Smith-Kettlewell Technical File VOL. 10 NO.3 Summer 1989

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-Summer-1989

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 Details regarding products, suppliers, and other contact information are original and may be outdated.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







Robert A. Pease, National Semiconductor Corp.

Reprinted from EDN, February 16, 1989,copyright 1989 CAHNERS PUBLISHING COMPANY a Division of Reed Publishing USA. Permission was granted for one-time-only reprinting herein; any unauthorized copying of this material is prohibited.

[Editor's Note: This came from a whole series of articles on troubleshooting in EDN Magazine, before and after. The rest are good too. However, the choice of capacitor for the right job has been a recurring question from readers. I hoped to do that research one day. It's a good thing I didn't--man, this article is unbeatable. Mr. Pease, I'd like to shake your hand some day.]

Capacitors are rather remarkable. We consider capacitors, like resistors, to be "passive." But if you charge up a good capacitor to 10V and take a two-week vacation, when you come back the voltage may not have decreased by as much as 20 or even 10%. The capacitor may have stored and retained enough energy to run a micropower circuit for hours or to light an LED for a shorter interval. Calling components with such exceptional properties "passive" is more than a little unfair.

Ordinary aluminum electrolytic capacitors are most often used for power-supply filtering and bypassing. In the old vacuum-tube days, electrolytic capacitors were often used at levels of 150, 300, 500 volts, or more. There are several basic problems with these old circuits. First, if the voltage across a capacitor is much higher than 350V, the part's reliability is not nearly as good as that of devices operated below 350V. Also, if a piece of old equipment has not been powered up for years, it is advisable to apply the AC power gradually by cranking up the line voltage slowly with a variable transformer so that the electrolytic film has a chance to "form" up. If you hit it with full voltage instantly, an old capacitor may fail. Of course, if you are hit by high voltage, you may fail, too.

At this point, I should remind you that when working on high-voltage circuits, probe with one hand only and keep the other hand in your pocket. Avoid grounding your body at any other place, and stand or sit on an insulating slab of dry material. These precautions can prevent a shock from causing you serious harm. When I start work on a high-voltage circuit, I solder a neon lamp in series with a 100K resistor across the high-voltage power supply as a glowing reminder that this circuit is powered by a voltage much higher than 15V.

After you operate a high-voltage power supply at full voltage, if you turn off the power and decide that for safety's sake you should short out the filters with a few hundred ohms, be careful. A few minutes later, the voltage on the capacitors may come back up to 60 or 80V and give you a shocking experience. The partial recovery of voltage on a discharged capacitor is caused by "soakage," or dielectric absorption, which causes the dielectric of the capacitor to "remember" the voltage it was recently at. In high-voltage equipment, it is wise to install a 2W resistor of a few hundred kilohms across each large high-voltage filter capacitor to bleed off the charge and decrease the chance of shocks.

The last problem with old vacuum-tube equipment is that the heat tends to dry up the capacitors' electrolyte, thus causing their capacitance to decrease. This decrease is evidenced by excessive ripple, or "hum," on various signals and, of course, on the power-supply output of unregulated supplies. Although I have presented these maladies as problems afflicting old equipment, you should consider them even in new designs.

In modern power-supply designs, it is critical that you choose a filter capacitor whose effective series impedance is low at all rated temperatures. Otherwise the rms filter current multiplied by the resistive component of the series impedance can cause excessive self-heating. And if the heat can't flow out of the capacitor, the temperature will rise and cause poor reliability. Excessive heating is one of the most common causes of failure in electrolytic capacitors.

For instance, at 120 Hz, which is the frequency of the ripple current flowing in the filter capacitors that follow a full-wave rectifier operated from a 60-Hz ac source, some manufacturers rate their capacitors at 2A rms for each 1000uF. Because the rms current in the capacitor is nearly 2A rms when the dc output is 1A, this rating is consistent with the rule of thumb for an ordinary full-wave bridge rectifier: Provide at least 1000uF of filter capacitance for each 1A of dc output. At 20 or 40 kHz, which is the ripple-current frequency in many switch-mode power-supply filters, the capacitor will have a higher series resistance. Thus, the capacitor won't be suitable for handling even 1A rms. If you insist on using a 120-Hz-rated capacitor as a filter in a switch-mode supply, you will probably have to contact the capacitor vendor for data or advice.

Of course, if you install an electrolytic capacitor with reversed polarity and apply working voltage, the reliability will be poor and the failure mode will probably be dramatic. So, please be careful working with big power supplies and big filter capacitors that store large amounts of energy. Wear protective goggles or glasses with safety lenses for protection because a capacitor in a high-energy supply might decide to blow up while you are peering at it. In fact, a friend of mine pointed out that a 6V electrolytic capacitor of even a few microfarads can blow out as explosively as a shotgun blast if you apply 6V dc of the wrong polarity or 6V ac to it. So, again, be very careful with your polar electrolytic capacitors.

Nonpolar Capacitors Can be a Bear

You can buy nonpolar electrolytic capacitors made of either aluminum or tantalum. They are bigger and more expensive than ordinary polar capacitors, so they are fairly uncommon. But, have you seen the little 3-leaded electrolytic types recently brought to market? The lead in the center is the positive terminal and the other two leads are negative. This configuration not only gives you lower inductance but also allows you to insert the device into a board two ways -- and both are correct.

Tantalum capacitors have many characteristics similar to those of aluminum electrolytic capacitors; and, for the extra price you pay, you can get less leakage and somewhat lower series resistance. Designers often try out a timing circuit using a tantalum capacitor and a high-value resistor. But when they try to buy a tantalum capacitor with leakage guaranteed low enough to make the circuit work every time, they get quite angry when nobody is interested in selling such a device. Of course, if you were a manufacturer of tantalum capacitors and someone asked you to measure the leakage, you would refuse the business, too, because testing is so difficult. Even though this leakage is usually quite low, nobody wants to have to measure it in production.

Wound-film and stacked-film capacitors cover wide ranges, from small signal-coupling capacitors to large high-power filters. The different dielectrics are their most interesting ingredients. Often a designer installs a polyester capacitor (technically, polyethylene terephthalate, often called Mylar -- a trademark of DuPont Corp.) and wonders why something in the circuit is drifting 2 or 3% as the circuit warms up. What's drifting is probably the polyester capacitor; its TC of 500 to 800 ppm/deg.C. is 10 times as high as that of a metal-film resistor.

If you give up on polyester and go to polystyrene, polypropylene, or Teflon, the TC gets better -- about minus 120 ppm/deg.C. Polystyrene and polypropylene have low leakage and good dielectric absorption -- almost as good as Teflon's, which is the best (Ref 1). But Teflon is quite expensive and much larger in package size than the other types. Be careful with polystyrene; its maximum temperature is plus 85 deg.C., so you can damage it during ordinary wave soldering. Polycarbonate has a TC of 150 ppm/deg.C. but has inferior soakage.

Foiled Again!

Now let's discuss the difference between a polyester foil capacitor and a metallized polyester capacitor. The foil capacitor is made of alternating layers of film and foil, where both the delicate film and the metal foil are just a couple of tenths of a mil thick. This construction makes a good capacitor at a nominal price and in a nominal size. The metallized-film capacitor is made with only a very thin film of polyester -- the metal is deposited on the polyester. This construction leads to an even smaller size for a given capacitance and voltage rating, but the deposited metal is so thin that its current-carrying capacity is much less than that of the metal in the foil capacitor. If a pinhole short develops in the metallized-polyester capacitor's plastic film, the metal layer in the vicinity of the pinhole will briefly carry such a high current density that it will vaporize like a fuse and "clear" the short.

For many years, metallized polyester capacitors were popular in vacuum-tube television sets. These metallized capacitors would recover from pinhole flaws not just once but several times. However, at low voltages, the energy stored in the capacitors would often prove insufficient to clear a fault. Thus, the capacitor's reliability at low voltages was often markedly worse than it was at their rated voltage. You could safely use a cheap, compact, metallized-polyester capacitor in a 100V TV circuit but not in a 2V circuit. Fortunately, there are now classes of metallized-polycarbonate, metallized-polyester, and metallized-polypropylene capacitors that are reliable and highly suitable for use at both low and high voltages.

When the old metallized-polyester capacitors began to fail in a TV set, the "clearing" of the shorts would make the signals very noisy. Likewise, when used as audio coupling capacitors, "dry" tantalum capacitors would sometimes make a lot of noise as they "cleared" their leaky spots. These parts have therefore become unpopular for audio coupling. Similarly, you can use an electrolytic capacitor with a small reverse voltage -- perhaps 0.5V -- with no harm or problems. But a friend told me of a time an electrolytic capacitor used as an audio coupling capacitor with 2V of reverse bias produced all sorts of low-frequency noise and jitter. So, excess noise is often a clue that something is going wrong.

Extended Foil Offers Extensive Advantages

Another aspect of the film capacitor is whether or not it uses "extended-foil" construction. The leads of many inexpensive wound-foil capacitors are merely connected to the ends of the long strip of metal foil. However, in an extended-foil capacitor, the foils extend out on each end to form a direct low-resistance, low-inductance path to the leads. This construction is well suited for capacitors that must provide low ESR (equivalent series resistance) in applications such as high-frequency filters. But if you substitute a capacitor without extended foil, the filter's performance will be drastically degraded.

So there are several methods of construction and several dielectrics that are important for most capacitor applications. If an aggressive purchasing agent wants to do some substituting to improve cost or availability, the components engineer or design engineer may have to do a lot of work to make sure that the substitution won't cause problems. If a substitution is made, the replacement part is a good place to start looking for trouble. For example, a capacitor with higher-than-planned-for ESR can cause a feedback loop to oscillate -- for example, when a capacitor without extended-foil construction is substituted for one with such construction. Substitution of capacitors with higher ESR than the designer intended can also cause filters to fail to properly attenuate ripple. Another consequence of excessive ESR is the overheating and failing of capacitors--capacitors may be passive components, but they are not trivial.

Not only does extended-foil construction lower a capacitor's ESR, it also lowers the component's inductance. As a friend pointed out, "Pease, you understand things really well if they are at dc or just a little bit faster than dc." I replied, "Well, that's true, but what's your point?" His point was that in RF circuits, and many other kinds of fast circuits, you should use capacitors and other components dressed closely together, so that the inductance is small and well controlled. He was absolutely right -- the layout of a high-speed, fast-settling or a high-frequency circuit greatly affects its performance. Capacitors for such circuits must be compact and not have long leads. Ceramic and silvered-mica capacitors are often used for that reason.

Ceramic Capacitors

Every year, billions of ceramic capacitors find their way into electronic products of all kinds. There are basically three classes of these parts: the "high-K" and "stable-K" types and the C0G or NP0 types.

The high-K types, such as those with a "Z5U" characteristic, give you a lot of capacitance in a small space -- for example, 10 to the 6th pF in a 0.3-inch square that is 0.15-inch thick. That's the good news. The bad news is that the capacitance of parts with this Z5U characteristic drops 20% below the room-temperature value at 0 and 55 deg.C.; it drops 60% below the room-temperature value at minus 25 and plus 90 deg.C. Also, the dielectric has a poor dissipation factor, mediocre leakage, and a mediocre voltage coefficient of capacitance. Still, none of these drawbacks prevents capacitors of this type from being used as bypass capacitors across the supply terminals of virtually every digital IC in the whole world. That's a lot of capacitors!

These ceramic capacitors have a feature that is both an advantage and a drawback -- a typical ESR of 0.1 ohms or lower. So, when a digital IC tries to draw a 50-mA surge of current for a couple of nanoseconds, the low ESR is a good feature: It helps to prevent spikes on the power-supply bus. However, when you have 10 ICs in a row and 10 ceramic bypass capacitors, you've got a long L-C resonator with the power-supply bus acting as a low-loss inductor between each pair of bypass capacitors. When repetitive pulses excite this resonator, ringing of rather large amplitude can build up and cause an excessively noisy power-supply bus.

The standard solution is to add 2uF of tantalum electrolytic bypass capacitors or 20uF of aluminum electrolytic capacitors for every three to five ICs. The ESR of the electrolytic capacitors, typically 1 ohm, is essential to damp out the ringing. I have read a few ads in which some capacitor manufacturers claim that their ceramic bypass capacitors are so good that ringing is no longer a problem, but I find the claims hard to believe. I invite your comments.

ESR, Friend or Foe?

Specifically, some capacitor manufacturers claim that the series resistance, RS (in proper mathematical representation, R subscript S), is so low that you won't have a problem with ringing. But low RS would seem to exacerbate the ringing problem. Conversely, I've heard that one capacitor manufacturer is proposing to market ceramic capacitors whose series RS has a lower limit -- a few ohms -- to help damp out any ringing. I'm going to have to look into that. But if you have bypass capacitors with a very low RS, you can lower the Q of the resonator you have inadvertently constructed around them by adding a resistor of 2.7 to 4.7 ohms in series with each capacitor. Adding resistance in series with bypass capacitors might seem a bit silly, but it's a very useful trick.

High-K ceramic capacitors also can exhibit piezoelectric effects: When you put a good amount of ac voltage across them, they can hum audibly; if you rattle or vibrate them, they can kick out charge or voltage. (Other types can do the same thing, but high-K types are worse.) Be careful when using these capacitors in a high-vibration environment.

The capacitance of stable-K capacitors, such as X7R, typically decreases by 15% from the room-temperature value over the minus 5 to 125 deg.C. range. These capacitors are general-purpose devices and are usually available in the 100- to 10,000-pF range; in the larger packages, you can get as much as 300,000 pF. However, you can buy a 10,000-pF capacitor in either a high-K or a stable-K type; and you can't be sure of the kind you're getting unless you check the catalog and the part number.

The last type of ceramic capacitor was originally called "NP0" for Negative-Positive-Zero and is now usually called "C0G." The C0G capacitor is really a high-grade capacitor with a guaranteed TC of less than plus or minus 30 ppm/deg.C. Their dissipation factor, dielectric absorption, and long-term stability are not quite as good as those of Teflon capacitors but are comparable to those of other good precision-film capacitors. So, if you want to make a S/H circuit usable over the military temperature range, you'll find that C0G capacitors are more compact and less expensive than Teflon parts. Many, but not all, ceramic capacitors smaller than 100 pF are made with the C0G characteristic. You can get a 22,000-pF C0G capacitor in a 0.3-inch-square package.

Recently, a customer called me about a drift problem: His V/F converter had a poor TC, even though he said that he had put in a C0G 0.01-uF capacitor as the main timer. Troubleshooting by phone -- it's always a wonderful challenge. I asked him, "This C0G-ceramic 0.01-uF capacitor -- is it as big as your little fingernail?" He said, "Oh no. It's a lot smaller than that." I replied, "Well, that's too small; it can't be a C0G." Problem solved.

One observed failure mode for ceramic capacitors can arise when the capacitor's leads are attached to the dielectric with ordinary, low-temperature solder. When the capacitor goes through a wave-solder machine, the lead may become disconnected from the capacitor. If this problem occurs, you'll have to switch to capacitors from a manufacturer that uses high-temperature solder.

Don't Forget Silvered Mica

Silvered-mica capacitors have many features similar to C0G capacitors. They have low ESR and a TC of 0 to 100 ppm/deg.C. They can also work at temperatures above 200 deg.C. if assembled with high-temperature solder. Unfortunately, they have poor soakage characteristics.

A major problem with silvered-mica capacitors is their marking. The silvered-mica capacitors in old radios had completely inscrutable markings -- six color dots. Some of the new ones have such odd codes that even if the marking on the capacitor hasn't rubbed off, you can never be sure whether "10C00" means 10, 100, or 1000 pF. You really need to use some kind of capacitance meter. Similarly, in the old days, some ceramic capacitors were marked in an inscrutable way. I remember two little capacitors both marked "15K." One was a 15-pF capacitor with a 'K' characteristic, and the other was a 15,000-pF capacitor -- yet they were both the same size and had the same marking.

I must also mention that, in the past, you could buy a pretty good capacitor that had never been tested for its own capacitance. About 99% of the time, they were excellent, reliable capacitors. But once in a while, some of the capacitors came through with a completely different value. One time I saw a whole box of "capacitors" in which the two leads were still made of one loop of wire that had not been snipped apart. Obviously, the manufacturer wasn't interested in testing and measuring these capacitors before sending them out the door. So, if you are buying capacitors to a 1% AQL (Acceptance Quality Level) and not 0.1% or 0.01%, you should be aware that some low-priced parts have not even been sample tested.

Variable Capacitors Have Finite Rotational Lives

Variable capacitors are usually made of low-K material with characteristics similar to those of C0G capacitors. Their performance is excellent. The dielectric doesn't cause much trouble, but the metal sliding contacts or electrodes on some types are very thin; after only a small number of rotations -- hundreds or even dozens -- the metal may wear out and fail to connect to the capacitance.

* * *

Trouble Shooting

In general, capacitors are very reliable components; and, if you don't fry them with heat or zap them mercilessly, the small-signal ones will last forever and the electrolytic ones will last for many years. (Old oil-filled capacitors aren't quite that reliable and have probably been replaced already -- at least they should have been replaced.) The only way you can have an unreliable capacitor is to use a type that is unsuitable for the task. And that's the engineer's fault, not the capacitor's fault. Still, some troubleshooting may be required; and if you recognize the clues that distinguish different types of capacitors, you've taken a step in the right direction.

What procedures are best for troubleshooting capacitors? I use two basic procedures, the first of which is the add-it-on approach. Most circuits are not hopelessly critical about capacitor values, as long as the capacitors' values are large enough. So, if there is a 0.0-uF capacitor that I suspect of not doing its job, I just slap another 0.01-uF capacitor across it. If the ripple or the capacitor's effect changes by a factor of two, the original capacitor was probably doing its job and something else must be causing the problem. But if I observe little or no change or a change of a factor of three, five, or ten, I suspect that capacitor's value was not what it was supposed to be. I pull the capacitor out and measure it. Of course, the capacitor substitution boxes I mentioned in the section on test equipment can be valuable here; they let me fool around with different values. But in critical circuits, the lead length of the wires going to the substitution box can cause crosstalk, oscillation, or noise pickup; so I may have to just "touch in" a single capacitor to a circuit.

Suppose, for example, that I have a polyester coupling capacitor that seems to be adding a big, slow "long tail" to my circuit's response. I don't expect the performance with the polyester capacitor to be perfect, but a tail like this one is ridiculous! So, I lift up one end of the polyester capacitor and install a polypropylene unit of the same value. I expect the new capacitor's characteristics to be a lot better than those of the old capacitor. If the tail gets a lot smaller, either my plan to use polyester was not a good one or this particular polyester capacitor is much worse than usual. It's time to check. But usually, I'd expect to find that the polypropylene capacitor doesn't make the circuit perform much better than the polyester capacitor did, and I'd conclude that something else must be causing the problem.

For either of these techniques to work, it is helpful to have a large stock of assorted capacitors. In our lab, we have several cartons of used -- but not too badly beaten up -- components left over from old experiments: One is a box of small mica and ceramic capacitors, one holds various electrolytic capacitors, and one is a tray of assorted wound-film capacitors. These boxes are extremely valuable because if I need an odd type, I can usually fish in one of those cartons and find something close. Or I can find some capacitors that give the right value if I parallel two or three of them. I can use these capacitors per the add-it-on or the substitution method to find out what my unhappy circuit is trying to tell me. In addition, I keep a couple of Teflon capacitors in my file cabinet for when I need a super-good capacitor.

A technique that nobody talks about but is as old as the hills is a favorite trick of mine. Sometimes it drives my technicians wild, but then they learn the trick and find it awfully useful. Let's say I want to compare a Mylar capacitor with a ceramic capacitor in a small, precision circuit. The technician starts to remove the Mylar capacitor and install the ceramic one. Wrong! Instead, remove one lead of the first capacitor and lift it up slightly. Then tack solder one end of the second capacitor to the circuit. At this point, neither capacitor is actually in the circuit -- both capacitors are just waving in the breezes.

After the solder thoroughly cools down, I can use the springiness of the leads to let me "touch in" one of the capacitors or the other or both, as needed. It only takes a second to go from one mode to the other. (Of course, I'm assuming there's not enough voltage to "bite" my finger.) If I actually desolder and resolder the capacitors and allow enough time for these temperature-sensitive components to cool off, I'll probably forget what the difference between them looks like. So, this technique can save a lot of time and greatly facilitates A-B comparisons -- it lets me use my eyeball to evaluate the nuances of small performance changes.

Of course, if I have two or three of these spring-loaded options at one time and they begin to get wobbly, it may be time to tack solder down the ones that I am not actively pursuing. In general, though, this technique is extremely valuable, and I've never seen it in any book. Use it with my compliments. It works with diodes, resistors, and transistors, too. Just make sure that solder flux doesn't prevent the spring-loaded component lead from contacting the conductor. And make sure that your finger doesn't add a lot of capacitance, impedance, or noise into the circuit. If you do have this problem, push on the component with the edge of a fingernail instead of a finger. A fingernail adds less than 1/2 pF.


1. Pease, R.A. "Understand capacitor soakage to optimize analog systems," EDN, October 13, 1982, pg 125.

Author's Biography

For more information about Bob Pease, see the box, "Who is Bob Pease, anyway?" in the January 5, 1989, edition of EDN.


by Bill Gerrey and Al Alden


These CMOS devices are digitally controlled analog switches. The 4051 is an 8-position single-pole switch which is operated (addressed) by a 3-bit set of control lines, the 4052 is a ganged pair of 4-position switches that are "addressed" by a 2-bit code, and the 4053 contains three separate SPDT switches that are thrown by three separate control lines. Their cross-talk is low, and their "on resistance" (about 1K in the worst of cases) is fine for audio applications. One of these might be just the thing to bail you out of some horribly expensive rotary switch arrangement.

Summary and Introduction

These really are analog switches. To accommodate voltages below VSS, they have a pin called VEE that can be taken below "logic ground." The way this setup works, and the principles imposed as limitations, can be described as follows:

The operating range of the chips' supply is from 3 to 18 volts. As is said in the Motorola literature, VEE must be less than, or equal to, VSS (equal to or below the negative of the logic supply). However, the voltage from VDD (the plus supply) to VEE cannot exceed 18 volts either. Thus, if you have a VDD of 5 volts, you can take the VEE pin down to minus 13 volts; if you have a VDD of 15 volts, VEE can only be taken down to minus 3 volts below VSS.

The absolute maximum bipolar signal you could present to the switch would be plus/minus 9V; this would only be permissible if the voltage from VDD to VSS were carefully limited to 9V, and if VEE were put at minus 9V. If your logic supply--VDD to VSS--were 5V, the switch could not accommodate any more than 5V excursions in the positive direction. On the other hand, if your logic supply is 12V, then VEE cannot be brought more than 6V below ground, and this would put a corresponding limitation on the negative swing of the analog signal.

The main point of all that is: If you have audio circuitry built around op-amps which are running from positive and negative 15V supplies, you could exceed the limitations of these switches if you're not cognizant of them. There is diode protection on all inputs; the diodes are good to a maximum current of 10mA.

They all have an "inhibit" pin (pin 6) which, when brought high, disconnects everything--all switches open. In the 4053 SPDT switches, you might look at the "Inhibit" function as a "center-off" position, except that they are all thrown to center-off simultaneously.

The "on resistance" in the typical case looks pretty good--250 ohms with a 5V VDD. However, for that same case, the maximum is 1050 ohms, so there can apparently be wide variation from chip to chip. On the bright side, the resistances are closely matched between switches--typically within 25 ohms for the 5-volt VDD case.

The maximum allowable power dissipation, per package, is 500mW.

Every specification gets better as VDD goes up. Using the example of VDD at 15V, the typical and maximum "on resistances" are 80 ohms and 280 ohms, respectively. (The match between poles slightly improves as well.) The "turn-on time" goes down as VDD goes up--perhaps from 400 nanoseconds to 200 nanoseconds as VDD is brought from 5V to 10V.

It is interesting to note that the resistance varies slightly, depending upon the voltage being switched. For example, with the switch set up for plus/minus 5 volts, the typical "on resistance" is about 100 ohms around zero; this goes up to about 120 ohms at plus 4V and minus 3.5V, then drops back down for wider swings. Therefore, it would make sense, when considering switching audio, to keep the load resistance high enough so that these variations will be insignificant.

Specifically addressing high-quality audio, the Motorola literature speaks of a load of 10K. A 2nd-harmonic distortion figure of 0.07% is given (this with VDD minus VEE at 10V, and with a 5V peak-to-peak 1kHz signal).

The "off leakage" at 25 dg.C. is "typically?" plus/minus 0.05 nanoamps. Again, however, extreme variations are possible; a maximum figure of plus/minus 300 nanoamps is given.

The propagation delay is very complicated, and is different for the three switch chips. First of all, as mentioned earlier, there is a delay caused by the control circuitry--one which goes down as VDD is increased. The worst is on the 4051 operating at 5V: the control propagation delay is typically 360 nanoseconds, with a maximum of 720. At a VDD of 15V, these typical and maximum values go down to 120 and 240 nanoseconds, respectively. The switches themselves have a propagation delay. Once again, the 4051 is the worst: With a VDD of 5V, this propagation delay is "typically?" 35 nanoseconds, and is 90 nanoseconds maximum. These figures drop to 12 and 30 nanoseconds for a VDD of 15V.

Pin Assignments

Gotten from the Motorola literature, the notation is different from that used for the CD4053 in the Nattering RAM and RAM-Talker articles. This Motorola notation fosters a consistency that appeals to the editor, so I'm gonna use it.

The individual switches (poles, if you will) bear the labels X, Y, and Z. The MC14051 (4051) only has pole X--with 8 positions. The positions are numbered, X0 through X7 on the MC14051, while the two positions of an SPDT switch in the MC14053 will be designated X0 and X1, or, Y0 and Y1, etc. The control lines will be A, B and C.

Note that on the MC14053, each control line is associated with one of the switches; "A" is associated with pole "X," for example. On the other two, the MC14052 and the MC14051, the control lines are binary address lines; an A, B, C address of 000 presented to the 14051 will connect the arm of X to position X0.

MC14051 or CD4051 Single-Pole 8-Position:

  • 7--VEE
  • 8--VSS
  • 16--VDD
  • 6--Inhibit
  • 11--Control A (least-significant)
  • 10--Control B
  • 9--Control C (most-significant)
  • 3--Arm of X
  • 13--X0
  • 14--X1
  • 15--X2
  • 12--X3
  • 1--X4
  • 5--X5
  • 2--X6
  • 4--X7

MC14052 or CD4052 Ganged Single-Pole 4-Position:

  • 7--VEE
  • 8--VSS
  • 16--VDD
  • 6--Inhibit
  • 10--Control A (least-significant)
  • 9--Control B (most-significant)
  • 13--Arm of X
  • 12--X0
  • 14--X1
  • 15--X2
  • 11--X3
  • 3--Arm of Y
  • 1--Y0
  • 5--Y1
  • 2--Y2
  • 4--Y3

MC14053 or CD4053 Triple SPDT:

  • 7--VEE
  • 8--VSS
  • 16--VDD
  • 6--Inhibit
  • 11--Control for X
  • 10--Control for Y
  • 9--Control for Z
  • 14--Arm of X
  • 12--X0 (connected when pin 11 is low)
  • 13--X1 (connected when pin 11 is high)
  • 15--Arm of Y
  • 2--Y0
  • 1--Y1
  • 4--Arm of Z
  • 5--Z0
  • 3--Z1


By Susan and Tom Fowle


A simple device is described which allows the sound from a radio to be silenced for brief repeating periods. The circuit illustrates switch debouncing, a toggling flip-flop and a timer circuit as well as relay and transistor switching.


Before venturing to work, while listening to a local news radio station in the shower to determine if the world still exists, what I don't need is overzealous and obnoxious commercials selling me products I don't want. This circuit was developed to allow a radio listener to pull a string and have 30 seconds--or a tiny bit less--of silence. At the end of this time, the audio is returned briefly so as to allow a quick sample of about 2 seconds to determine if useful program content has returned. The circuit repeats this on/off cycle endlessly until the string is pulled again, at which time the killer is disabled.

The circuit timing is done by the ubiquitous NE555 running in a stable mode and driving a relay or other switching device. The 555 is enabled, or disabled, by a CD4013 dual clocked D flip-flop. The first half of the flip-flop is set up as a switch debouncer. This debounce circuit is given a good long debounce time, since a pull string is bouncier than other switches.

The second half of the CD4013 has its Q-NOT output connected to its D input, causing it to toggle its output high and low with every incoming clock pulse from the debounce circuit. The reset terminal of this toggling flip-flop is connected through a resistor to ground and a capacitor to plus to form a "power-up reset" circuit, thus insuring that the system comes up with the timer disabled. The Q output of this toggling circuit controls the enable pin of the timer.

Another interesting option is to control the debounce flip-flop not with a lever switch and string, but by taking its "set" input to an ungrounded metal surface to serve as a "touch plate." In this case, the input goes through a 20 megohm resistor to ground (instead of the specified 10K unit). In our experience this works fairly well, if it is simply touched and released, but severe chatter is introduced if the hand is left in contact for long. There is also the real possibility of falsing if there is any significant RF energy nearby.

The output of the 555 timer can control the audio system in several ways. A small single-pole double-throw relay can be used, with its arm connected to the output of the audio amplifier and its normally closed contact going to the hot speaker lead. The normally open contact of this relay can be taken through a 10-ohm resistor of suitable wattage to the other side of the speaker. This provides an alternative load for the amplifier, and lessens the "pop" of switching by keeping the output capacitor active and avoiding having to recharge it when the speaker is reconnected.

(For some reason, in all the relay-controlled units we have built, we could not make a 555 drive a relay without causing "latch-up" in the chip's output stage, presumably from the back EMF developed by the collapsing field across the relay coil. This problem occurred even with series and shunt protective diodes around the relay. Although direct driving of the relay is given later as an option, we had to use an output transistor. (If anyone can figure this one out, the authors would love to hear about it.)

In the transistor-driven system, the timer's output drives the base of a transistor with the relay in its collector lead.

Another means of controlling audio is at the low-level point present at the wiper of the volume control in many amplifiers. Depending on your desire to attack an existing piece of equipment, and on the delicacy of the output amplifier, you may not wish to switch the speaker leads. In this case, the timer can control the base of one or two transistors across the bottom half of the volume control. When saturated, the transistor will short out the audio signal. (More than one transistor can be used so as to control any number of audio channels.)

Power for the Killer can be taken from the device receiving the implant if a convenient source of from 5 to 15 volts--at about 100 mills--is available. Otherwise, a battery or other supply can be used. If the transistor output switch is to be used, care must be taken to be sure that no conflict occurs between any extra power supply ground and circuit ground of the "patient." This is because Killer circuit ground goes to the ground end of the patient's volume control.

In our first installation, we mounted the Killer on a small piece of vector board and simply put it in a recess in the unused battery compartment of a medium-sized portable "TV" radio which lives in our bathroom. The power was taken from the terminals intended for the battery (the designers didn't even switch them off when using AC). One speaker lead was broken and attached to the relay as described later. The leads for the switch were brought out of the cabinet with the power cord, and a momentary-contact lever-action microswitch was attached with string to the far end of a towel rack. Strings were then run from the switch lever to a couple of appropriate points for easy access while performing such functions as one may enjoy in these environs.


As set up here, the circuit stays "active" (silencing the audio) for about 30 seconds, then inactive (audio back on) for about 2 seconds. This is, of course, very rough, and timing may be adjusted as follows:

In the NE555 circuit, the 110K resistor from pins 2 and 6 to pin 7 adjusts the "on" time, or the time of returned noise. The 1.5 megohm unit from pin 7 to plus V adjusts the quiet time. These interact, of course, and it might be a good idea to make them up with series rheostats as shown in parentheses having a value of one-tenth the total for trimming. A little fiddling and luck will give you a burst of noise just as the 30-second commercials change, thereby allowing you to "unkill" without missing anything important.


The positive supply lead, 5 to 15V DC, is taken to the circuit's plus V point, and this point is bypassed to ground by a 100UF electrolytic, negative of the cap at ground. The negative supply lead is grounded. The CD4013 flip-flop has pin 14 going to plus V, while pin 7 is grounded.

The 4013's clock 1 terminal, pin 3, goes through 10K to ground (or 20 megohms--see text). Pin 3 also goes to one side of the normally open single-throw single-pole "kill" switch, with the other switch contact taken to plus V. The reset 1, pin 4, goes to the positive of a 1UF electrolytic whose negative end is grounded. This pin 4 also goes through 510K to the Q1 output of this flip-flop, pin 1. Pin 1 is the output of the debouncer.

Pin 6, set 1, is grounded, while pin 5, D1, goes to circuit plus (providing a logic high to be clocked through by the switch.) The Q output, pin 1, goes to the clock of the second half of the CD4013, pin 11. Set 2, pin 8, is grounded, while reset 2, pin 10, goes through 10K to ground and to the negative of a 1UF electrolytic whose positive is at circuit plus. The CD4013's Q2-NOT, pin 12, goes to its D input, pin 9. The Q2 output, pin 13, goes to the enable of the NE555, pin 4 of the 555.

The NE555 has its pin 1 grounded, and its pin 8 going to plus V. Between pins 1 and 8 is 0.1uF (close to the chip). Pins 2 and 6 are tied together and taken through 22uF to ground (negative of the cap at ground). Pins 2 and 6 also go through 110K (or 100K in series with a 20K rheostat) to pin 7. Pin 7 goes through 1.5 megohms in series with an optional 200K rheostat to VCC.

Output Option 1, Direct Relay Control

(See text above; should work but doesn't.) Pin 3 goes through R1, if necessary, to the anode of a 1N4000 diode. The cathode of this diode goes to one end of the coil of a single-pole single-throw relay whose other end is grounded. Across the relay coil is another 1N4000 with its anode at ground.

Note that if a 5V relay is used, and the circuit is run off a 9V battery, a dropping resistor of perhaps 27 ohms should be put in series with it; this is the "R1" referred to above.

Output Option 2, Transistor-Driven Relay

Pin 3, the output, goes through 3.3K to the base of a 2N2222 whose emitter is grounded. The collector of the transistor goes through R1 (if necessary), then through the relay coil to plus V. The relay coil is shunted by a 1N4000 diode with its anode at the collector.

In both these cases, the arm of the relay goes to the hot output of the amplifier, while the normally closed relay contact goes to the speaker lead which was removed from the amplifier. The normally open relay contact may be taken through a 10-ohm resistor to the unmodified speaker contact to keep a load on the amp (see above).

Output Option 3, Transistor Switching

The output of the 555, pin 3, goes through 33K to the base of a 2N2222 whose emitter is grounded. The collector of the transistor goes to the arm of the volume control of the invaded audio amplifier, while the bottom of the control goes to killer circuit ground. Be sure that the volume control has its bottom to amplifier ground, and that all power connections are non-conflicting.

This transistor switch can be duplicated for stereo systems--as many transistors as you want, as long as grounds are common. If more than a couple of these switches are to be run off of one 555, a 0.1UF bypass from base to emitter on each transistor might be a good idea.

RCA CD4013 Dual D Flip-Flop:

  • 7--VSS
  • 14--VDD
  • 1--Q1
  • 2--NOT-Q1
  • 3--Clock1
  • 4--Reset1
  • 5--D1
  • 6--Set1
  • 13--Q2
  • 12--NOT-Q2
  • 11--Clock2
  • 10--Reset2
  • 9--D2
  • 8--Set2


by Bob Norris, W6EAA


This material is about my answer to a real need that was vexing me: orientation in a swimming pool in the dead of night. Its scientific label might be a "sonic beacon," but to me it's "The Tinker." Use it as you see fit.

Introduction and Description

One of the nice things about this place was its almost-Olympic-size swimming pool, heated year-round.

I always paid my rent a half-month in advance of due-date . . . a matter of MY convenience. This must have charmed management, because I came away with my own key to the pool area, as well as permission to swim "after hours." I felt like a millionaire.

My swim-time quickly settled on four A.M., the quietest hour of all. I can swim very quietly (frogman techniques) and some of my best design work has been done during those quiet swims. BUT . . .

About four-fifteen A.M. an automatic timeclock would switch off the filter pump and the remaining yardlights. The loss of light didn't bother me a bit, but the sound of the pump was another matter. It provided a very useful orientation bearing, and without it I was forced to pay more attention to my location in the pool, an interference to my freedom-of-thought. I needed a sound-maker that produced a distinct sound that blended into the noises of the night. My situation? One complaint and I might be out of business.

I needed a sound that was high in amplitude, short in duration, repeated monotonously, and well spaced in time.

My first thought was the sound of the cricket, but that'd sound odd in the winter. I thought of the WWV time-ticks. Pretty good, but not natural enough. I built a circuit just for a test.

Once every second a highly asymmetrical oscillator keyed on a one-kilohertz source for ten milliseconds. It sounded okay but not as good as WWV.

Then came a period of dorking with higher frequencies, considering phase, and measuring ear-travel as the head "sniffs out" a bearing. Great theory, but even more unnatural in sound.

In an effort to get away from the "dry" sound of the WWV ticks, I dropped the frequency almost an octave, more than doubled the pulse length, and spaced the result out to two seconds or more and said "the heck with it, that'll have to do." I packaged it up, and here is where serendipity took a hand.

The circuit required a nine-volt battery and an inch-and-a-half speaker, all of which was stuffed into a metal Band-Aid can. The result was a real surprise. The sound had changed from a "tick" to a "tink." It appears that the fatter, more tuneful "tick" found enough resonance with the container to produce a decided ring--though well damped into a sharp tail-off.

The "tink" sounds very much like a drop of water, falling from house-eave-height onto a tin can or rainspout.

In use, I place the unit at the shallow end of the pool, and all orientation problems disappear. Though the "tink" is loud, it disturbs no one. Lately it has seen service as a "homing beacon" in support of my midnight strolls, and I hear it easily from a block away; on still nights, two blocks. Admittedly, my area is quieter than most.


The circuit was housed in a Band-Aid can measuring 3-5/8 by 3-1/8 by 1-1/8. The hinged lid mounted the nine-volt battery and the miniature single-pole single-throw power switch. The front major surface was drilled with a pattern of #11 holes as a sound-port for the speaker. The very thin sheet metal of the can tends toward ragged holes, so both surfaces, inside and out, were dressed smooth with a file. To mask this butchery, a three inch by three and a half inch piece of Vector board was cemented to the boxfront as a grill.

A hand-drawn 2-inch by 2-inch circuit board was etched and drilled. The foil side of the stuffed board was sticky-taped to the back of the speaker magnet, and the combined assembly (speaker and circuit board) was securely held against the speaker port by a piece of kitchen sponge, jammed between the board components and the rear wall of the case. There are no screws used; the battery was sticky-taped to the inside of the hinged lid.

The Tinker uses only one integrated circuit: the RCA CD4049BE, a CMOS hex inverter/buffer. There are doubtless some who will have a cardiac over such duty as a speaker driver. RCA says don't parallel and don't drive speakers. I've been doing it for years.

The speaker load has been softened up with a 47-ohm resistor, not in consideration of the IC but because the signal was much too loud.

The Circuit

An oscillator running at a kilohertz or so is keyed by a timing waveform generated by another oscillator.

Basically, both oscillators are of the R-C phase-shift type. The Kilohertz Oscillator (KO) is composed of Inverters E and F, along with R-5, R-6, and C-2. The Timing Waveform Oscillator (Inverters B and A) is identical except that the charge and discharge paths are modified by switching diodes D-1 and D-2.

Diode D-3 inhibits the KO except during the short-pulse portion of the timing waveform. The keyed output of the KO is buffered by Inverters C and D connected in parallel with their combined outputs driving the small inch-and-a-half loudspeaker.

A word about the diodes. These are of the small signal-switching type such as the 1N914. These are silicon diodes and the back-resistance should be quite high, but be sure that the back-resistance of D-2 is especially high.

Circuit Description

The negative side of the 9V battery is grounded; the positive battery terminal goes through an on-off switch to the VCC line. Pin 8 of the CD4049 is grounded, while pin 1 (the chip's positive supply pin) goes to the VCC line.

Inverters B and A are in cascade; the output of B, pin 4, goes to the input of A, pin 3. Pin 2, the output of A, goes to the positive end of C1 (4.7uF). The negative end of C1 goes to one end of each of the following three resistors: R1 (3.9meg), R2 (680K), and R3 (1.1K). The far end of the 3.9meg, R1, goes to pin 5, the input of B.

The far end of the 680K, R2, goes to the cathode of D1 (1N914); the anode of D1 goes to the junction of the inverters, pins 3 and 4. The far end of the 1.1K, R3, goes to the anode of D2 (1N914); the cathode of D2 goes to the junction of the inverters, pins 3 and 4.

Inverters E and F are also connected in cascade; pin 12, the output of E, goes to pin 14, the input of F. The output of F, pin 15, goes to one end of C2 (0.0047uF). The far end of C2 goes to one end of each of the following resistors: R5 (470K), and R6 (56K). The far end of the 470K, R5, goes to the input of E, pin 11. The far end of the 56K, R6, goes to the junction of the two inverters, pins 12 and 14.

The first oscillator's output is used to inhibit the second oscillator; pin 2 goes through a diode (D3, 1N914) to pin 11 (the anode of D3 at pin 11).

Inverters C and D are paralleled. Their inputs, pins 7 and 9, are tied together and go to the output of F, pin 15. The outputs, pins 6 and 10, are tied together and go to the positive of a coupling capacitor (unspecified, but how about 10uF). The negative end of this cap (C3) goes through 47 ohms (R4), then through the speaker to ground.

Pin Assignments

CD4049 Hex Inverter:

  • Pin 1--VCC
  • Pin 8--Ground
  • Pins 13 and 16--No Connection
  • Pin 3--In A
  • Pin 2--Out A
  • Pin 5--In B
  • Pin 4--Out B
  • Pin 7--In C
  • Pin 6--Out C
  • Pin 9--In D
  • Pin 10--Out D
  • Pin 11--In E
  • Pin 12--Out E
  • Pin 14--In F
  • Pin 15--Out F

As to battery life, after six years of use I replaced the original alkaline battery, though it was still going strong, because it was like waiting for the other shoe to drop.