SKTF -- Spring 1988

The Smith-Kettlewell Technical File

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

William Gerrey, Editor

Issue: [current-page:title]

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 Details regarding products, suppliers, and other contact information are original and may be outdated.

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by Richard Oehm


This article was written to augment and accompany the previous article on the Smith-Kettlewell scope (SKTF, Summer 1984). The information presented here includes: a vertical metering circuit, a tried-and-true autoscan circuit with tone bursts to indicate when the left or right edge of the display has been reached, and solutions to noise problems which occurred in some configurations (noise which modulates or distorts the sound of the graphics VCO). Circuit boards or completed instruments can be gotten from Oehm Electronics, whose address appears at the end of this article.

It really only begins to express my affection for this remarkable instrument when I say that Al Alden's auditory oscilloscope is the most useful and flexible single tool available to the blind electronics technician or engineer. In my past eight years of employment in the electronics industry, I've used this scope every day to make accurate measurements on all sorts of parameters which would otherwise be cumbersome, difficult, or downright impossible for me to do without this device. By connecting this auditory scope to the horizontal and vertical outputs provided on many test instruments, it provides me with information from sweep generators, vector scopes, curve tracers, spectrum analyzers and network analyzers. In the field of electronics, the Smith-Kettlewell auditory oscilloscope is the most useful tool I have in my arsenal of "enabling technology," and it is the key to my employability in the industry.

I have constructed five auditory oscilloscope adapters in the past few years. Two were for my own use, and three were sold to other blind individuals entering the field of electronics. I have also produced a pair of printed circuit boards that contain the scope adapter with all the changes and refinements described here. Information about how you can obtain these boards, or a ready-made adapted scope, will be given at the end of this article.

Description and Features

[Editor's note: Writing a manual on how to use oscilloscopes cannot fit in this magazine--nor is this entirely necessary. Each make and model will come with its own manual. In addition, so much educational material for students of electronics has been done by Recordings for the Blind and other transcribers' groups that a little research will unearth the instructions you need.]

In general, an "oscilloscope" consists of a cathode-ray tube monitor with driving circuitry that can "graph" electrical events. A typical setup is to plot a single-valued voltage against time--the amplitude of the signal deflecting the cathode-ray tube's beam vertically while the beam's horizontal position is guided by a carefully timed linear sweep. Visual oscilloscopes are sometimes used in other modes; for example, harmonically related signals are sometimes presented with their amplitudes plotted on opposing axes. In this latter mode, the curve may have more than one intersection with a given axis.

[It should be noted that this auditory system can only be used to present single-valued functions. This is true, since the vertical deflection signal cannot be indicated as being in two places at once. Furthermore, the horizontal position at which a sample is taken is only sensed when the trace moves from left to right (which is always the case when time is on the abscissa).

With the present technology, lissajou patterns and dual-trace presentations are out of the question. (By using his head, though, a clever technician can set up a switching arrangement with which an A/B comparison of two synchronized signals can be made, which gets around the dual-trace limitation.)]

The adaptation consists of two circuit boards. One contains what you might call "display circuitry"; such circuits as the auditory oscillator, amplitude metering system, automatic scanning, and audio amplifier are built on this board. The other board contains interface circuitry which, for reasons of susceptibility to noise and minimizing stray capacitance, should be incorporated into the cabinet of the modified oscilloscope.

The finished instrument is comprised of an ordinary-looking scope, along with an extra box that contains two Braille dials and an assortment of other switches and controls. Features of the instrument are as follows:,

Obviously, the audio output has a volume control. The way this auditory attachment is silenced is by turning this control all the way down.

There is a Braille-calibrated scanning control whose position determines the point on the abscissa at which the trace is "sampled." (The scale of this control is in centimeters and fractions of centimeters; these markings correspond to lines on the screen.)

Another Braille-calibrated pot is a "null-type indicator" with which the amplitude of any part of the trace can be measured. (This, too, is calibrated in centimeters and fractions of centimeters.)

An "amplitude/graphic" mode switch (SPDT toggle) selects either a VCO which "describes" the shape of the curve in the graphics mode, or the "null-type" amplitude measuring circuit.

A "manual/auto" switch (a DPDT toggle) allows a free-running ramp generator to be substituted for the Brailled horizontal scanning control. This is of value when circuit adjustments are being made--where free-hands operation of the auditory scope is desirable.

A "scan-direction" switch (SPST toggle) sets the automatic scanning circuit to recurrently scan from left to right, or to scan back and forth in both directions.

Also part of the automatic scanning feature is a "scan-rate" control. With this, you can adjust the speed of scanning, depending on what sorts of features on the curve you are listening for.

As mentioned in the original article, it is possible, by modulating the "Z-axis" (intensity) of the visual display, to create a "dot" on the trace at the position where the sample is taken. Although this circuit may not be of use to many of you, I have provided for it on the PC-board, just in case a sighted instructor wants to know the position of the scanning control for training purposes. (This feature is also very useful in demonstrating the auditory oscilloscope to sighted people, which might be important in a job interview.) An SPST toggle switch allows this feature to be turned on or off. (Unfortunately, a visible dot can only be created at slow sweep rates on waveforms of low frequency; this is as much caused by limitations in the bandwidth of the Z-axis input as by limitations in the circuit shown here.) Note that the intensity control on the scope may have to be adjusted to accentuate the dot.

It is necessary to have two "speeds" of sample-and-hold. (The reason for this will be described shortly.) Therefore, a "sweep-rate selector" switch (DPDT toggle) is provided; the user must remember to change this selection when extreme sweep rates are being used.


In the "graphic" "manual-scan" mode, you scan the horizontal control from side to side; an audible tone then "describes" the waveform by presenting a varying pitch which is a function of vertical amplitude. In other words, the pitch you hear at any given position of the scanning control represents the instantaneous vertical amplitude at the instant of time represented by that horizontal position.

The peak and valley amplitudes of the wave can then be individually noted. While still in the "graphic" mode, the scanning control is moved to a point where a desired peak or valley resides--indicated by a corresponding "peak" or "valley" in the pitch of the graphics tone. Then, the "amplitude/graphic" mode switch is flipped to the "amplitude metering" position, whereupon the null-balance meter reader becomes audible. The Brailled amplitude control is then turned to the point of "null" (meaning that the tone disappears). The two Braille controls can now be read to ascertain vertical amplitude and horizontal position.

I chose to use the null-balance Smith-Kettlewell "Meter Reader" for measuring amplitudes. The Braille scale can be tailored to match the divisions on the graticule of the scope being used. In most instances, this arrangement can provide you with amplitude information more quickly than a spoken-word output can do. I chose the null-balance circuit over the "Fowle Gimmique" variable frequency system so that the never-changing 500Hz tone could not be mistaken for the graphics tone; there is never any doubt as to which position the "amplitude/graphic" switch is in.

On the other hand, the "vertical metering amplifier" described will accommodate a talking meter as an output, and its adjustable scaling will permit the spoken voltages to represent centimeters of the graticule.

The "sweep-rate selector" switch does not change a horizontal sweep rate, but it must be set to the appropriate position based on the visual scope's sweep rate being used. It changes the value of the capacitor in the sample-and-hold circuit. At fast sweep rates, a small sampling capacitor is used to minimize charging time, thus maximizing the bandwidth of the adapter. On the other hand, a large sampling capacitor is needed at slow sweep rates--of the scope's trace--so as to minimize sagging of the held value between sweeps. In other words, at faster sweep rates, the small sampling capacitor is refreshed often enough so that "sagging" of the held voltage between sweeps is not significant. At slow sweep rates, using this small capacitance, the VCO would drop in pitch between sweeps; the result would be noise (flutter) in accordance with the sweep rate superimposed on the tone. Therefore, the large sampling capacitor should be switched in for sweep rates below 1 millisecond per division.

At very slow sweep rates (10 milliseconds per division or so), the rate of refreshment will be quite apparent as you scan past a feature with a steep slope. On a sinewave, for example, the "slope" (rate of change) is maximum at the zero crossings; in scanning past these points, the VCO will jump in steps. These "steps" will be smaller as you scan slower, since the differences in sampling will be less. If scanning is done steadily enough, the intervals of the steps can be an actual indication as to the degree of slope.

Causing the sweep circuit to trigger on the waveform is very easy with the auditory scope. If the sweep circuit is triggering on something unrelated to the input signal, the VCO will make warbles, random music, or rough noise. As soon as proper triggering is established, a pure tone is the result. (Of course, this assumes several essential conditions: the tone will be as pure as the signal is free of noise, and the amplitude of the signal must be within the workable range of the VCO. If no definite indication of triggering is heard, these conditions should be sought after.)

Technical Description

[Note: This section is largely borrowed from Al Alden's original article.]

In the adaptation, the vertical and horizontal signals are fed into differential amplifiers to provide signals which are referenced to ground potential. The output of the horizontal amplifier and the voltage at the wiper of the graphics scanning control are the inputs to a comparator. When the horizontal signal (a ramp) falls below the scanning control voltage, the comparator's output rises and triggers a one-shot multivibrator. (Note the use of the ramp with a negative slope.)

The one-shot produces a short pulse which activates a sample-and-hold circuit. This circuit samples the vertical signal and stores its amplitude until the next pulse from the one-shot. The next pulse occurs with the next horizontal ramp (sweep).

The effect of moving this horizontal scanning control is to pick the point along the horizontal axis at which the vertical signal is sampled. The "dot" described in the previous section is gotten by modulating the brightness of the visual display with a differentiated pulse off the comparator's output; this generates a sort of "cursor" by which sighted observers can see the point at which the auditory system is set.

The output of the sample-and-hold is fed to the voltage-controlled oscillator. The voltage-controlled oscillator (VCO) includes an exponential converter circuit. This causes equal changes of vertical amplitude on the oscilloscope to produce equal ratios of output frequencies (constant musical intervals). The output of the VCO is fed to a power amplifier in order to drive the loudspeaker.

The vertical amplitude at any point (not just at a maximum or minimum, but at any point on the trace) can be measured by looking at the output of the sample-and-hold circuit.

The auto-scan circuit merely substitutes a ramp generator for the horizontal scanning control. Bi-directional scanning, vs. scanning only from left to right, is accomplished by radically changing the duty cycle.

Modifications to the Original Smith-Kettlewell Circuit

The first few times that I built this adapter, I was plagued with all sorts of noise components that would either ride on or modulate the tone. I hit on a combination of fixes that seem to work together to eliminate this problem entirely. All of the editions and changes discussed here are incorporated into the printed board version of this adapter.

Most of what was needed turned out to be "decoupling"; you will notice quite a few new bypass capacitors on both plus and minus supplies. Besides these, you will notice an RL filter in the sample-and-hold circuit.

I was able to considerably improve the bandwidth of the adapter system by assuring that all high-frequency signals from the deflection plates go through short direct leads. This made it necessary to create two boards, one being mounted inside the visual oscilloscope. Only DC voltages are sent to the external adapter box.

The board inside the scope contains: both differential input systems, the sample-and-hold circuit, the "dot" circuit, the vertical metering amplifier (the one to which a digital voltmeter can be connected, and whose output feeds the null-balance meter reader), and the power supply regulators.

As far as power is concerned, 15-volt regulators on the board inside the scope provide the regulated dual supply. A uA7815 (or LM340-15T) supplies plus 15V, and a uA7915 (LM320-15T) supplies minus 15V. Thus, "raw" (unregulated) DC is needed to power the system--from plus/minus 18V to plus/minus 40V. If this is not available from the visual scope's circuit, a suitable transformer--say with a 36- or 48-volt center-tapped secondary--will have to be installed. (Ground loops can be avoided if the adapter's transformer gets AC from the same primary circuit as the scope.)

The board inside the scope measures 4.7 by 5 inches, while the one intended for the external adapter is 5 by 5 inches. There are a myriad of calibration adjustments. All of these use multi-turn trim pots; they are the oblong style with the screw adjustments at one end.

I designed my own "auto-scan" circuit; for some reason, I could not get the one in the original article to work. Further, I have included marker tones that tell the user when the margins of the screen have been reached--a low pitch for the left side of the screen and a higher pitch for the right side. Using a pair of free-running oscillators in a 556 dual timer chip, the beeps produced by this circuit are distinct and stable pitches, and can in no way be confused with the other graphics or amplitude-metering tones. PC-mount single-turn "volume controls" are provided to set the levels of these beeps.

Circuit Descriptions

Power Requirements

A dual supply, providing regulated plus and minus 15V, is required. Provisions for two complementary 3-terminal regulators (LM340-15T and LM320-15T) have been included on the interface board. The "Common" terminal of each regulator is grounded. The "Input" of the LM320 goes to an unregulated minus voltage; its "Output" terminal is the minus 15V line. The "Input" terminal of the LM340-15T goes to an unregulated plus voltage; its "Output" terminal is the plus 15V line. It is unlikely that local bypassing will be necessary at the regulators, since so much decoupling has been added throughout the circuit.

Besides this, a plus 5V supply is required. This can be derived from the plus 15V source by using an LM340-5 as follows: Its common terminal is grounded, while its input terminal goes to plus 15V. Between input and common is 0.1uF; the output terminal goes through 10uF to ground (negative of the capacitor at ground).

Universal Interface Circuits

Two carbon copies of this circuit must be built--one for the vertical deflection plates, and another for the horizontal ones. They each consist of two LM310 followers and three transistors (2N2222A's).

The plus deflection plate (or other differential signal source) goes through a 510K resistor, then through a "selected resistor" (perhaps 10K) to ground. The junction of these two resistors goes to the input (pin 3) of an LM310 follower. The output, pin 6, goes to the base of a 2N2222A. The collector of this transistor goes through 2.2K to plus 15V. This collector becomes the "negative signal output"; this is true because of the inversion that takes place by way of the transistor.

The negative deflection plate goes through 510K, then through an identical "selected resistor" to ground, with the junction of these resistors going to the input of another LM310 (pin 3). The output of this follower (pin 6) goes to the base of another 2N2222A. The collector of this transistor goes through 2.2K to plus 15V; this becomes the positive output of the differential interface circuit.

Each of the above transistors has an emitter resistor of 510 ohms; the far ends of these are tied together and go to the collector of a third 2N2222A. The emitter of this third transistor goes through 270 ohms to minus 15V. Its base goes through 30K to ground, as well as going through 10K to minus 15V.

[Note: The vertical circuit, and the horizontal circuit, each use this system of two LM310's and three transistors; there will be six 2222's and four 310's in this front-end section.]

To power these four LM310's: Their pin 7 goes to plus 15V; each pin 7 is bypassed to ground by 10uF (negative at ground). Their pin 4 goes to minus 15V; each pin 4 is bypassed to ground by 10uF (positive of these electrolytics at ground).

The above "selected resistors" should be chosen to give less than a 4V bias at the inputs of the LM310's, and so that the peak signal does not cause limiting at the collectors of the 2N2222A's.

Sample and Hold Circuit

The "positive vertical output" is used; this comes from the collector of the transistor whose base is driven by the buffered signal from the negative vertical deflection plate. This collector goes through a "selected resistor" (perhaps 36K) to the top of a 10K pot (this is the scaling adjust for the VCO), the bottom of which is grounded. This resistor is selected so that the maximum peak signal at the top of the pot is less than 5V.

The wiper of the 10K pot goes to the input (pin 3) of another LM310, with the output of this 310 (pin 6) going to pins 3 and 2 of a CD4066 analog switch. Pin 7 of the LM310 goes to plus 15V; pin 4 goes to minus 15V. A 50uF electrolytic capacitor goes between pins 7 and 4 (negative end at pin 4).

Pins 2 and 3 of the 4066 are each one terminal of an SPST switch; the other terminals of these switches, pins 1 and 4, are tied together and go through 100pF to ground. (In other words, these two switches are in parallel.)

Pins 1 and 4 not only go through the 100pF unit to ground, but they also go through 0.01uF to pin 8 of the 4066 (the terminal of another switch). Pin 9 of the 4066 (the other side of this switch) is grounded. Pin 14 of the 4066 goes to plus 5V, and pins 7 and 11 are grounded.

Pins 1 and 4 of the 4066 also go through a 0.33mH choke to pin 3, a non-inverting input of an op-amp (one of two contained in a CA3240). Also on this 3240, pins 1 and 2 (output and inverting input) are tied together to make a follower. Pin 1 is the output of the sample-and-hold system which drives the VCO.

Vertical Amplitude Metering Section

The second half of the 3240 is the vertical metering amplifier. Pin 6, the inverting input, goes through 20K to the arm of a 10K pot, the top of which goes to plus 5V and the bottom of which is grounded. Between pins 6 and 7 is a 39K feedback resistor shunted by a 1uF nonpolarized cap. Pin 5 goes to the arm of another 10K pot; pin 5 is bypassed to ground by 25uF (nonpolarized cap). The bottom of this pot is grounded; the top of this pot goes to pin 1 of the same 3240. The first pot adjusts the offset, and the second pot the scale factor.

On the CA3240, pin 8 goes to plus 15V; pin 8 is bypassed to ground by 10uF (negative end at ground). Pin 4 of this 3240 goes to minus 15V; pin 4 is bypassed to ground by 10uF (positive at ground).

Pin 7 of the 3240 goes to pin 4 of an H11F3 opto isolator, pins 4 and 6 being the channel of the FET. Pin 6 connects through 47K to the arm of the 10K linear precision "amplitude metering" pot (fitted with a Braille scale designed to match the major divisions of the graticule on your scope). The top of this pot goes through a 50K 10-turn rheostat to plus 5V. The bottom of the 10K pot goes through another 50K rheostat to a minus 5.1V reference, constructed as follows:

A 5.1-volt zener has its cathode grounded; its anode goes through 470 ohms to minus 15V. The anode of this zener is the desired reference and, as mentioned, goes through the lower 50K rheostat to the bottom of the calibrated pot.

The junction of the 47K resistor and pin 6 of the H11F3 goes through 0.1uF to the "amplitude metering" contact on an SPDT switch. (This selects between metering of vertical amplitude and the "graphic mode" of the scope.)

A 555 timer chip drives the H11F3 chopper. Pins 2 and 6 are tied together and go through 0.01uF to ground. Pins 2 and 6 also go through 100K to pin 7, with pin 7 going through 100K to pin 8 (which is plus 5V). Pin 3 goes through 2.2K to pin 2 of the H11F3, the cathode of its LED. Pin 1 of the H11F3 goes to plus 5V.

Pin 1 of the NE555 is grounded. Pins 4 and 8 go to plus 5V; they are bypassed to ground by 47uF (negative end at ground).

Horizontal Sensing and Triggering Circuit

The negative horizontal signal is used to trigger the system; this is gotten from the collector of the transistor whose base gets the buffered version of the positive horizontal deflection plate. This collector goes through 15K to the plus input of an LM311 comparator (pin 2). From pin 7 to pin 2 (from output to non-inverting input) is a 1.5meg resistor. Pin 3, the negative input, goes to the scanning system--be it by manual control or automatic scan.

On the LM311, pin 8 goes to plus 15V, while pin 4 goes to minus 15V. The 311 output is arranged to go between plus 5V and ground. Pin 1, its negative reference pin, is grounded, while pin 7, its output, goes through 1K to plus 5V.

Pin 7 of the 311 goes to both pins 1 and 9 of a 74LS123; these are the inputs of two one-shots. On the first section, pin 3 is connected to plus 5V, while pin 2 connects to the arm of a sweep-rate selector switch to be described later. Between pins 14 and 15 is a 100pF capacitor. Pin 15 goes through a 10K rheostat to plus 5V (this is adjusted for a 200 nanosecond pulse when this one-shot is triggered).

Pin 13 of the 74LS123 (its output) goes to pin 13 of the CD4066 (one of the switch's control pins).

On the second half of the 74LS123 (whose pin 9 input already goes to the comparator), there is a capacitor of 0.001uF connected between pins 6 and 7, with pin 7 also going through 27K to plus 5V. Pin 11 goes to plus 5V. The output, pin 5, goes to the control pin of the other parallel-connected switch (pin 5 of the 4066).

To power the 74LS123, pin 8 is grounded, while pin 16 goes to plus 5V.

A sweep-rate selector switch is required to select appropriate "sampling durations"; a double-pole double-throw toggle is used. Pin 10 (the enable of the second one-shot) of the 74LS123 goes to the arm of one pole on this selector). Along with the enable of the one-shot, the control of pin 6 of the 4066 also goes to the arm of this pole. The "fast" position of this pole is grounded, while the "slow" position goes to plus 5V. To disable the 200-nanosecond pulse when the "slow" position is chosen, pin 2 of the 74LS123 goes to the arm of the second pole. On the latter pole, the "fast" position goes to plus 5V, while the "slow" position is grounded.

Manual Scanning Control

The scanning control (fitted with a Braille scale marked in centimeters) is a 10K linear precision pot. The top of this pot goes through a "selected resistor" (perhaps 20K), then through a 10K trimmer rheostat to plus 15V. The bottom of the pot goes through a similar "selected resistor," then through another 10K trimmer rheostat to minus 15V.

Automatic Scanning Circuit

An ICL8038 is used to generate linear ramps which replace the voltage from the manual scanning control. Pin 11 of the 8038 goes to minus 15V, while pin 6 goes to plus 15V. Pin 7 goes through a 250K pot to plus 15V. The arm of this pot goes to pin 8. A panel-mount unit, this pot is the "scan-rate control," and operates the VCO input of the 8038.

On the 8038, each current-source pin (pin 4 and pin 5) goes through 560K in series with 10K to plus 15V. A "scan-direction" switch, an SPST unit, shorts out the 560K resistor off pin 5. When this switch is closed, the circuit scans from left to right; when it is open, it scans both ways--right and left.

Pin 10 goes through the timing capacitor, a 2.2uF tantalum unit to minus 15V (positive of the cap at pin 10.)

Pin 3, the triangle output, goes to the top of a 10K potentiometer ("scan width" control); the bottom of this pot is grounded. The wiper goes through 47K to pin 2 of a 741 op-amp; from pin 2 to pin 6 is a 56K feedback resistor. Also from pin 2, there is a 100K resistor going to the arm of a 20K pot (offset adjust); the ends of this pot go to plus and minus 15V.

On the 741, pin 3, the non-inverting input, is grounded. Pin 7 goes to plus 15V, while pin 4 goes to minus 15V. Pin 6 of the 741 is the scanning voltage, equivalent to that on the arm of the manual control pot.

A DPDT switch is used to select the method of scanning--"manual" or "auto."

The arm of the first pole goes to pin 3 of the LM311. The "manual" contact of this pole goes to the arm of the 10K Braille-calibrated scanning pot. The "auto" position goes to pin 6 of the 741 which carries the triangle wave (see above).

The second section of this switch couples end-of-scan beep tones into the audio amplifier. Thus, its swinger goes to the swinger of the aforementioned "amplitude/graphics" switch. The "auto" contact of this pole has two 0.01uF capacitors connected to it; the free ends of these capacitors go to pins 2 and 9 of another 4066 switch chip that is used to switch in the tone bursts.

Pin 8 of the 4066 goes to the arm of a 10K single-turn pot; one end of this pot is grounded, while the other end goes to pin 9 of a 556 (used to generate the burst tones). Pin 1 of the 4066 goes to the arm of another 10K single-turn pot; the bottom of this pot is grounded, while its top end goes to pin 5 of the 556.

Pins 4, 10 and 14 of the 556 go to plus 5V, and these pins are bypassed to ground by 47uF (negative end at ground). Pin 7 is grounded. For one oscillator: Pins 8 and 12 are tied together and go through 0.01uF to ground; 8 and 12 also go through 100K to pin 13. Pin 13 goes through 100K to plus 5V. For the other oscillator: Pins 2 and 6 are tied together and go through 0.01uF to ground; 2 and 6 also go through 12K to pin 1. Pin 1 goes through 100K to plus 5V.

Margin Tones

Next, the squarewave output of the 8038, in conjunction with two one-shots, triggers beeps at the margins of the screen.

Pin 9 of the 8038, the squarewave output, goes through a 10K pot to ground; the arm of this pot goes through 47K to pin 2 of a 741 op-amp (the inverting input). The 741 pin 2 also goes through 100K to the wiper of another 10K pot; the top of this pot goes to plus 15V, while its bottom end goes through 10K to ground. (The first of these pots is "gain" (amplitude) of the squarewave output, and the latter pot is "offset.")

Between pins 2 and 6 of the 741 is a 56K feedback resistor. Pin 3 of the 741 is grounded. Pin 4 goes to minus 15V; pin 7 goes to plus 15V.

Pin 6 of the 741, the corrected squarewave, goes to pins 2, 3 and 9 of a 74123 one-shot. An 8uF non-polarized capacitor goes between pins 6 and 7 of the 74123. Pin 7 also goes through a 10K rheostat, then through a 10K resistor to plus 5V. (This rheostat adjusts the length of the first tone burst.)

Pins 10, 11 and 16 of the 74123 go to plus 5V; pins 1 and 8 are grounded. Between pins 14 and 15 is another 8uF nonpolarized capacitor. Pin 15 also goes through a 10K rheostat, then through a 12K resistor to plus 5V. (This rheostat adjusts the length of the second tone burst.)

The output pins of these two one-shots are pins 5 and 13. Pin 5 goes to pin 6 of the 4066 tone-burst switch, while the one-shot's pin 13 goes to pin 13 of the 4066. (Pins 6 and 13 are control terminals for the analog switches contained in this chip.) On the 4066, pins 5, 7 and 11 are grounded; pin 14 goes to plus 5V.

Cursor Circuit

By modulating the Z-axis (intensity) of the scope, a visual indication of scan position can be made visible (creating a "dot" on the trace). This "dot" is only good for slow time bases, and low-frequency input signals. Its bandwidth is not only limited by shortcomings of the following circuit, but its usefulness is also impaired by the bandwidth of most Z-axis inputs.

The 12-millisecond output of the sample-and-hold one-shot, pin 5 of that 74123, goes through an SPST "dot off/on" switch to pins 10 and 11 of another 74123 (dot one-shot). Pins 8 and 9 of this 74123 are grounded; pin 16 goes to plus 5V. Pins 6 and 7--normally connected to a timing capacitor--are left open; the internal capacitance of the chip is sufficient for the desired short pulse.

[For scopes whose Z-axis input intensifies the trace with a negative signal, the "Q" output, pin 5 is used. For scopes whose trace brightens with a positive Z-axis signal, the "Q-NOT" output, pin 12, is used.]

The appropriate output of the one-shot goes to the arm of a 10K pot; the bottom of this pot is grounded, while its top end goes through 27K to pin 2 of a 741 op-amp. Pin 3 of this 741 is grounded. Pin 4 goes to minus 15V; pin 7 goes to plus 15V. Pin 2 also goes through 100K to the arm of another 10K pot; the top end of this pot goes through 10K to plus 15V, while the bottom end goes to minus 15V. Between pins 2 and 6 is a 56K feedback resistor. Pin 6 is coupled through a 0.01uF capacitor to the Z-axis input.

Exponential Converter, Oscillator, and Speaker Amplifier

One half of a CD3240 is used as an inverting amplifier with offset; this is in order to set the "base-line pitch" of the VCO system. The second half has a diode in its gain-determining circuit; the non-linearity of the diode creates an amplifier gain which is approximately exponential.

Pin 4 of the exponential converter's IC goes to minus 15V; pin 4 is bypassed to ground by 10uF (positive end at ground). Pin 8 of this 3240 goes to plus 15V; pin 8 is bypassed to ground by 10uF (negative at ground).

Pin 3 of this IC, a non-inverting input, is grounded. The inverting input, pin 2, goes through two 4.7K resistors in series back to pin 1 of the sample-and-hold follower (an aforementioned 3240). The junction of these 4.7K units is bypassed to ground by a parallel combination of three non-polarized capacitors: 0.1uF, 1uF, and 5uF. Pin 2 also goes through 3.9K to the wiper of a 10K pot (this is the offset adjust for the VCO). The top of this pot goes to plus 5V, while the bottom end is grounded. Between pins 1 and 2 (the output and inverting input) is a 1.3K feedback resistor, this 1.3K being shunted by 0.1uF and 10uF (both nonpolarized capacitors).

The output of the above stage, pin 1, goes to the cathode of a diode, the anode of which goes to the inverting input (pin 6) of the other op-amp in the chip. (The diode is a 1N4148 silicon unit.) Pin 5, the non-inverting input, is grounded. The feedback resistor from pin 7 to pin 6 is 47K.

Both sections of another CD3240 comprise the oscillator. In this case, the 3240 is operated from 5V; pin 8 goes to plus 5V, while pin 4 is grounded. Pin 8 is bypassed to ground by 10uF (negative end at ground).

The output of the previous stage, its pin 7, goes through a 100K, 1% resistor to pin 2 of this 3240 (an inverting input). Between pins 1 and 2 (output and inverting input) is a capacitor of 820pF. Pin 7 of the previous IC also goes through a 49.9K 1% resistor to pin 3, the non-inverting input of the oscillator's 3240. A second 49.9K 1% resistor goes from this pin 3 to ground. The inverting input, pin 2, also goes through another 49.9K, 1% resistor to the collector of a transistor (2N2222A). The emitter is grounded.

Pin 1, the output of the first half of the 3240, goes to pin 6, the inverting input, of the same package. The non-inverting input, pin 5, goes through 39K to the junction on a voltage divider--two 4.7K resistors in series between plus 5V and ground. Between pin 5 and the output of its op-amp, pin 7, is another 39K resistor. Pin 7 also goes through 7.5K to the base of the 2N2222A.

The output of the oscillator is actually pin 1 of its op-amp. This pin 1 goes to the positive end of a 1uF coupling capacitor, with the negative end going through 39K to the "graphic mode" side of the SPDT "amplitude/graphic" switch. The arm of this switch goes to the top of a 10K volume control. The bottom of this control is grounded.

The wiper of the control goes to pin 3 of an LM386. Pins 2 and 4 of the 386 are grounded. Pin 6 goes to plus 5V; this is bypassed to ground by 47uF (negative end at ground). Pin 5, the 386's output, goes through 10 ohms in series with 0.068uF to ground. Pin 5 also goes to the positive end of a 100uF capacitor, the negative end of which goes through the speaker to ground.

Choosing the "Selected Resistors"

First, resistors in the initial voltage dividers (the ones going to ground off the 510K units) must be chosen for two criteria. First, the base-line deflection should create a bias at the LM310 inputs which does not exceed 4 volts. Second, make sure that with deflections whose peaks fill the screen, no limiting occurs at the output collectors. You can check these criteria as follows:

Arrange a horizontal line to be present in the center of the screen. (Do this by first causing the horizontal sweep circuit to trigger on something--the ac line for example; then, with the input shorted, set the vertical position control to place this line in the center.) Now, pick the resistors (the ones immediately following the 510K units) so that less than 4V appears at pin 3 of the LM310's. Next, move the horizontal line from bottom to top of the screen (using the vertical position control of the visual scope), and with a voltmeter connected to one of the output collectors, verify that no "bottoming out," or "limiting," occurs as these extreme positions are approached. If limiting does occur, reduce the value of these resistors until this ceases to be a problem.

Without another oscilloscope, equivalent tests are harder to make for the horizontal circuit. In most cases, however, the voltages seen by the input circuits will be identical in character; proper choice of the vertical components will give you sufficient information to install these in the horizontal circuitry. Different resistors must be chosen where the screen is not square. For example, if the screen is only 8 centimeters high, but is 10 centimeters wide, the "selected resistors" in the horizontal circuit will have to be reduced by 20% or so.

A resistor at the top of the 10K scaling potentiometer (at the input of the Sample-and-Hold Circuit) must now be chosen so that the voltage at the top of this control does not exceed 5V. With a voltmeter at the top of this pot (the negative meter lead being grounded) and with the horizontal line still being generated on the scope, choose the resistor so that, as the line is moved from top to bottom of the screen, a maximum voltage of 5V is not exceeded.

Finally, resistors at the ends of the 10K scanning potentiometer must be selected. Determine the voltage swing which the ramp applies at the 15K input resistor (this resistor comes off pin 2 of the LM311 comparator). (This can best be done with another oscilloscope, but it can also be done with an analog voltmeter if the sweep is slow enough.) Then, with the 10K trimmers (fine adjustments) set to their mid-positions, choose these resistors so that the wiper of the scanning control (and hence pin 3 of the LM311) roughly covers this same range of voltages.


VCO Offset and Scaling

The VCO can now be "calibrated" so that its frequency excursions fall within a comfortable range of audio frequencies. Ideally, the center of the screen should produce a tone of about 500Hz; then, for our units, the arbitrary scale of an octave per 2 centimeters was chosen. For a screen whose height is 8cm, the VCO will span 4 octaves--from 125Hz to 2000Hz. (The exponential converter is not perfect. Depending on the characteristics of your diode, "tracking" may be better in one direction than the other.)

With a horizontal line in the center of the screen, set the "offset" potentiometer (off pin 2 of the CA3240 in the Exponential Converter) for a tone of about 500Hz (about 4th-octave B). Then, move the line up and down to get a rough idea of where the scaling is at this point; if the oscillator moves wildly, decrease the setting of the pot on the input of the Sample-and-Hold system. When you change the scaling, a shift in the offset will result; these two adjustments profoundly interact. Once again, put the line in the center of the screen and set the offset for about 500Hz--then check the scaling. After several iterations, you should be able to set up a condition where the oscillator drops about an octave as the line is brought down by 2cm, and goes up an octave when the line is brought to 2cm above the center.

Vertical Amplitude Calibration

The second half of the CA3240, the one associated with the sample-and-hold system, can be adjusted to accommodate a wide variety of external meters. The adjustments off of pin 6, the inverting input of this op-amp, can be used to create any desired offset or scale factor. Used with the internal null-balance circuit here, however, the offset for center screen is meant to be 0 volts, and amplitudes of not more than plus/minus 4.5V can be accommodated. (If other levels are required--and if the operation of the internal null-balance circuit is to be retained--some redesign will be necessary; the voltage on the Braille pot will have to be increased, the audio level may have to be attenuated, etc.)

If the internal meter is the only system to be considered, a 3V peak-to-peak range with no offset is recommended. Use the horizontal line on the display to make these adjustments, with a voltmeter monitoring pin 7 of this 3240. With the trace at center screen, adjust the offset pot (the one off pin 5, the non-inverting input) for zero volts on pin 7. Then, move the line up and down, noting the voltmeter readings for top and bottom scale. The pot off pin 1 of the 3240 is the gain adjustment. Juggle these adjustments (which interact) so that the voltmeter reads plus 1.5V when the line is at the top of the screen, and minus 1.5V for the bottom of the screen (going to zero for center screen).

The null-balance meter can now be calibrated. One way of doing this is to disconnect the 47K resistor from the arm of this Brailled pot, and adjust the rheostats at the end terminals so that the voltage on its wiper spans the exact range of pin 7 of the 3240. Another way is to juggle these rheostats with the null meter working, arranging for the pointer knob to be at center scale for center screen, and nulling at the Braille scale's end points for top and bottom of the screen.

Horizontal Axis Calibration

A periodic waveform with distinctive features is best suited for this purpose--a squarewave, a sawtooth, or even a triangle. Start out with a signal whose frequency is perhaps 10kHz, and with a sweep rate of 50 microseconds per centimeter. Synchronizing the sweep externally is also desirable, since this will afford maximum flexibility in placing the waveform on the screen. Arrange the repetition rate of the wave, or the horizontal scaling on the scope, to put the distinctive features in coincidence with marks on the graticule.

The trimmers in series with the scanning control can now be adjusted so that the corresponding markings on the tactile scale line up with those on the graticule. Once again, these two trimmer adjustments interact; start out by lining up a mark near the left end, then line up an appropriate mark near the right end. Repeat this procedure until you are satisfied with the results.

Autoscan Circuit Adjustments

There are eight trimmers associated with this circuit. Two of these affect the actual "calibration" of the automatic ramp; the rest are associated with the tone-burst feature and will be dealt with shortly. Note that there is a 741 op-amp off pin 3 of the ICL8038 function generator. Pin 6 of this op-amp contains the voltage which must be calibrated.

Off pin 2 of this op-amp are the two trimmers. They are juggled until the range of the output ramp matches that of the manual scanning control. Measure the voltages at the end points of the manual control; then, with the auto-scan "scan-rate" knob set to the slowest scan, arrange for the ramp (or triangle, if you are in the bi-directional position) to cover that same range.

In order for the tone bursts to trigger reliably, the squarewave output of the system must be set up for a 5V logic level. This can best be done with the ramp generator set for bi-directional scanning. There is another 741 op-amp off pin 9 of the 8038; its output, pin 6, contains this squarewave signal. Pin 2 of this op-amp is associated with two 10K pots. One of these, the "offset pot," has its top end going to plus 15V. This can be adjusted for an output of 0 volts when the 8038 is unplugged, or for 0 volts during the time that the 8038's collector (pin 9) is open. The pot connected to the 8038's pin 9 is then adjusted for 5V pulses.

Two single-turn pots (comprising volume controls) off the outputs of the 556 (pins 5 and 9) can then be turned up to suit the user. Besides these, duration adjustments allow setting of the "length" of the beeps to your liking; one is off pin 7 of an associated 74123, and the other is off pin 15.

Adjustment of the "Cursor" Circuit

As mentioned in the circuit, a 741 op-amp is used to amplify either the Q or the NOT-Q output (pin 5 or pin 12) of a 74123 one-shot. (If the trace brightens with a negative-going signal, the Q output, pin 5, is used.)

Pin 2, the inverting input of this 741, has two pots associated with it. The one going to the supply lines--feeding pin 2 through 100K--is the offset adjustment. Adjust this trimmer so that 0 volts rests at pin 6 between pulses. The pot whose wiper goes to the one-shot is the gain of this amplifier; this is adjusted to get a nice-looking "dot" on the trace (possible only at slow sweep rates).

Parts and Parts and Parts List

Resistors (1/4-watt 5%):

  • 1--10 ohms
  • 1--270 ohms
  • 1--470 ohms
  • 1--510 ohms
  • 3--1K
  • 1--1.3K
  • 5--2.2K
  • 1--3.9K
  • 4--4.7K
  • 1--7.5K
  • 4--10K
  • 2--12K
  • 1--15K
  • 1--20K
  • 3--27K
  • 2--30K
  • 4--39K
  • 3--47K
  • 3--56K
  • 8--100K
  • 1--470K
  • 4--510K
  • 2--560K
  • 1--1.5meg

Selected Resistors (estimated values, see text):

  • 4--10K?
  • 2--20K?
  • 1--36K?

Precision Resistors (1% low-wattage):

  • 1--100K
  • 3--49.9K

Trim Pots (multi-turn, unless otherwise specified):

  • 2--10K Single-turn
  • 14--10K
  • 1--20K
  • 2--50K

Panel-Mount Potentiometers:

  • 2--10K Precision Linear (fitted with Braille scales and pointer knobs)
  • 1--250K Nonprecision Linear
  • 1--10K Volume Control

Electrolytic Capacitors (50V, unless otherwise specified):

[The editor has made this terrible decision: Not all of the bypasses are across the full 30 volts; therefore, some can be purchased for lower voltage ratings. For the advanced builder with a junk box having assorted "working voltages," the 50V constraint can be relaxed as seen fit. Otherwise, they should be ordered in bulk so as to be interchangeable.]

  • 1--100uF 10V
  • 1--50uF (can be lumped with the 47uF ones coming up) 3--47uF 10--10uF 1--2.2uF 25V tantalum 1--1uF 10V

Non-Polarized High-Value Capacitors (25V):

  • 1--25uF 2--8uF
  • 1--5uF 1--10uF
  • 2--1uF

Low-Value Capacitors (of various types):

[Note: All of these are non-polarized. Where that feature is specified in the circuit, the editor has specified Mylar here, since those are of good quality]:

  • 2--100pF mica (such as "silvered mica")
  • 1--820pF Mica (silvered mica will do)
  • 1--0.001uF disc ceramic
  • 5--0.01uF disc ceramic or Mylar
  • 2--0.01uF Mylar (whenever "non-polar" was specified)
  • 1--0.068uF disc ceramic or Mylar
  • 2--0.1uF disc ceramic or Mylar
  • 2--0.1uF Mylar (whenever "non-polar" was specified)


  • 1--0.33 millihenries

Transistors and Diodes:

  • 1--5.1V low-wattage zener (4.7V or 5.6V will do)
  • 1--1N4148 general-purpose small-signal silicon diode
  • 7--2N2222A

Chips Ahoy!:

  • 1--LM320-15T or uA7915 negative 15V regulator 1--LM340-5T or uA7805 5-volt regulator
  • 1--LM340-15T or uA7815 15V regulator
  • 1--555 timer
  • 1--556 dual timer
  • 3--741 individual op-amps
  • 2--CA3240 dual op-amps
  • 5--LM310 followers
  • 1--LM311 comparator
  • 3--74123 dual one-shots
  • 2--CD4066 quad analog switches
  • 1--ICL8038 function generator
  • 1--LM386 audio amplifier

Toggle Switches:

  • 2--SPST1--SPDT2--DPDT
  • One external cabinet,
  • And a very loudspeaker.

Printed boards, documentation on their layout, and complete instruments--built into an appropriate visual scope--are all available from:

Oehm Electronics: 944 Fletcher Lane, No. 17, Hayward, CA 94544; Phone (415) 537-5635.


By Al Alden and Bill Gerrey


Based on the idea published here in the Winter 1987 issue, the resultant soldering iron is a quick-heating, fast-cooling iron which can be put into position when cold, then energized by operating a foot pedal. Its very small size allows the user to hold it only 1-1/2 inch away from the work, and it is of feather-weight. Like many quick-heating irons, however, this low-mass soldering tip cannot, by itself, dissipate the power it consumes, and the uncontrolled version was prone to overheating. This controller promotes safer operation, and it is expected that the life of the tips will increase.

This soldering iron uses the so-called "tuner extension tip" available for cordless soldering irons of the Wahl Clipper Corp. This tip is intended to fit into a handle containing a 2.5V rechargeable battery and a pushbutton switch. Besides the limitations of battery operation, this cordless instrument has two disadvantages: The battery makes the handle heavy, and its shape does not permit the user to "choke up" near the business end to be close to the work. Second, the operation of a pushbutton puts muscles in tension which destabilize hand position.

Nevertheless, a lot of blind technicians liked the Wahl cordless iron. Because its tip heats up rather quickly, you can place it against the work pieces when it is cool. Bernie Vinther fits the tip with little solder "preforms" (little one- or two-turn coils which he makes from rosin-core wire solder). With the tip loaded with this bit of solder and put in place before it is turned on, the problems of coordinating a lot of activities with only two hands are eliminated.

The extension tip discovered by Bernie Vinther has about a four-inch insulated portion that remains cool. By attaching a two-wire cable to the conductors in this extension, the iron can then be powered by a source from the mains (a 2-1/2 volt transformer was used in the original article). The foot switch can be wired in such a way as to avoid carrying the tip current; it was in the primary circuit of the transformer in the original article.

The "tuner extension tip" (Wahl part No. 7556) uses the same element as the standard-length "fine tip" (part No. 7545); this

is nice to know, since the former is nearly twice the price of the 7545. The extension is made by crimping 3-1/2-inch long copper tubes onto the pins of the 7545, taping these tubes to a fiber separator and encasing them in heat-shrinkable tubing. Save the copper tubes and the separators from burned-out 7556's, since you can re-use bits of them to extend the cheaper standard-length tips.

The editor has gotten a couple of suggestions about how to attach wires to these tips. However, nothing seems to be free of intermittent contact; the tip draws about 8 amps when first energized--4 amps when hot--so you can imagine how "positive" these connections need to be. (Various push-on connectors, including parts of tube sockets, have always intermittently failed me, darn them.)

Pins of these tips are steel, and they don't take solder very well. However, the copper tubes are easy to solder to, and that is what the editor advocates currently. Peeling away half an inch of the heat-shrinkable tubing at the back end of the 7556 reveals that pins--just like those on the heating element--have been inserted into the tubes to "adapt" them to the cordless handle. I pull these out with pliers; then I solder the flexible cable to the tube ends and wrap the assembly with tape.

A serious disadvantage of the original design was that, if you lost track of melting of the solder, the iron would frequently be left on too long and overheating would occur. Our temperature controller completely solves the problem of overheating; damage to the work is now infrequent, and the tips (which are rather expensive at $7 each) should last longer.

Theory of Operation

The controller works by putting the tip's heating element into a Wheatstone bridge and measuring its resistance. This resistance increases as a function of temperature; it starts out being about 0.3 ohms when cold, and gets up to perhaps 0.5 ohms when soldering temperature has been reached. An integrator senses the difference between the voltage on the tip and that of a "temperature-control potentiometer," and the output of the integrator is then used to control the tip current.

A new component is used to vary the tip current; introduced by Motorola, it is called a "sense FET." In principle, a power FET is comprised of 3600 FET's in parallel. (All the channels are in parallel, and all the gates are connected together.) However, if we then disconnect two of the "sources" and bring them out to a separate pin, we can use an external "sensing resistor" to monitor one eighteen-hundredth of the current. Thus, instead of providing the whole mess with a source resistor, which would put a lower limit on the total "on resistance" of the control circuit, a resistor of comfortable value (100 ohms) can be used to sample a small branch of the total current.

The sense FET chosen is the Motorola MPT10N10M. With a gate voltage of 10V (positive with respect to the source) and with a drain current of 10 amps, its "on-resistance" is a maximum of a quarter of an ohm--typically 0.175 ohms.

Its TO220 package has five pins. Three of these (pins 1, 3 and 5, respectively) are "Gate," "Drain" and "Source" terminals. As mentioned, however, only 3,598 of the sources go to pin 5; two of the 3600 elements have their sources both going to pin 2, the so-called "Mirror" terminal. Pin 4, the so-called "Kelvin" terminal, is a way of looking at the source voltage, but not on pin 5 which passes all that heavy current. Pin 4 is a separate light-duty wire that goes to the 3,598 sources.

In operation, a "sampling" resistor (we used 100 ohms) goes between the "Mirror" and "Kelvin" terminals. When the FET is operated, a voltage (E equals I times R) reflects the fact that 1/1800th of the drain current is flowing through that resistor.

Conceptual Model of the Wheatstone Bridge Circuit

(Don't build the circuit in this paragraph, or it will leave your iron cold.) The negative supply point of the bridge is grounded. One side of the bridge consists of the following: A 10K resistor goes from the plus supply point to the top of a 10K temperature-control pot. The bottom of this pot goes through 91K to ground. The arm of the pot is the take-off point for this branch. As for the other branch: The positive supply point goes through 100 ohms, then through the iron to ground. The take-off point for this branch is the junction of the iron and the 100 ohms.

In order to make that circuit work, all you need is a way of multiplying the current that flows through the 100 ohms by 1800 times and apply it to the iron. This is accomplished by supplying the iron with current from another branch. The iron gets most of its current from the "Source" terminal of the sense FET. The "Mirror" terminal, which is at the positive supply point of the bridge, goes through that 100 ohms to the "source" (or more correctly, to the "Kelvin" terminal). The result is that the companion resistor in the iron's half of the bridge has a simulated value of 100/1800 ohms (0.056 ohms). (Don't worry that this is less than the "on-resistance" of the FET; remember, it is dissipating lots of power and getting supplied from a higher source.)

"Control" is affected by pulling up on the supply to the bridge. When the pedal is first pressed, the integrator gently suggests, then emphatically demands, that more and more power be supplied; it does so because this is the only way of getting the heating element's resistance to climb. When the iron's resistance causes its voltage to exceed that on the wiper of the control pot, power to the bridge is reduced until cooling changes the condition of balance, at which point the bridge gets more power.

Without a little help, it is possible that the integrator would not have anything to compare initially, since its output powers the bridge. Thus, a "boot-strapping" capacitor from the non-inverting input (and the arm of the control) goes to the supply line that gets operated by the foot pedal; this causes an immediate unbalance that brings the sequence of events into play.

Finally, there are two 1meg resistors which are protective in nature. Imagine, if you will, what could happen if you forgot to install the chip and the gate drifted aimlessly into the region of conduction. The FET might turn fully on and burn out the iron. Thus, the gate goes through 1 megohm to ground to prevent this. Likewise, if the wiper of the pot got dirty and opened temporarily, the non-inverting input to the integrator could drift high, causing the integrator to turn the FET fully on. Another 1meg resistor to ground prevents this latter disaster.

Construction Details

While it might be done differently--in a fancy enclosed box--our controller was built onto a 6 by 8 by 2 inch chassis. As the sense FET is called upon to dissipate over 16 watts, a heat sink is required. This chassis has a couple of transformers on it and is naked aluminum; it serves as part of the heat sink, while a small additional heat sink was affixed to the outside for good measure.

Two DC voltages are required by the circuit. First, the iron is supplied by a heavy 5-volt filament transformer working into a bridge rectifier (and filtered by 40,000 microfarads). A light 12- to 30-volt DC supply is needed for the op-amp. (Since the op-amp only draws 0.8mA, a 12- or 18-volt battery could be used instead. Furthermore, the op-amp is only powered when the foot switch is depressed, so a battery would last its shelf life.) We chose to use a separate small transformer for the op-amp's supply. It's not that having 5-volt and 12-volt secondaries on the same unit is a rare combination--it used to be quite common. But if you come across a filament transformer whose 5-volt winding is for a 10-amp rectifier, it will have been made with the assumption that the 12-volt filaments add up to a lot of power as well, and such a unit will be heavier than it need be.

The two transformers were mounted on top of the chassis, along with the 40,000uF filter cap. The rear panel has a large bridge-rectifier module mounted on it. There are two fuses, a line fuse and a 10-amp fuse for the 6-volt supply; these can be internally placed, or put on the back panel.

The front of the chassis is rather interesting. On the left front corner is the female spring clip for a cabinet door latch, the part which usually is mounted up under the middle cabinet shelf. If you reshape it a bit, it makes a fine clamp for the "extension" part of the iron. Our clip needed no reshaping; it was one of the double-roller types, and it squeezes the handle of the iron just fine. There is a wrong kind with rollers; some of them have metal "bridges" that hold the rollers' axes, and these would not permit the iron's handle to slip down in. The rollers, or simple spring flippers, should be placed so as to face upward, beyond the top of the chassis. When the iron is in place, it will lie on top of the chassis parallel to the left edge.

Also on the front panel is the temperature-control pot and the on-off switch. A grommet in the lower middle permits the cord for the iron to emerge; this cord should not come out the back, as it would then be in the path of the iron more often. On the lower right, opposite the iron stand, is the jack for the foot pedal.

Since the foot pedal supplies power to the op-amp, it cannot be common to ground. Therefore, its jack must be insulated from the chassis. This is more trouble than you would immediately think; the standard jack used for such foot pedals is the tiny 1/16th inch size, which is generally too short to accommodate insulating washers. This jack can be mounted on a piece of plastic behind a large hole in the chassis, but it was decided that this was more trouble than adding a control transistor which would permit grounding the jack. The jack ended up at the lower-right corner of the front panel--out of the way.

Other than the 6V high-current supply, all circuitry was put on a piece of perforated board measuring 1.8 by 4.5 inches. It was mounted up underneath the top of the chassis--with one of its long edges butting against a side panel. By placing it thus, the sense FET, which is in a TO220 package, can be stood up on the board (near the long edge) and bolted to the side of the chassis when the board is in place. The case of the sense FET is hot--plus 6V--so standard insulating hardware for the TO220 package must be used to insulate it from the chassis, which is ground.

The same bolt that holds the sense FET to the chassis also secures a heat sink to the outside of the side panel; the insulating shoulder washer goes on the outside of this heat sink, putting the heat sink at chassis ground and leaving the head of the mounting bolt hot. Our heat sink ended up on the left side--the same side as the makeshift iron stand. This was probably a mistake, since that heat sink can get pretty warm.

Rather than putting binding posts on the outside for connection to the iron, we fashioned binding posts on the circuit board. We did so using 1/2-inch 6-32 bolts with two sets of nuts, the top set being used to secure the ends of the iron's cable.

Circuit for the Controlled Vinther Iron

The two transformers have their primary windings connected in parallel. One side of this combination goes to the cold side ("neutral") of the AC line, while the other primary leads go through a 1/2-amp slow-blow fuse, then through the on-off switch to the hot side of the line. If a 3-wire plug is used, the "ground" prong goes to the chassis and to circuit ground. (The neutral side of the plug is the right-hand prong as they point at you with the ground at the bottom.)

The 12-volt transformer's secondary feeds a small bridge rectifier unit. The negative output of the bridge is grounded. The positive output (marked with a slightly longer lead, usually) is bypassed to ground by a 47uF 25V electrolytic (negative end at ground). The output will be perhaps 16 to 18 volts.

The positive output of the 18V supply goes to the emitter of a 2N2907 PNP transistor. The base of this transistor goes through 10K to its emitter; this base also goes through 10K to the tip of the foot-pedal jack. The sleeve of this jack is grounded.

The collector of this transistor goes to pin 8 of an LM358 dual op-amp. Pin 4 of this 358 is grounded. One op-amp is not used: pins 6 and 7 are jumpered together, while pin 5 is grounded.

Between pins 1 and 2 of the 358 is a feedback capacitor of 0.01uF (Mylar). Pin 2 (the inverting input) goes through 10K to the iron side of the Wheatstone bridge (the non-grounded side of the iron). Pin 3 goes to the arm of the temperature-control pot. For protection, pin 3 goes through 1 megohm to ground. Pin 3 also goes through the boot-strapping capacitor of 0.1uF (disc or Mylar) to the collector of the 2N2907. Pin 1 of the 358, the output of the integrator, goes to the gate (pin 1) of the sense FET. For protection, this gate also goes through 1 megohm to ground.

The 5-volt transformer's secondary feeds a high-current bridge rectifier. The negative output of this bridge is grounded. Its positive output (usually marked by a cutout on its adjacent corner) is bypassed to ground by a 40,000uF 10V electrolytic (negative terminal at ground; and for your safety, be assured of correct polarity). Because of voltage drops in the bridge, the output of this supply will be perhaps 6.5V, dropping to 4V under full load.

The output of the 6V supply goes through a 10-amp fuse to the drain (pin 3) of the sense FET. The FET's source (pin 5) goes through the iron to ground.

The supply point of the Wheatstone bridge is the "Mirror" terminal (pin 2) of the sense FET. This pin 2 goes through 10K to the top of a 10K pot (temperature control); the bottom of this pot goes through 91K to ground. Between pins 2 and 4 of the sense FET (between the "mirror" and "Kelvin" terminals) is a 100-ohm sampling resistor.

Temperature Setting

The temperature-control setting that everybody seems to like is with the pot about two-thirds of the way up (60 to 70 percent). It should be noted that when the control is in the first quarter of its rotation, the control circuit starts to warm things up--then collapses. Pressing the control again brings on a fresh start, but there is definitely a lower limit to the successful operation of the circuit at low settings.


A lot of heads were together on this one. It should be said that a co-worker, Manfred Mackeben, was after Smith-Kettlewell to design such an iron for darn-near a decade. It took Bernie Vinther's vigilance to find the extension tip that made this all possible. Al Alden's creative use of the sense FET to simulate an otherwise impractically low resistance value is sheer genius.

Parts List

Resistors (1/4-watt 5%):

  • 1--100 ohm
  • 3--10K
  • 1--91K
  • 2--1 megohm
  • 1--10K panel-mount pot with non-precision linear taper


  • 1--0.01uF Mylar
  • 1--0.1uF disc ceramic or Mylar
  • 1--47uF 25V electrolytic
  • 1--40,000uF 10V electrolytic (Digi-Key P6422, 47,000uF 16V)


  • 1--AC mains to 5V 8- or 10-amp (such as the Signal A41-43-10 which has two 5-amp secondaries that can be parallelled, or the Signal 241-8-10 with a 10-amp center-tapped 10V secondary)
  • 1--AC mains to low-current 12V (such as the Signal 241-3-12, or the Radio Shack 273-1385)


  • 1--Low-current 50-PIV bridge rectifier (such as the Radio Shack 276-1161)
  • 1--8-or 10-amp low-PIV bridge (such as the Jameco MDA990-3 12-amp unit, or the Radio Shack 276-1181 8-amp one)
  • 1--2N2907 PNP transistor
  • 1--LM358 op-amp chip
  • 1--Motorola MTP10N-10M Sense FET


  • 1--1/2-amp slow-blow fuse
  • 1--10-amp slow-blow fuse
  • 2--Appropriate fuse holders
  • 1--SPST on-off switch
  • 1--1/16th inch open-circuit footpedal jack
  • 1--General-purpose tape-recorder foot switch
  • 1--7556 Wahl Clipper Co. "Tuner Extension Tip" (available from Fordham Radio in Farmingdale, NY.)
  • 1--Small heat sink with insulated mounting kit for TO220 packages (Note that the chassis box should be bare metal, so that it can be the main heat sink.)
  • 1--Suitable bare-metal chassis or box

Sense FET Pin Arrangement

The MTP10N-10M comes in a TO220 package from which five leads emerge. With the mounting surface toward you and the leads pointing up, the pins are numbered 1 through 5 from left to right. When you get them, the pins are bent in a rather bizarre way: 1, 3, and 5 are positioned rather far forward (away from the mounting surface), while 2 and 4 have only slight "dog legs" in them and appear further back.

  • Pin 1--Gate
  • 2--"Mirror" (The Sources of two "cells")
  • 3--Drain (common to the case)
  • 4--"Kelvin" (connected internally to the main Source junction)
  • 5--Source

Address List

Digi-Key Corp., P.O. Box 677, Thief River Falls, MN 56701; (800) 344-4539.

Fordham Radio, 855 Conklin Ave., Farmingdale, NY 11735; (800) 645-9518.

Signal Transformer Co., 500 Bayview Ave., Inwood, NY 11696: (516) 239-5777.