2/3-VCC point. By loading pin 5 with a selection of resistors (the bottom ends of which are at ground), we can reduce this "threshold of oscillation" to other values we might want. For example, putting 68K between pin 5 and ground would load the top 100K with a combined value of 51K (the 51K coming from putting 68K in parallel with the bottom two 100K elements in the chip). This should mean that pin 5 would now be at 3 volts instead of 6; the Blue Box should now begin to sound as point "IN" is brought above 3 volts.

For some reason, probably several reasons, the math doesn't predict very well where this threshold will be: perhaps the resistor elements in the chip are nonlinear, perhaps the comparators have significant offset voltages, or maybe the comparators draw current from the divider. For whatever reasons, I have had to choose these resistors experimentally. I have done so to get threshold voltages of 0.1V (heavy loading indeed), 0.5V, 1.5V, 3V; leaving pin 5 open to detect 6V is still useful at times.

If you do have to hand-pick these resistors, it's easy to do, since I have made pin 5 available through a jack on the panel of the cabinet. This has been done, not so much to make choosing these resistors handy, but so that an external voltage can be impressed on pin 5. With this feature, the instrument can be used to compare voltages; for example, impressing 2.4 volts on this jack will make the tester tell you if you have a "good" logic high in troubleshooting TTL logic circuits.

It should be mentioned here that there is a good solid reason why (besides extending battery life) the CMOS version of this chip must be used. Other 555's (even the so-called "low-power" bipolar ones) control the discharge cycle with a conventional transistor. This means that there is a much higher practical limit to your available threshold. With those, the best you could hope for is a threshold above the saturation voltage of the pin 7 transistor, probably higher than 0.3 volts. The FET, on the other hand, acts as a very low resistance from pin 7 to ground. Therefore, the practical limits with the CMOS device are probably set by the offsets of the comparators, not the depth to which the discharge cycle can go.

The lowest threshold voltage possible with these IC's seems to be 100 millivolts. Yet there is a way to make even this disappear. (This is what you'd want if you were using the Blue Box as a meter reader; many meters develop less than 100 millivolts across them over their full range.) This modification entails lifting the negative test lead off of ground and tying it to an adjustable bias voltage to "correct" for the threshold. A toggle switch is used to select this feature.

Evolving the Continuity Meter

Any current path between point "IN" and VCC will cause the circuit to oscillate--however slowly. Already, the frequency is an inverse function of the resistance; shorting point "IN" to VCC will make the Blue Box "sing." Moreover, as the "threshold" is decreased by loading pin 5, the Blue Box will detect leakage paths of higher and higher resistance. With the threshold for point "IN" set at 0.1 volt, you can hear charging of the stray capacitance between the test leads as you swing them toward each other. (In most cases, the charging resistor, which determines the "sensitivity" of the oscillator to its charging current, should be at minimum--1 megohm.)

This is all fine for testing continuity paths of several hundred kilo-ohms, but how can we "desensitize" the circuit to make lower values sound different from short circuits? The answer lies in providing a set of resistors to shunt point "IN" to ground. With these resistors, we can provide the continuity circuit with heavier loads which must be overcome before the IC will oscillate. The lowest of these will be 1000 ohms, the next will be 3.3K, then 10K, etc. By switching the three lowest ones in (they now being in parallel from point "IN" and ground), the test element must pull up on about 700 ohms. It is in this position that 10 ohms causes a discernible frequency difference from a direct short. The frequency of the dead short can be chosen by setting the sensitivity and/or changing the threshold.

Evolving the Current Meter

Actually, we've already got it. The load resistances for the continuity section now appear across the terminals when detecting voltages or currents. With the 1K load resistor switched in, a 100-microamp current will make the tester speak, provided its threshold is set at minimum. With a 10K load resistor in place, 10 microamps will develop the necessary 0.1 volts at point "IN" to make the Blue Box start to speak. Of course, it would be a simple matter to include load resistors of lower value for measuring higher currents; however, this could lead to costly mistakes. For example, if you were checking the continuity of something and you happened to switch in a load resistance of 100 ohms (or worse), you could possibly damage the item you're testing, and the strain on the 9-volt battery would be tremendous. My advice is to "play it safe"; make yourself use external shunt resistors when measuring high currents.

It should not go unnoticed that these load resistors can be used to apply light loads to small batteries under test. For example, with the three lowest resistors switched in (the parallel combination yielding 700 ohms), you will draw 12mA from a 9-volt battery, and 2mA from a small button cell. Comparing the pitch of the unknown unit with one which is known to be good, you have a fine battery tester which is more telling than a high-impedance voltmeter.

Blue Box Circuit

A 7555 (CMOS 555) of any brand is used. Pin 1 is grounded, as is the negative side of the 9-volt battery. Pins 4 and 8 are tied together; these are bypassed to ground by the parallel combination of 47uF and 0.1uF (the negative side of the electrolytic at ground). The on-off switch is a so-called "on-off-on," or center-off double-pole double-throw switch. On one pole, positions 1 and 2 are jumpered together and go to the positive of the battery. The arm of this pole goes to the VCC line of the project, and to pins 4 and 8.

The second pole of the on-off switch selects "voltage" or "continuity." The arm of this switch goes to the sleeve of the input jack. The position marked "continuity" goes to the VCC line. The position marked "volts" goes to the arm of section A of another double-pole double-throw switch (this unit having no center-off position). A position marked "normal" on this section A is grounded. (The "normal" position of section B is left open.) The other position of section A (for threshold correction) goes to the arm of a 10K screwdriver-adjust trim pot. This arm is bypassed to ground by 10uF (negative at ground). The bottom of this pot is grounded, while its top end goes through 220K to the "corrected" position of section B. The arm of section B goes to the VCC line (or to the arm of the power side of the on-off switch). (If you wish to omit this switch for "correcting" or accounting for the 100-millivolt minimum threshold, merely ground the "volts" position of the on-off switch's second pole.)

Pin 3, the 7555 output, goes through 2.7K, then through 18K to one side of the piezoelectric speaker; the other side of this speaker goes to the VCC line. Across the 18K resistor is a single-pole single-throw switch (volume switch).

On the 7555, pins 2 and 6 are tied together and go through a good-quality 680pF capacitor to VCC. (Silver mica is recommended. A disc ceramic simply will not do because of its leakage.) Pins 2 and 6 also go through 100K to pin 7. Pin 7 goes through 1 megohm, then through 2.2 megohms, then through 6.8 megohms (all in series) to point "IN." SPST DIP switches are shunted across the 2.2- and 6.8-meg units. These switches set the "sensitivity"--the rate of frequency change for delta V or delta R. [Not using DIP switches, the editor settled for 1 meg in series with 10 megs, with a single toggle switch across the latter. This seems to be enough for many applications, and it takes away some of the need for profound thought when using the instrument.]

The "tip" contact of the input jack goes to point "IN." Also going to point "IN" are the following nine load resistors: 1K, 3.3K, 10K, 33K, 100K, 330K, 1meg, 3.3megs and 10megs. (All are 1/4-watt, except the 1K unit, which is 1/2-watt.) One side of each of nine DIP switches goes to ground. The free ends of the resistors then go to their respective switches in this string of nine. A rotary switch could be used; its arm would be grounded, and the resistors would go to their respective positions. A ten-position rotary switch would be needed, since a position which is "open" is required.

Pin 5 of the 7555 is bypassed to ground by 2.2uF (negative at ground). Pin 5 also goes through 1K to the "tip" contact of the "threshold input" jack; the sleeve of this jack is grounded. This jack is of the closed-circuit type, and its "switch" contact goes to one end of each of the following resistors: 330 ohms, 8.2K, 33K, and 100K. Note that these were chosen experimentally, and yours may need to be slightly different to obtain thresholds of 0.1V, 0.5V, 1.5V and 3V. (You could use trim pots of 2K, 20K, 50K, and 200K; this would permit setting of the threshold voltages exactly. However, these thresholds will get lower as the battery runs down, and precision may be too much to hope for.) One side of each of four DIP switches is grounded. The free ends of the resistors go to their respective DIP switches in this string of four. A rotary switch could be used; its arm would be grounded, while the resistors would go to four consecutive positions. However, a 5-position unit would be needed, since one position should leave pin 5 "open."

Finally, to facilitate use of the Blue Box as an ohmmeter, sample resistors are placed on the board that holds the DIP switches. Adjacent to each "load" switch is an equivalent resistor, one end of which goes to VCC; the free end of each comes up through the board so that it can be "sampled" (touched with the test probe) to see what pitch it gives you for a particular arrangement of Blue Box settings. Once again, the resistors are (and this time, they are all 1/4 watt): 1K, 3.3K, 10K, 33K, 100K, 330K, 1meg, 3.3megs and 10megs.

Note: With this arrangement of the on-off switch, the polarity of the probes in the "continuity" position is reversed from that in the "volts" position. This should be familiar to old-timers who had to know this about their VTVM's.

Possible Circuit Variations

The "volume switch" can be replaced by a volume control if desired. In this case, pin 3 goes through 2.2K, then through a 10K volume control connected as a rheostat to the speaker. If this volume control contains a switch, this can be used to switch the hot battery line; this would make it possible to get rid of the DPDT unit with center off and replace it with a cheaper DPDT switch with no center off.

As mentioned, different shunt resistors could be included (including one as low as 330 ohms, 1 watt). This is a dangerous arrangement, however, since the Blue Box can supply heavy currents to test items with the wrong switches flipped. Other speakers can be used. However, the CMOS 555 is not very tolerant of low impedance loads, so the speaker used should present an impedance of 2K or more.

Testing and Calibration

Does it work? The easiest way to find out is to turn the Blue Box on in the "continuity" position and put your fingers across the "volts/ohms" input jack. When you do this, however, make sure that all the "load" resistors are switched out, and it might help to have the "sensitivity" at maximum (only the 1 megohm charging resistor in place). If this test works, then everything works (unless there is an error in the switching networks).

As mentioned, switching in any of the four resistors on pin 5 will drastically reduce the voltage at which the tester begins to sound. The values given are ballpark figures; you can install these and learn by experimentation where the thresholds turn out to be. If you wish to hand-pick these four threshold resistors, this can be done after the rest of the circuit is built and working. To do so, you will need a voltmeter and an adjustable power supply. The adjustable supply can be comprised of a 9-volt battery, a 10-turn pot, and a resistor of twice the pot's value; i.e., a 10K 10-turn pot and a 20K resistor to put in series with it. In other words, connect the bottom (counterclockwise end) of the 10-turn pot to the negative of the battery, and its top end through the resistor to the battery's positive. Then, connect the arm of this pot to the tip of a plug which will fit into the "volts/ohms" input jack; the sleeve of this plug should go to the bottom of the 10-turn pot.

Arrange a way of connecting experimental resistances (a resistance-substitution box, if you have one) to another plug that will fit into the "external threshold" jack. For starters, select one of the values recommended, perhaps 33K. Then, with the Blue Box turned on in the "volts" position, and the "offset correction" switch in the "normal" position, manipulate the 10-turn pot until you find the point at which the Blue Box just starts to click. (This setting will be most critical when the "sensitivity" is at maximum, that is, with only the 1 meg charging resistor.) Then, measure the actual voltage on the arm of the 10-turn pot. The 33K resistor here is listed as creating a threshold of 1.5 volts; change this 33K as necessary. Now, the point is to obtain thresholds that are easy to remember; I chose values of 0.1, 0.5, 1.5, and 3 volts.

If you have included the switch to correct for, and eliminate, the offset, make this adjustment with the threshold at the 0.5V setting (with the 8.2K resistor in place). (This setup could be adjusted for use at higher thresholds because of improved noise immunity.) With the "volts" jack shorted, adjust the 10K trim pot until the Blue Box just begins to click.

Notes on Using the Blue Box

Measuring Voltage and Current

This is the easiest to describe, and you can hardly go wrong here. Any voltage over the selected threshold will make the tester sound. However, depending on the sensitivity setting, the tester may oscillate out of the range of your hearing. Therefore, if you hear nothing from the tester and you're fairly sure that a voltage is present, lower the sensitivity by including all the charging resistors (which add up to 10 megohms).

If you want to "measure" a voltage, see if it is near one of your selectable threshold points. If the tester "sings" for one threshold setting and makes no sound on the next higher setting, you at least know that the unknown is between those two points.

For many relative indications, what you want to achieve is a comfortable setting whose frequency you can hear--perhaps noting that pitch so that you can compare it with other unknowns. To accomplish this, the threshold should be below anything you don't want to miss, and the sensitivity should be adjusted for a pleasant audible tone.

Note that the threshold setting will also influence the pitch of the tone. Remember, you are actually measuring the amount of voltage that exceeds the threshold; if you lower the threshold, naturally the tone will be higher (much as if you increased the input by that difference in thresholds).

If the threshold is to be eliminated--as it should be for "reading" voltage drops across meters--two settings must be made. First, the threshold should be set in its 0.5V position (by loading pin 5 with the 8.2K resistor). Second, the toggle switch for "threshold correction" should be thrown accordingly. Just for fun, now, set the Blue Box for its lowest threshold; it should sing for an input of zero volts. Maybe this zany arrangement would be good for something?

When using the Blue Box to measure the voltage across a meter, two things must be carefully noted. If the switch is thrown to the continuity position, current will be passed through the meter; this can be disastrous if load resistances are switched in. The voltage position is the one which should always be used. Here too, however, switching in load resistors will put the bias voltage of the 10K trimmer in series with the meter. Leave all load resistors switched out.

Usually, when measuring voltages you will want to leave the load resistors out of the circuit. However, as mentioned earlier, you can use these loads to influence a circuit as desired (just like testing a battery under load). With these loads switched in, you can use the Blue Box as a current meter, placing its test probes in series with a circuit to be measured.

The Continuity Section

It may seem as though all adjustments and switch settings interact. They do, but once you take a little time to experiment and to understand the system, it's not as bad as all that. Turn the Blue Box on in the "continuity" mode, short the test leads, and try the following:

First note that the tone can be adjusted to a comfortably high pitch by manipulating either (or both) the sensitivity and the threshold. Raising the sensitivity (meaning reducing the charging resistor) shortens the charging cycle, thus raising the frequency. Lowering the threshold reduces the amplitude swing of both charge and discharge cycles--decreasing their time periods and raising the frequency. Find some combination that gives you a moderately high pitch--2kHz or so. Now throw in some load resistance (one or more of the nine resistors from point "IN" to ground). With a dead short between the test probes, you will notice that these resistors have no effect; if there were any test resistance at the probes, applying a significant load would lower the pitch.

A good start, then, is to set the tester up to give you a good range of frequencies first, with the probes shorted. Then, on the basis of what you're testing, dial in an appropriate load; for example, if you are testing a speaker or other low-impedance item, the load will need to be as heavy as you can make it, whereas if you're experimenting with photoresistors, the load should be moderate or light. If you have no idea as to what the unknown is going to be, you can just switch in load resistance until you get a significant drop in pitch. Once again, first set the Blue Box up so that you can comfortably hear it with the probes shorted, then adjust the load as appropriate for the item under test.

As shown in the circuit, my latest model of the Blue Box has standard resistors which can be touched with the probe to get a pitch comparison. When testing something that is completely unknown, find a load resistance which causes the pitch to drop perhaps an octave. Then, by sampling the standards, you can at least guess at a ballpark figure for the unknown resistance.

Capacitor Testing

As discussed in other Smith-Kettlewell reports, continuity testers can be used to watch capacitors charge. The Blue Box, having ten ranges of continuity sensing, can do this with an extremely wide variety of capacitors. For example, with the threshold at minimum and the load circuit open, you can hear a unit of 0.01uF charge--it takes almost a quarter of a second. With the load of 1K, I have used it on electrolytics as large as 2000 microFarads.

On electrolytics rated at 10 volts or higher, it can be used to tell which lead is positive and which is negative. A reverse-connected electrolytic cannot accept a full charge. Therefore, if things are set up right, the tone emitted with an improperly connected test capacitor will descend for a while, but eventually level off and not go any lower. When properly connected--so as to charge the capacitor in the "forward" direction--the tone will descend to zero. First, however, the threshold must be lowered somewhat, or either polarity might look okay. Switch in a threshold resistor (perhaps the 33K); then connect the capacitor and apply load resistance until the tone descends at a reasonable rate. If the tone eventually goes to zero, the capacitor has been charged in the forward connection. Also with electrolytics, you can now monitor their internal leakage by opening the load circuit, whereupon you will hear a tone rising to signify decay of the charge.

It is important to restate here that the polarity of the test leads reverse when switched from "volts" to "continuity." In the above example, therefore, a successful charge on the capacitor means that the sleeve of the test-lead plug is going to the positive lead of the capacitor.

Leakage Testing

The Blue Box is so sensitive that it can detect reverse leakage in any diode I have yet to find--even "hot-carrier" types. (The leakage of germanium units is so bad that they look like a short until some load resistance is applied.) An interesting experiment is to set the Blue Box up to register the leakage of a diode whose envelope is glass, then subject the diode to a direct light source (such as a flashlight). By raising the energy level of the carriers around the diode junction, the leakage is increased. Finally, various capacitors can be compared for leakage as well. For example, a marked difference can be noted between silver mica and ceramic types, or between mylar and metal-oxide types.

For making sensitive leakage checks, lower the threshold by switching in the smallest resistor (330 ohms) on pin 5, turn the sensitivity up by minimizing the charging resistor (setting it to 1 megohm), and disconnect all load resistors. A low level of leakage will make the tester emit slow clicks--one or two per second. If the leakage is bad enough to generate a fairly high tone, switch in some load resistance until this tone drops perhaps an octave; then measure it by comparing this sound with one gotten from known standards.

The Blue Box as a Light Probe

Because of its wide range of continuity loads, the Blue Box can be used with any kind of light sensor (even a solar cell, in the "volts" position). I've used it to judge the level of gas stove flames, and I've even used it as a mobility aid (more on this later). The appropriate switch settings will depend on which type of sensor you are using. Phototransistors, especially photo "Darlingtons," work best when sourcing currents into low-impedance loads. Therefore, one of the lower value load resistors would be most appropriate with these (from 3.3K to 100K). Although phototransistors are fairly "fast" in their response to changes in light intensity, they are also fairly insensitive, as compared to cadmium-sulphide photoresistors.

With a cadmium-sulphide cell, even starlight is detectable. For this, the threshold is set at minimum, the sensitivity is at maximum, and the load is either left open or set to 10 megohms. Taking this setup out into the country, you can tell the difference between starlight and covering the cell with your finger. (At these low levels, the photoresistor is extremely slow, sometimes taking minutes to decay in darkness.) The best you can do with a phototransistor is bright moonlight.

With a photoresistor mounted in a very opaque tube, small fields can be sampled for light intensity. For example, with such a device I can "watch" the white line of the road when riding in a car. I live in an area where there are no sidewalks, and where neighbors'' driveways are not very apparent. With such a sensitive light probe, I can find out which house is mine by noting the pattern of the windows.

I have further improved the mobility-aid version of this circuit by making one that compensates for ambient light; this arrangement uses two opposing photoresistors. One looks through a very opaque tube at the end of my cane and is connected in the regular continuity circuit--from point "IN" to VCC. The other is partially exposed to ambient light (it is mounted in a translucent housing which defuses and cuts out perhaps 25% of the ambience). This latter is in the "load" circuit--going from point "IN" to ground. In this way, by automatic lightening of the load, the "probe" looking at the ground becomes more sensitive when conditions are less well lit, and in bright daylight the "load-imposing" sensor keeps the continuity circuit from saturating.

This system is very experimental. Settings of the various switches will depend on the physical arrangement and the photosensors you use. (You could use almost any type of sensors, photoresistors, phototransistors, etc.) Also, as you can see, the system of input jacks will have to be modified somewhat. Have fun, and I encourage you to just try things.

With this mobility light probe, I can follow the edge of the sidewalk, and can easily hear changes in pitch for light and dark areas. For example, as a wheelchair ramp meets the street, I can immediately tell when the tip of my cane has crossed from concrete to the dark street. (Subtle changes like going from old pavement to new are discernible, and you can count the "expansion joints" in the sidewalk, if you're so inclined.)

[Editor's Note: There is a "Help Line" available. Bernie Vinther has made the offer that anyone who wants to can call him (please, no collect calls). If you need any advice or want to hear his personal comments, the number is (509) 586-8060.]

Parts List


  • 1--47uF 10V electrolytic
  • 1--10uF 3V electrolytic
  • 1--2.2uF 10V electrolytic 1--0.1uF disc ceramic 1--680pF silver mica

The following resistors (1/4 watt, 5%) pertain to the 555 oscillator and offset circuits:

  • 1--1K
  • 1--2.7K
  • 1--18K
  • 1--100K
  • 1--220K
  • 1--1 megohm
  • 1--2.2 megohm
  • 1--6.8 megohm

The following threshold-determining resistors (1/4 watt, 5%) are suggested values; they could be hand-picked or replaced with trim pots (see text).

  • 1--330 ohm
  • 1--8.2K
  • 1--33K
  • 1--100K

The following "load" resistors are all 1/4 watt, 5% unless otherwise specified. If standards for resistance comparison are included, another set should be procured (all of these latter being 1/4 watt, 5%):

  • 1--1K 1/2 watt
  • 1--3.3K
  • 1--10K
  • 1--33K
  • 1--100K
  • 1--330K
  • 1--1 megohm
  • 1--3.3 megohm
  • 1--10 megohm

Toggle Switches:

  • 1--DPDT with "center-off" position
  • 1--DPDT 2-position
  • 1--SPST

DIP Switches (all SPST):

  • 1--Bank of nine; suggest two 18-pin units with alternates clipped off.
  • 1--Bank of four; suggest a 14-pin unit with even numbers clipped off.
  • 1--Set of two; suggest an 8-pin unit with alternates clipped off.


  • 1--1/8-inch "miniphone," open-circuit type
  • 1--1/8-inch "miniphone," closed-circuit type


  • 1--Intersil ICM7555 of commercial grade, or equivalent
  • 1--4-pin DIP socket (I solder to wire-wrap sockets in all my projects.)
  • 1--Radio Shack 276-076

"Piezoelectric Transducer"

  • 1--highly blue cabinet, Radio Shack 270-222
  • 1--9-volt battery connector


by Bernie Vinther


Using the soldering tip of a battery "cordless soldering iron," this instrument is of the quick-heating fast-cooling type; it can be guided into place while it is still cool, and then energized by pressing a foot pedal. Moreover, an external transformer has been substituted for the large handle and heavy battery of the cordless unit. The particular tip assembly chosen happens to have an extension arrangement which provides a ready-made handle. The result is that, even with its supply cable, the finished soldering iron is of feather weight, it can be put into position directly by feel (before it is energized), and it extends very little beyond the fingertips--hence the descriptive term, "fingertip soldering iron." [While seeming simple enough, it is cleverly implemented, and the right tip had to be found, and the editor considers this to be a major advance in soldering tools for the blind.]

"All right, gentlemen, push the pedal to the metal and let the soldering begin!" Formerly being a sighted electronics technician, now as a blind person I've been frustrated by whatever approach at soldering I tried; I always seemed to leave something melted or burned, especially my fingers. In trying to use various methods (described in Smith-Kettlewell reports), locating the parts to be soldered with a hot iron and applying the solder, the materials would accidentally be jostled out of position, or things would become overheated while I was trying to feed the solder. Therefore, I most often chose to use a "cordless soldering iron" (first made by the Wahl Clipper Corporation, and then also marketed by Radio Shack). This iron has the following advantages:

First, the tip is cool to start with, and heats up quickly when you're ready to solder. Therefore, the tip can be guided into position with fingers of your free hand, after which the iron is energized when you are out of the way. You no longer need a system of landmarks to guide the iron to the connection. After the connection is made, the button is released, and the iron cools down by the time you are ready to make another.

Because it is only energized when you need it, there is no danger of leaving a hot iron lying around where it could burn something or someone, or present a fire hazard. There is no chance of leaving it on to oxidize badly overnight, which would give you a lot of cleaning work to do on it.

An important advantage over regular irons is the distance from your fingers to the soldering point. The tips of standard irons are normally 3 inches or more beyond the tips of your fingers; this makes for a "wand"--which is also hot--that is hard to control. The cordless iron at least reduces this distance to about 2 inches, and the proposed iron described here further reduces this length to 1-1/4 inches.

Also, the cost of a temperature-controlled iron is outrageous in comparison. The ones used by the Smith-Kettlewell group cost over $120.

I found that I could overcome the frustrations of feeding the solder by wrapping a small length of it around the tip of the cordless iron when it is cold. The length of solder needed depends somewhat on the size of the connection and the diameter of the solder. However, I usually use only 3/8 of an inch--perhaps as much as 1/2 inch--of 23-gauge solder; this is fine for most applications. This eliminates the need for intricate manipulations or complex solder-feeding systems that can plug up or move the parts out of position.

Yet the cordless iron is not without disadvantages. First, it is not very rugged, and its tip can be damaged by mishandling (a problem accentuated by the fact that the battery and handle assembly are heavy). Next, its primary way of dissipating heat is in the connection you are soldering; therefore, it can overheat if left on too long. Pushing down on the button in the handle often caused extra undesirable movement of the hand--my hand would tend to move away from the connection at times. Finally, it seemed that the battery would always run down, just when I was almost finished with a project I was dying to try out. These many frustrations were, however, more than overcome by the quick-heating fast-cooling feature, and that the solder was applied without complicated techniques or feeding systems.

All this led me to devise the following improvements, which even offset some of the disadvantages.

Describing the Vinther Fingertip Iron

The Wahl Clipper Corporation makes a variety of tips for their instrument. One of these is known as the "Tuner Extension Tip" (apparently designed for making long reaches into TV tuners). Bearing the Wahl Part No. 7556, it is the same as the standard-length "fine" tip No. 7545, except that it is mounted on long stiff leads--about 3-1/2 inches long. Moreover, being held apart by an insulator, the conductors are encased in heat-shrinkable tubing. The resultant lead assembly resembles a piece of flat 300-ohm "twin-lead" TV lead-in wire.

You might ask, "But what good is a 4-inch-long extension to a soldering iron tip? Weren't you just complaining about length a little while ago?" The 4-inch long extension serves as the handle. The fact that it is insulated means that it doesn't heat up very much, and you can easily get to within 1-1/4 inch from the soldering point. Voila, we have a fingertip version of the quick-heating fast-cooling iron.

Instead of running this thing with a heavy and expensive nicad battery pack, why not choose an appropriate transformer? This would solve another problem; the on-off button can be in the primary circuit of the transformer, and I chose to make this a foot control on the floor. No longer does the battery run down. No longer does pushing a button cause accidental movement of your hand. Finally, being of such light weight, it is much easier to be gentle with this fragile tip construction, and the tips will last longer.

The battery in the Wahl cordless iron is a 2-cell unit, supplying 2.5 volts. I simply went down to my local supplier and got a 5-volt transformer whose secondary centertapped. (My transformer is only good for about 3 amps. Though the tip draws about 7.5 amps when cold and settles to about 6 amps as it reaches soldering temperature, its intermittent use has caused me no problems. Nevertheless, a 6-amp transformer is specified here.)

The two-wire cord that supplies the tip should be as flexible as possible. In hardware stores, rubber-covered "heater cord" is sometimes available which is more flexible than standard plastic-covered "zip-cord." Belden Wire Company does not regularly distribute non-shielded rubber-covered two-wire cable. For now, the Belden zip-cord listed here is 22-gauge. While they rate this at only 5 amps, intermittent use with this iron should not cause a problem. [If you have any better suggestions, the editor is interested in announcing them--be specific, with catalog numbers, please!]

Also still somewhat in doubt is the best way to attach the 7556 tip to the cable. The pins provided are steel, and do not take solder readily (although, if you wrap them with several turns of flux-core solder and heat them before making any connection, soldering them will be much easier). These pins are 0.037 inches in diameter, and they are spaced at 1/4-inch centers. A socket such as the inside portion of the Amphenol 6175 two-lead "UHF" antenna connector might work. I found some automotive push-on connectors which I was able to modify to do the job. As will be seen in the section "Tip Construction," these male pins on the back are actually crimped into copper tubing, which you can get at by removing a small portion of the heat-shrinkable tubing of the tip assembly. This tubing can easily be soldered to, and the wires of the cable could even be fitted into the tube ends after the pins are extracted with healthy pliers.

A straight SPST foot switch in series with the primary will work. However, as mentioned, this type of iron can get very hot if left on too long where the work pieces are small--the heat capacity of the work not being enough to dissipate the iron's energy. I thought, "Wouldn't it be nice if there were a way of operating the iron at reduced power?" I tried using a sewing machine foot control which has a variable resistor inside it; however, I soon found that the resistance of this control wasn't high enough to make an appreciable difference.

Another way of reducing the power might be to build a double-ended foot-control switch, one with a button at either end. Thus, by pushing down on the "toe end" of the control, full power would be applied; by rocking back on the "heel end" of it, it would be run at about half power, which would be fine for maintaining the tip temperature. This could be done by having a pushbutton operate the primary circuit directly at the toe end, and by putting the pushbutton at the heel end in series with a 15-watt 120-volt lamp, or perhaps a 500-ohm 20-watt wire-wound resistor.

At present, if I need to maintain the heat for a while, but not supply full power, I merely push up and down on the switch about once a second, and this seems to work fairly well.

One more improvement completes the design. The diameter of the soldering point on this tip is 0.070 inches (before tinning). The spacing between pins of IC sockets is often 0.075 inches; thus, the tip can sometimes contact two pins at once, causing a solder bridge. The surface of these tips cannot be ground down or harmed with abrasives in any way; they are clad with iron and tinned at the factory for long life, and this cladding must not be harmed. While visiting at Smith-Kettlewell, we found that you can flatten the last eighth inch gently in a vise without cracking the iron-clad coating. Don't mash any farther back than this eighth inch, or you will damage the heating coil inside. This gives the tip a screwdriver-shaped end whose thickness is perhaps 0.050 inches, which can easily pass between pins of an IC socket. When you flatten the tip, orient it in the vise so that the flat sides coincide with the flat faces of the "handle," as we now shall call the lead extension assembly.

Tips on Using the Fingertip Iron

The way I use it, this iron permits soldering to be basically a one-handed operation. As mentioned earlier, I wrap a little piece of solder around the tip before I put it into position; pieces 3/8 or 1/2 inches long will do nicely for IC sockets, while longer pieces could be used for larger terminals. To save time when I am doing a lot of connections (one after another), I prepare precut lengths, form them into little rings, and stack them on a nail; they can quickly be picked off the nail and installed on the iron's tip as needed.

If you find short pieces of solder difficult to deal with, you may prefer just wrapping a turn and a half around the tip, then nipping this assembly free of the spool. The amount you use will actually depend somewhat on the diameter of solder you have, as well as the size of connection. The amounts advocated here are based on my experience with 23-gauge solder.

Caution! It is important to hold onto the metal body of the tip when wrapping it with solder, not the handle or the ceramic insulator that the leads go through to reach the tiny heating element inside. If you do not hold on to the metal portion, mechanical damage to the heating element can occur.

Being cool to the touch (most of the time), I place the iron on the target using an index finger of either hand. [Editor's Warning: As with the old soldering gun, cooling of the instrument takes at least more than half a minute. If you are doing a series of connections, don't let your zeal get you in a hurry; you can still get a serious burn if the tip has not been allowed to cool. The iron, once a trusted friend, can turn on you.] A wet cleaning sponge should be kept on hand to wipe off excess solder after every few connections (this is not necessary each and every time, as it is with a constantly hot iron which is always building up oxides.) If you have a connection to do right away, wiping the tip on the cleaning sponge will help cool it faster.

(At Smith-Kettlewell, we tried using the sponge as a storage place for the soldering iron. This got us into trouble in two ways. First, the tip is apparently not watertight, and I got a serious burn one time as steam came out of it. Second, the frequent drastic heat cycling was hard on the tips; we had repeated failures due to the metal portion of the tip breaking loose from its ceramic insulator.)

Because soldering is now a one-handed operation, you can use fingers of your free hand to hold parts in position. In fact, I often do not use a vise or board holder; oftentimes, I just hold the board still with the heels or palms of my hands.

Once everything is in place, I step on the foot pedal, and in five seconds or so, soldering takes place. You know when soldering occurs by two indications. One is that, when the flux has done its cleaning job, a typical "squeakiness" can be felt as you make small motions with the iron. Since you have free fingers on your other hand, the other can be when you feel the quick rise in temperature of a component which happens when "wetting" of the metals has occurred.

Viola! What a breeze! Soldering is no longer a complicated process. No more scorched parts or burned fingers. Because it's so easy to hold things while you're using this system, it makes experimenting with projects much easier; I can just "tack" parts temporarily in place without formally mounting them in some way.

If you don't like the idea of applying the solder to the iron, you can still use your favorite system for feeding solder with your free hand. [The editor actually prefers feeding solder to the work for two reasons: First, by letting it run off the iron, you stand some chance of using up the flux before it has a chance to clean the work pieces--although I've seen this system work like crazy for ol' Bernie. Second, however, I like to use the solder as my "pyrometer," as you might call it; the solder's melting tells me when the parts are hot enough to accept it. However, the squeakiness and rapid heat transfer described above are good indications of success as well, so do as you like.]

When working on old hand-wired equipment (such as an old ham receiver I had), I found this soldering iron to be a real boon for getting into tight places. The fattest part of its hot portion is less than a quarter inch in diameter, and since it is cool to start with, it is easy to find free room for this before turning it on. With a standard pencil iron, it is easy to burn nearby components with its hot barrel.

Still, though, the constantly hot pencil iron has its place. As I mentioned before, these "cordless" tips are very fragile in their construction. A little fingertip tool simply will not do heavy jobs--on large items. For unsoldering, these fragile tips cannot withstand prying, twisting, and other such procedures for separating parts.

When working with CMOS circuits, I've tried grounding the centertap of the transformer. So far, I have had no problem, neither due to static charges nor due to the AC supply. (I have ruined CMOS devices by using the battery iron only, since it wasn't grounded.) Grounding the centertap does not actually ground the iron's tip; it is insulated from its heating element. However, not enough static charge apparently builds up to damage the CMOS devices.

As listed in the next section, there are other tips available which, with a little mechanical engineering, could also be used. The smallest unit has an almost needle-like soldering point on it. I don't like it, though, because its tip is so fine that I can't put enough solder on it, and it slips off the connection more easily. One use for this tip is for cleaning out plugged-up holes in PC boards; it is so fine that it can actually reach right through them.

Tip Construction and Availability

One problem with these "Tuner Extension Tips" is their scarcity, even among dealers of Wahl products. A good source for them is Fordham Radio (see "Address List"), where they can be ordered under their Wahl number 7556.

Another disadvantage is their cost, about $7. In dissecting one, it was found that they are actually made by adding the extension assembly to the "fine" tip, Wahl No. 7545. If you have the right tools--a small crimping tool and a heat gun to shrink tubing around the assembly--you could precisely duplicate the 7556 from the 7545 (a cheaper tip which costs from $3.25 to $4.00 and which can even be gotten from Jameco). With some mechanical engineering, one could machine a lightweight handle with screw-type binding posts so that the more common short tips could be used instead of this rare one.

The pins on the short-legged tip are fitted into 2-3/4-inch lengths of copper tubing, which is then crimped down tightly onto them. Likewise, steel pins are fitted into the far ends of the tubes and crimped in place, leaving about 7/16 inches protruding. (The tubing used is 1/16 inch o.d., and 0.040 inch i.d.) A strip of fiber insulation--often called "fish paper"--is placed between these conductors, whereupon they are wrapped with "transformer tape" (which is fairly resistant to heat). (This fish paper insulator may be the hardest thing to duplicate; it is 1/16 inch thick, which is unusual. Bakelite would work, as would the fish paper salvaged from burned-out units. Just about any tape would do; just apply it an inch back from the forward end so that it won't get warm.) With the fish paper strip being 3/16 inches wide, the conductors are then spaced at 1/4-inch centers. Finally, heat-shrinkable tubing (capable of a "recovered size" of 5/8 inches) is slid up to within 1/2 inch of the soldering tip and secured, leaving 3/8-inch-long steel pins to protrude at the back end.

An exact replacement for the tubing has not been found in small quantities. "Small Parts" has 1/16-inch o.d. copper tubing, but its inner diameter is 0.035 inches. (The pins of the tips are 0.037 inches.) However, their 3/32-inch tubing (being 0.066 inches i.d.) would probably work. This is Small Parts number TTRC-2, and comes in 12-inch or 36-inch lengths.)

Other Wahl Tips

  • 7566--"Micro" Tip, 0.020 inches
  • 7545--"Fine" Tip, 0.070 inch (Often comes as standard equipment with the cordless iron.)
  • 7535--So-Called "Regular," 0.156 inches

Vinther Iron Circuit

Belden No. 9712 two-wire zip-cord is used to power the Wahl 7556 tip. One side of the tip goes to the centertap of the transformer's secondary; this centertap is also grounded to the anti-static pad on your bench and/or a wrist strap, where necessary. The other side of the tip goes through a 15-amp fuse to one end of the secondary. (This transformer should be capable of delivering 2.5 volts at an intermittent current of 7.5 amps. A suitable unit is the Stancor 6455, which has a 6-amp 5-volt centertapped secondary, and a 107- or 117-volt primary.) One side of the primary goes through an on-off toggle switch to one side of the mains. The other lead of the primary goes through the foot switch to the other side of the mains. (The foot switch should be normally open, and can be one such as the Line Master 491S. Also, foot switches with a standard three-prong male/female end on them could be used, such as the Line Master 491SC360; a three-prong electrician plug from the transformer can just be plugged into the latter type of foot switch.)

Address List

  • Fordham Radio: 855 Conklin St., Farmingdale, NY 11735; (800) 645-9518.
  • Jameco Electronics: 1355 Shoreway Rd., Belmont, CA 94002; (415) 5928097.
  • Small Parts, Inc.: 6901 N.E. Third Ave., Miami, FL 33138; (305) 751-0856.


I. The News is Tragic

The news traveled with the impact of lightning; Lloyd Rasmussen (a blind engineer with the National Library Services) informed Smith-Kettlewell that our storm trooper of "equal access," Bob Gunderson, passed on on January 11, 1987. He died as a result of complications following a gall bladder operation.

In the week that followed, another dynamo was running: his wife, Linda Beer Gunderson. Sharing his ideals, her first priority was to document his life, and his principles, for us to learn from. She sent a 17-page biography to World Radio, and I hope to run pieces from this material for a long time to come. I consider this material to be vital to blind people; we are confronted with attitudinal barriers from outside all the time, and we could use the cheering news that someone from inside really believed in us. Moreover, to see true conviction in a champion of our cause can only incite us to positive action.

I learned of Linda's efforts when I called her on a following week. We talked about what should be made known of Bob's work, and how his example could be used outside our small sphere to change public attitudes to our advantage. In a strong commanding voice, she demanded that his exemplary life and ideals be shown to the people whose attitudes could be broadened, not just holding his model sacred among our own kind ("preaching to the converted," as the expression goes).

I know what Linda Gunderson is getting at, and I agree with her. It will do the world good to hear about the direct efforts of Robert W. Gunderson. Yet, my current state of grief drives me to depart from her wishes--just a little--and summarize the effect of this maverick on the "converted." Beyond his accomplishments are the standards and the pace that he set for blind people involved in science. In formalizing the scheme by which things are described (namely circuits and mechanical arrangements), by designing measuring instruments whose accuracy was better than visual equivalents, and by getting us a lot of material fast (because his methods were so efficient), he literally put us in the fast lane. In these ways, he was special to his converts.

He wouldn't like this discussion one bit. The suggestion that he was "special" means, to the world of unbelievers, that the exceptional blind person is marvelous and okay, but when it comes to the run-of-the-mill member of our group, our limitations are well known and clearly defined. Given this "popular view," he would be considered the special exception; we would be ordinary, and there would have been no worthwhile students to teach and no audience for his magazine of worldwide scope (the Braille Technical Press). Therefore, as I praise him to you converts, let us do him the honor of downplaying his utter uniqueness to those who cannot understand it in its true perspective. But to our own, let us just see how he was "special."

II. A Frenetic Source of Energy

Ironically, this piece can be rather objective, since I never got to meet him. I never got to know anything about his physical presence, or just how loud he really was in person. In a conversation with Jim Swail, he assured me that Gunderson would make himself heard, and that he was so physically impressive that if a refrigerator was to be moved, the best you could do was be out of the way. What's more, he was full of opinions--strong ones, unflagging ones; he had determined what was right, and he could make the case for why.

What did this giant Gunderson fellow do with these qualities? He took his own experience and inventiveness to heart, and he forced them on the world as follows:

Before Gunderson, blind people had been involved in electronics alright; Dr. T.A. Benham was probably the first to get his full broadcaster's license in 1931. No doubt, schematic diagrams were noted down by those people in a short form (what I call "good college notes"). Without drawing aids like we have today, drawn diagrams were laborious to do, and they all had to be done upside down. Drawings were not the sort of quick communication link you would want with your sighted interpreter. Restrictions imposed by raised drawings further included their inaccessibility to the fingers--where complicated--and the fact that their production delayed the issuance of braille study materials.

Gunderson reasoned that, since a nonvisual scheme (good college notes) worked fine for him, he'd print a flock of stuff in that form. By 1955 (using a style which is still employed today), he had run off so much study material (circuits for consumer goods in the series "Information for the Serviceman," an electronics course called "Fundamental Electronics," and the publication of his metering inventions) that in order to stay abreast in electronics, you simply had to use his method. The rest of the literature was far behind, and it fell into a deserved obscurity.

The design of "meters" for the blind was, before Gunderson, a business of "garage inventors." Some of these must have worked just fine; however, Gunderson devised circuits which could be made with very few components--they were cheap and accurate. (The "null-type" meter using a surplus mechanical vibrator and headset as a detection system was published in 1950, and the 2-transistor meter reader called "The Transistorized Auditory Gimmick" was published in 1954.) All of a sudden, with his publications you could build them yourself. Moreover, they were so commercially viable that variations on their design can be bought commercially to this day.

Being a high school teacher for 37 years (at the New York Institute for the Education of the Blind), he polluted the airwaves with young ham operators. He wrote the books we all studied, and he declared it to be a good hobby which could teach blind people how to work with their hands, how to make friends and be sociable, and how to be competitive (certain optional aspects of ham radio are rather like competition sports). If you wanted to feel needed, you could join public-service networks where you could help rush aid to emergencies or help overseas military personnel call home. While there are good agencies recruiting handicapped hams now, Gunderson was the first to sell this principle to our population on a large scale, as well as to the community at large.

Gunderson meant for us to study--"no free lunch." That's a great cliche, but he set things up to make it actually possible to do so. Using his system of verbal diagrams, he was able to print up-to-date license manuals (for ham radio) which, unlike today's, contained circuit information. (Although he was demanding of his students, Vito Proscia told me that when he was in Gunderson's class, the lab was rather freewheeling; you could come and go as you pleased, as long as you were working on something. For all his dogmatism, then, Gunderson knew that this is the atmosphere in which true learning most comfortably rests.)

The hardest part, when you think of it, is what he had to do to get grist for our mill. Besides his inventions and the instructional materials inspired by his teaching career, he worked multiple jobs--repair, design, and purchasing for Hudson Radio, for example. That's where he got all those circuits for "Information for the Serviceman," etc.

Gunderson knows how he managed to get all that reading done or when he got his sleep. One thing is certain: that big fellow did more work than was proportionate to his size. You could not ignore him, even on the West Coast or overseas.

III. Portrait of a Maverick

We all have heroes; we look up to these and imagine ourselves in their places. As I read the BTP (Braille Technical Press), I had several heroes, none of whom were Gunderson exactly. Degree'd engineers wrote for the BTP; these people had regular jobs and regular work schedules, both of which I could imagine. I loved Gunderson and took to heart everything he wrote, but things about that guy were not compatible with my temperament and disposition. I would never be a frenetic dynamo. All I wanted to be was moderately successful--with regular hours--so that I could go home, work ham radio, and play with tape machines. Gunderson didn't fit my model, and I wager he wouldn't fit most of yours.

Who was this Bob Gunderson, and why was he so driven? The answer is, he was not a "role model" or hero, he was a maverick.

In modern times, change in every field is a given condition. We pool our knowledge and creativity and gradually press on in our endeavors. Yet sometimes the rate of change is tempestuous--almost uncontrolled--and you will usually only understand the changes after they have taken place. During these rare periods, new music is created, new scientific theories are established, and the "games" are basically changed. The driving functions for these flagstones are "the mavericks."

In every field, there are heroes: Mozart, Liszt, and Count Basie, for example. Mavericks are different in that they are sure of themselves. They are not well governed by the hesitance of science, the fear of acceptance, or the bounds of tradition. There are probably a lot of people like this; the majority, no doubt, have ideas which are wrong, and these people fall into obscurity (or learn the disciplines of their more cautious counterparts). When you get a right one, though, his ideas are correct and he makes a splash.

Freud took fragments of knowledge, did a few biased experiments, and synthesized a whole new way of looking at the human psyche. He had more reserved colleagues who would make better role models and heroes to their successors. Carl Jung, for example, followed organized method more closely and held what you would consider regular jobs. Many contemporaries hated Freud, and his successors still do. Hate him or not, they can only argue with his precepts in language which he helped to define. Freud changed the game they were playing.

There once was a frenetic cyclone of a man called Beethoven. He could not leave traditions alone. He used new instruments, expanded music notation (writing down metronome markings and crescendos); he took the molds of melodies and broke them. You can hate Beethoven or call his music boring, but you can't have a modern orchestra without his instruments and without using his music notations. Beethoven changed the music they were playing.

Mavericks leave us new and useful traditions. From Gunderson, we now know how to read buckets of material on tape--describing diagrams in a writing style that he formalized. To indulge a fantasy, what is the worst thing that could happen to us now? Suppose computers became so graphic in nature that we could no longer read their screens by any mechanical or electronic means. If this happened, there would be ordinary blokes among us who would get their "screens read" by way of interpretation (having a sighted associate put the information into the verbal form of "good college notes"). Blind people would still be involved in computers, and Gunderson, who never had a computer, would be the one to save us.

Although they usually come along every few generations, one can really hope to be in that time--the time of a maverick. It's quite an experience to note that when the winds die down, when the dust settles and your mind clears, the procedures are somehow different, and the game you're playing will never be quite the same . . . thank Gunderson.