Smith-Kettlewell TECHNICAL FILE

Published by Rehabilitation Engineering Center Smith-Kettlewell Institute of Visual Sciences

Bill Gerrey, Editor

Supported, in part, by Smith-Kettlewell Eye Research Foundation and National Institute of Handicapped Research

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TABLE OF CONTENTS

The Intersil ICL 8038 Function-Generator Chip

A Highly Portable Function-Generator using the ICL 8038

The Smith-Kettlewell Universal Tone Indexer

The Smith-Kettlewell Speaker Phase Tester

The Silkiest MCW Signal on the Air

Save Your Batteries with a Timer Switch

Summary of SKTF Survey

Editor's Crystal Ball

THE INTERSIL ICL 8038 FUNCTION-GENERATOR CHIP

Abstract

The INTERSIL ICL 8038 is capable of producing sinusoidal, square, triangular, saw tooth and pulse waveforms over an extremely wide range of frequencies. In addition, the frequency can be voltage-controlled so as to be made easily tunable over a range of 10-to-1 or more. Though its outputs are not low impedance, a full-fledged function generator can be built with the inclusion of a buffer amplifier. Other applications include use as a highly linear VCO and as special test oscillators; we have used this IC in our tape indexer and various Morse Code devices. Sample circuits of a general sort will be given here, while specific hookups appear in articles to follow.

Brief (?) Description

The ICL 8038 is a 14-pin package, two pins of which are not used (13 and 14). It has three output pins, one for sine, one for triangle, and one for squarewave (the square- wave needs an external pull-up resistor). It has a high-impedance voltage-control pin, in addition to which it provides a VREF on still another pin so that you can anchor the VCO pin to VREF if external electrical control of the frequency is not desired.

The oscillator works by charging and discharging an external timing capacitor. The charge and discharge times are each controlled separately by resistors; these can be made equal or vastly different to generate a wide variety of wave shapes. A near approximation of symmetry between the two half-cycles can be had with these resistors combined into one, resulting in a circuit of extreme simplicity.

Fine "shaping" of the sinewave can be accomplished by attaching trim pots to two pins provided for this purpose. Compromise- trimming with a single fixed resistor is also commonly done.

Besides taking care of the VCO input (for example, jumpering it to its VREF pin), a usable test oscillator can be made with a single resistor and capacitor. Beyond this, the versatility of this chip affords connection in many configurations; this, not its complexity, is what makes this article so frighteningly long. Please read on in good spirits.

Theory of Operation

[Ah, something simple for a change.]

The heart of the 8038 is a triangle-wave oscillator based on the charge and discharge of a capacitor. The workings of this oscillator are best illustrated by looking at an equivalent circuit:

Equivalent Circuit of the Triangle Generator

Two "active current sources" act on a timing capacitor, the bottom end of which goes to the -V line. The top end of the capacitor goes to the output of a current source "I" coming off the plus supply. The top of the capacitor also goes through a single-pole single-throw switch to a current sink of -2I, which comes off the -V line. With the switch open, the capacitor is charged by current "I," while closing the switch will discharge the capacitor with the net current of -I (the sum of the source and the sink).

One input each of two comparators senses the voltage at the top of the capacitor; one comparator is set to trigger at 2/3V, while the other is set to trigger at 1/3V. The outputs of the two comparators operate a flip-flop, one output of which is used to open and close the SPST switch (turning the current sink on and off).

The capacitor is charged by the current source until a voltage of 2/3V is reached, at which point the upper comparator directs the flip-flop to close the switch and establish the discharge current. The capacitor is then bled off by a net current of -I until the lower comparator directs the flip-flop to disengage the current sink, whereupon the charge cycle resumes.

Squarewave Generation

The free output of the above flip-flop drives the base of an NPN transistor in the common-emitter connection. The collector of this transistor is open (a pull-up resistor must be connected externally if a squarewave output is desired).

Sinewave Generation

The sinusoidal output is gotten by performing a "wave-shaping operation" on the triangle. Progressive loading is imposed on the triangle wave as it approaches its peaks--"rounding it off" to get a simulated sinewave. (Under the best conditions where trimming of the circuit has been done externally, a total harmonic distortion of better than 1/2% can be expected.)

A string of eight switching transistors connects load resistors of various values across a high-impedance version of the triangle--four progressive loads being imposed on each half of the cycle. The points at which these various loads are applied are determined by the voltages found along a string of nine resistors in a voltage divider contained in the chip. The top and bottom junctions on this divider string are made available on pins 1 and 12, respectively, so that further "trimming" can be done to minimize distortion.

Wave Symmetry and Frequency Adjustments

The actual value of the charge and discharge currents can be set individually with external resistors. Therefore, a fast charge and slow discharge, or vice-versa, can be purposely set up; in this way, the triangle can be made into a sawtooth and the squarewave can be made into pulses. The variations afforded can be extreme--the duty cycle can be varied from less than 1% to greater than 99%.

Frequency determination depends on three elements listed below:

  1. Different capacitors can be connected; a wide-range function generator might have several such units to be selected by a "range switch."
  2. Since the charge and discharge currents are "programmable" using the aforementioned resistors, changing these will directly affect the frequency.
  3. The charge and discharge currents are also voltage-controlled via a VCO input pin. Furthermore, once the duty cycle is chosen, the charge and discharge times "track" very well over a range of greater than 10-to-1 as the voltage on this pin is varied. Finally, a voltage divider within the chip provides a reference (VREF on pin 7) to which the VCO control pin can be tied when external voltage control of the oscillator is not intended.

Specifications

[Editor's note--Much confusion is apparent in looking through the literature at specs which refer to the quality of waveforms, distortion and the like. This is probably due to the fact that several grades of this chip are available, and that no consistent mention as to which chip is being demonstrated in the spec is apparent. The most commonly available unit (which one is likely to find at Radio Shack and JAMECO, for example) is the unit of commercial grade having the suffix CC; i.e., ICL 8038CC.]

The ICL 8038 can be used over a range of frequencies from 0.01Hz to 1/2MHz. (The distortion of the sine output begins to increase above 10kHz; some circuit configurations only guarantee a distortion figure of 10% as 1/2MHz is approached.) The frequency will be relatively independent of the supply voltage if the VCO input is tied to its VREF pin (both the reference voltage and the triggering points will track so as to hold the frequency constant). If the VCO input voltage is controlled independently from an external circuit, the supply voltage will directly affect the frequency, thus making a regulated supply necessary.

Although advertisements boast that the VCO will operate over a range of 1000-to-1, this range is hard to achieve in practical terms; the control pin must be pulled slightly above VCC to cover this range, and mistracking of the current source and sink lead to asymmetry in the waveforms. It seems to the Editor that a different chip, such as the VCO section in the CD4046 PLL IC (See SKTF, Summer 1982) would be a better choice where a wide range of frequencies as a function of voltage is required. By my experiments, I have found that a more realistic statement of VCO frequency range is about 16-to-1 or less.

Supply Voltage

The range is stated to be +5 to +18 volts. Actually, the chip runs off a single supply of from 10 to 36 volts (drawing a minimum of 12mA, typically 20mA); however, since the output waveforms are "centered" within this region, a split supply can be used, the common connection of which can then be considered a "signal ground." In other words, the waves should be symmetrical in amplitude about 1/2VCC. On the other hand, if capacitor coupling is used on the outputs, the split supply can often be avoided.

The range of the VCO will to some extent depend on the supply voltage used. Mistracking of the current source and sink occurs as the VCO pin approaches VCC; it is caused by the junction potentials in the active devices becoming significant. At higher supply voltages, the region in which these problems can be expected becomes less significant as compared to the acceptable range of input voltages on the VCO. It is probable for this reason that they list the lowest supply voltage as 10V. [Out of ignorance, I have run these devices off 9 volts in countless projects, and I have never had a chip misbehave. In these projects, however, I have usually affixed the VCO input to the VREF pin, thereby avoiding the troublesome region.]

Output Amplitudes

From the section "Theory of Operation," it can be seen that the output amplitudes vary in direct proportion to the supply voltage. The peak-to-peak values are listed below:

Frequency Formulas and/or the timing of each half-cycle:

Recommended Loading

The literature says very little about "output impedance." In most examples, however, the sine and triangle outputs are loaded with 100K. By experimenting I have found that load resistances of less than 100K degrade performance; the output of the triangle begins to fall off and clearly audible distortion of the sinewave is noticeable. Perhaps a load resistance of 500K is optimal. Specifications for the squarewave's pull-up resistor are not discussed directly in the literature, but the rise- and fall-time specs are given for a collector current of 4mA. This would dictate using a 4.7K pull-up resistor for a supply voltage of 20V. VCO Input and the VREF Pin--The VCO input is very high impedance; though never specified, the internal circuit of the chip shows this pin as being the input of an emitter follower (more or less, the internal circuit being very complicated). My experience is that this input impedance is very high, well over a megOhm.

The lowest output frequency is gotten with this pin being taken to, or slightly above, VCC (clamped with a diode to limit this voltage at 0.6V above VCC). The frequency increases and varies directly as the control voltage drops (measured with respect to VCC and not with respect to ground). In other words, the frequency goes up as this control pin is brought down from VCC. The lowest voltage which should be allowed to appear on this pin (resulting in the highest frequency for any given hookup) is 2/3 of VCC plus about 2 volts. For example, with the supply of 15V, the VCO pin will function over a range of VCC down to 2/3 of 15, +2--12V above the minus supply (a range of 3V below VCC).

The VREF pin (to which the VCO can be directly tied) consists of a voltage divider; VCC goes through 10K, then through 40K to -V, with the junction of these resistors being VREF, available on pin 7. Even with the control pin tied to pin 7, a modulation signal for FM'ing the chip can be applied to the control pin through a coupling capacitor; the resultant "input impedance" of this arrangement is the parallel combination of the 10K and 40K resistors, about 8K. This impedance can be increased by running the control pin through an external resistor to VREF; the resultant impedance at the control pin will be 8K plus R, where R is the value of the external resistor.

Using the ICL 8038 Illustrated by Exemplary Circuits

Simple 1kHz Test Oscillator

The supply pins are pins 11 and 6; pin 11 goes to -V, while pin 6 goes to +V (VCC). Pin 10 goes through a 0.015uF timing capacitor to pin 11 and to -V. Pins 4 and 5 (the current source and sink programming pins) are tied together and go through 10K to pin 6 and to VCC. (This being a single resistor common to both halves of the cycle, the formula for determining frequency is: f = 0.15/RC.) Pin 8, the VCO input, goes to pin 7 (VREF).

Pin 2 is a pleasant sounding sinewave (its distortion has not been minimized), while pin 3 is a triangle wave. In order to get a squarewave from pin 9, its output, a pull-up resistor from pin 9 to VCC is necessary; let us run pin 9 through 10K to pin 6 and to VCC. Now that we have connected up the squarewave, however, switching spikes from this section will no doubt be present on the power supply and affect the performance of the rest of the chip. It is recommended, therefore, that the VCO pin be bypassed to VCC through 0.1uF.

Pins 2 and 3 (the sine and triangle outputs) can each go through 100K, then through 0.1uF to their respective hot output terminals. The squarewave is less susceptible to loading (in our example it will have a 10K source resistance in the positive direction) and can directly go through a coupling capacitor to its output jack. Since the outputs are capacitively coupled, the cold side of the output can be taken as -V. If a split supply were used (perhaps made up of two 9V batteries in series), the common connection could be taken as ground; the coupling capacitors could be eliminated, since the waveforms are nearly symmetrical with respect to one-half of the supply voltage.

A noticeable improvement in harmonic distortion of the sinewave output can be gotten by running pin 12 through 82K to -V; this constitutes a very rough adjustment of the sinewave shaping network. In later circuits where care is taken to equalize the charge and discharge time (thus tuning for temporal symmetry), more complicated external networks for fine trimming of the sinewave circuit will be given. The next step up from the above would be to change the 82K resistor to a 100K rheostat.

Variable Frequency Test Oscillator

We can make the above circuit adjustable in frequency so as to cover a range of perhaps 100Hz to 1kHz. There are two ways to achieve this:

  1. We can make the current source resistor off pins 4 and 5 variable. (The literature specifies practical limits on resistors associated with pins 4 and 5; they should not be less than 500 Ohms or greater than 1 megOhm.) In our example, pins 4 and 5, which are tied together, will go through 10K, then through a 100K rheostat to VCC. The disadvantage of this system is that the frequency does not vary linearly as the rheostat is adjusted; this adjustment becomes very critical at the high-frequency end.
  2. Another way of doing it is to use the VCO control pin. Instead of going to VREF, pin 8 (which is still bypassed to VCC by 0.1uF) goes to the arm of a pot. The bottom of this pot goes to VCC, while its top end goes to VREF, pin 7. To get the maximum range from this system, the resistance of the pot should be high enough so as not to pull VREF up to VCC; I usually use a linear pot of about 250K. A slight loading effect (and hence lowering of the frequency) can be offset by decreasing the value of the resistor on pins 4 and 5, perhaps to 9.1K in this example. This system will give you a good linear frequency range of greater than 10- to-1. Failure to go to 0 frequency, as well as asymmetry in the output waveforms, are unavoidable effects which dictate that the very bottom end of this adjustment be ignored.

Minimizing Distortion

There are two items on this agenda. First, we must equalize the charge and discharge currents in the timing capacitor so that the upper and lower halves of the cycle are of equal duration. Second, we can adjust the "sine converter" for optimum wave shaping. Both procedures require embellishments on the basic circuit. The improved circuits and adjustment procedures are listed below:

Temporal Symmetry

With pins 4 and 5 jumpered together, the current source and sink share the same sampling resistor; there is no way of "balancing" them or adjusting them individually. We could connect each pin through a separate rheostat; pin 4 (dedicated to the "charge" half of the cycle) would go through a rheostat to VCC, while pin 5 (dedicated to the "discharge" half of the cycle) would go through another identical rheostat to VCC. A system more commonly used, however, is to run pins 4 and 5 each through a resistor, with the far ends of these resistors going to the ends of a single pot which is shared between them. The arm of this pot goes to VCC. Let us rebuild our 1kHz test oscillator to incorporate this feature:

Model 2, 1kHz Test Oscillator with Temporal Symmetry Adjustment

As before, pin 6 goes to VCC, while pin 11 goes to -V. Pins 7 and 8 are tied together and go through a 0.1uF bypass capacitor to VCC. Pin 10 goes through a 0.015uF timing capacitor to -V. Pin 4 goes through 18K to the top of a 2K pot; pin 5 goes through 18K to the bottom of this 2K pot. The arm of the pot goes to VCC.

[The resistance values off pins 4 and 5 and the timing capacitor were this time chosen by using the formula:
f = 0.3/RC
(where R is either Ra or Rb).

In this case, Ra is the resistor from pin 4 through its portion of the pot to its wiper. Rb is the resistor from pin 5 plus its portion of the pot to its wiper. Using the above formula, the frequency would come out to be 1kHz if Ra and Rb were 20K. Since standard values of resistors and capacitors preclude this possibility, the above values are given; considering the nature of the VCO pin and its associated VREF, it is easier to lower the frequency than to raise it.]

We will have need for the squarewave output; it goes through 10K to VCC.

The usual way to assess the symmetry is to look at the squarewave on an oscilloscope; the pot is then adjusted so that the top and bottom portions of the squarewave are of equal length. However, until we all have "scopes," I recommend the following alternatives:

  1. Pad the timing capacitor with a large value unit, say from 0.5uF on up through 5uF. Connect the squarewave output to a test amplifier so as to permit adjustment by ear. The 5uF unit will produce a series of clicks which you can adjust for equal timing, similar to leveling a mechanical metronome. I prefer a fast buzz, such as the 60hZ gotten with a 0.5uF unit. This latter system will emit a pure buzz when the adjustment is correct; any deviation from this point will give the illusion of producing two frequencies (sort of like listening to the echoes of the old LORAN system on 160 meters).
  2. The charge and discharge currents to which the timing capacitor is subjected can be measured with a sensitive milliammeter, and the adjustment can be made so as to make these equal in magnitude. With a pot connected across the supply (I used a 5K unit between pins 6 and 11), connect the arm of the pot through a milliammeter to pin 10 (the top of the timing capacitor). This can be done with the timing capacitor left in place. Turn the pot down so that you are sure its wiper has gone below 1/3 VCC and then advance it to about mid-position; take note of the current. Next, advance the pot until you are sure it has gone above 2/3 VCC, and then back it down to about mid-position; take note of this current. (Of course, the polarity of the meter will have to be alternated, since these currents will be in opposite directions.) Make the necessary adjustment of the pot on pins 4 and 5 so as to equalize these currents. Caution--do not believe a reading while the pot is outside the limits of 1/3 to 2/3VCC; the circuits in the chip do funny things when forced to their extremes.

Once temporal symmetry has been accomplished, the square and triangle waves will be as pure as the driven snow.

Shaping the Sinewave

Eight junctions on a voltage divider determine the "breakpoints" or points of progressive loading which make a sinewave out of a triangle. The top and bottom junctions (pin 1 and pin 12, respectively) are made available so that this divider string may be externally influenced to achieve a more precise replica of a sine- wave. Left untouched (without any trimming), you can expect the harmonic distortion to be about 5% or so. Two schemes, one simple and one elaborate, are commonly used to vastly improve the above distortion figure.

The simple setup merely consists of running pin 12 through 82K to -V. One might expect that this would offset the "sine converter," and that another resistor should be used at pin 1; this is not the case, and the distortion can be reduced to perhaps 0.8% with this simple approach. Replacing this resistor with a 100K rheostat is often done to get an additional slight improvement.

The fanciest scheme is as follows:

Pin 12 goes to the arm of a 100K pot; the bottom of this pot goes to -V, while its top end goes through 10K to VCC. Similarly, pin 1 goes to the arm of another 100K pot; the bottom of this pot goes to VCC, while its top end goes through 10K to -V. These two units are then jockeyed in combination to get minimum distortion, better than 0.5%.

The usual way of assessing the distortion and trimming it for minimum is to examine the wave on an oscilloscope, especially while comparing this with a true sinewave superimposed on it. An alternative would be to run the sinewave output through a notch filter which is tuned to the fundamental frequency; the remaining signal will be comprised of the harmonics which are to be minimized. The residual signal can be tuned for a minimum output, either audibly or with the help of an AC VTVM.

A notch filter can be made by combining a second-order low-pass and a second-order high-pass filter (both with the same cutoff frequency) in a summing amplifier (an inverting op-amp configuration with two equal input resistors). [The low- and high-pass filters are described in "Op-Amps III"; the summing amplifier is discussed in "Op-Amps I" (Summer 1983 and Winter 1983, respectively).] On the other hand, a notch filter using the Reticon 5620 (Summer 1983) is easy to build and is tunable to boot.

A 20-to-20,000 Hertz Audio Oscillator

This comes from an Intersil pamphlet, "Everything You've Always Wanted to Know about the 8038." The intention was to show how the device could be used over its advertised frequency range of 1000-to-1. I would never build this circuit; it calls for such items as a logarithmic rheostat (which would be expensive), and it is loaded with trimming adjustments. However, it is an outstanding example of how to hang every imaginable thing onto this chip; any one of these tricks may be individually useful to you.

Circuit

Plus and minus 15V supplies are used; their common connection is grounded. Power to the chip is taken across the whole 30V. Pin ll goes to -l5V, while pin 6 goes through a diode to +l5V (cathode toward pin 6). (This diode will be explained later.) Pin l0 goes through a 0.0047uF timing capacitor to -l5V. Pins 4 and 5 each go through 4.7K, with the far ends of these resistors going to the ends of a lK symmetry adjustment pot. The arm of this pot goes to pin 6, the plus supply pin. Pin 5 also goes through a l0meg rheostat to -l5V (to be explained later). For frequency adjustment, pins 7 and 8 are tied together and go through a l00K log pot, connected as a rheostat, to +l5V. Pins 7 and 8 are also bypassed to +l5V by 0.luF (made necessary only when the squarewave pin is connected).

The squarewave output, pin 9, goes through a l5K pull-up resistor to +l5V.

For trimming distortion in the sinewave, this circuit shows pin l2 going through a l00K rheostat to -l5V. For completeness here, however, we will restate the fanciest arrangement: Pin l2 goes to the arm of a l00K pot; the bottom of this pot goes to -l5V, while the top of the pot goes through l0K to +l5V. Pin l goes to the arm of a l00K pot; the bottom of this pot goes to +l5V, while the top of the pot goes through l0K to -l5V.

Notes on the Above Circuit

The diode in the plus supply line enables you to take pins 7 and 8, used here for frequency control, to about 0.6V above the supply pin, pin 6. This allows you to get very low frequencies (without changing the timing capacitor, which is another way of getting them).

The l0meg rheostat on pin 5 is necessary because, as pins 7 and 8 approach +l5V, the duty cycle shifts due to transistor mismatches in the current source elements. These mismatches can be compensated for by bleeding a small amount of current away from pin 5, hence the l0meg rheostat.

Intersil recommends making the duty cycle adjustment (using the lK pot off pins 4 and 5) with the oscillator set to its highest frequency--with the frequency control wide open. Then, with the frequency control set to generate the lowest frequency of interest, decrease the l0meg rheostat to again balance the temporal symmetry.

Finally, they also caution that, unless the supply has excellent dynamic regulation, transients from the squarewave will find their way onto the triangle, and hence onto the sinewave. It may be necessary to bypass both supply pins to ground near the chip. They don't recommend a particular value; try 0.l's for starters.

A HIGHLY PORTABLE FUNCTION-GENERATOR USING THE ICL 8038

I have never had the time to build this instrument; I designed it using the information contained in the cogent, well-written article above. It is an instrument which I would love to have; perhaps an ambitious reader might build one for me for presentation on one of my birthdays (March l2, April l, October 3l, etc.)--Thank you very much.

As designed, this instrument produces sine, square, and triangle waveforms whose peak- to-peak amplitude is adjustable over two ranges--0 to 10V, and 0 to 3V. Its five tunable frequency ranges are: 0 to 10Hz, 0 to 100Hz, 0 to 1kHz, 0 to 10kHz, and 0 to 20kHz. If a semi-precious linear pot is used for the frequency control, it is well worth attaching a 10- or 20-division braille dial so that the frequency can be read directly. (The bottom 10% of the scale will be extremely inaccurate and should be ignored.)

Circuit

A split supply is used, ±9V to ±15V. The negative side of the plus supply is grounded, while its positive terminal goes through one pole on a DPST on-off switch to the +V line. The positive side of the negative supply is grounded, while its negative terminal goes through the other pole on the switch to the -V line. The +V line is bypassed to ground by l00uF (negative at ground). The -V line is also bypassed to ground by l00uF (positive at ground).

Pin ll of the 8038 goes to -V; pin ll is also bypassed to ground by 0.luF (located near the chip). Pin 6 goes to +V; pin 6 is also bypassed to ground by 0.luF (located near the chip). Pin 8, the VCO control pin, is bypassed to the +V line by 0.luF (located near the chip).

Pin 7, the VREF pin, goes through a l0K calibration rheostat, then through a l00K linear pot to the +V line. The arm of this l00K unit (the frequency control) goes to pin 8.

Pin 4 goes through 24K to one end of a l0K symmetry-adjustment pot; pin 5 goes through 24K to the other end of this pot. The arm of this pot goes to the +V line.

Pin l0, intended for the timing capacitor, goes to the arm of a 5-position rotary switch (range switch). Position l goes through luF to -V (25V tantalum with its positive toward position l). Position 2 goes through 0.luF (Mylar) to -V. Position 3 goes through 0.0luF (Mylar or mica) to -V. Position 4 goes through 0.00luF (mica) to -V. Position 5 goes through 500pF (mica) to -V.

The fanciest distortion trimming is connected. Pin l2 goes to the arm of a l00K pot; the bottom of this pot goes to -V, while the top goes through l0K to +V. Pin l goes to the arm of another l00K pot; the bottom of this pot goes to +V, while the top goes through l0K to -V.

A 3-pole, 3-position switch is used to select the functions; one pole disables the squarewave output when not in use, one pole feeds the desired oscillator output into the buffer amplifier, and the third changes the gain of the buffer to make their amplitudes equal.

Pin 9, the squarewave output, goes through l0K to position 3 of pole No. l; the arm of this pole goes to +V.

On pole No. 2, position l goes to the sine output, pin 2 of the 8038. Position 2 goes to the triangle output, pin 3. Position 3 goes to the squarewave output, pin 9. The arm of this switch goes through a 500K output level control to ground. (This control can be log, linear, audio taper, as you wish.)

On pole No. 3, position l goes through Rl to ground, position 2 goes through R2 to ground, and position 3 goes through R3 to ground. These resistors will be tabulated later on the basis of supply voltage.

A fast op-amp is used as a buffer; I have always like the Signetics NE535 which does not need external compensation. Pin 4 of the op-amp goes to -V; pin 7 goes to +V. Between pins 6 and 2, from output to inverting input, is a selectable feedback resistor to provide two gains. Pin 6 goes through 43K, then through 20K to pin 2. Across the 43K resistor is an SPST gain switch.

Pin 2 of the op-amp, the inverting input, goes to the arm of pole No. 3 of the waveform selector switch. Pin 3 of the op-amp, the non-inverting input, goes to the arm of the output level control.

The cold output terminal of the instrument is grounded. The hot output terminal comes directly from pin 6 of the op-amp.

Table of Input Resistors R1, R2, and R3

For a split supply of ±9V:

For a split supply of ±l2V:

For a split supply of ±l5V:

Happiness is a Warm Screwdriver

With the frequency control turned up, the temporal symmetry adjustment (the l0K pot off pins 4 and 5) should be adjusted first while looking at the squarewave output. (The range switch can be in any position as appropriate given the technique used; see previous article. My favorite technique is the trick with the current meter, although the symmetry can be adjusted audibly with the range switch in one of the two lower positions.)

Next, adjust the l0K rheostat off pin 7 so that, with the frequency control turned all the way up, frequencies close to the maxima specified are gotten. A compromise here or there may be necessary due to distributed capacitance, etc. Because of loading on the VREF pin by the frequency control circuit, it may be impossible to reach the desired maxima, even with the calibration rheostat closed. If you encounter this problem, put a slight load on the VREF pin in the opposite direction; i.e., put a resistor of 470K or 680K from pin 7 to -V.

Trimming of the sinewave is perhaps best done visually on an oscilloscope. However, feeding the sinewave through a notch filter tuned to its fundamental frequency will present the distortion products audibly so that they can be tuned for a minimum. If worse comes to worst, listen to the sinewave with your naked ear and tune the trim pots for the purest tone (maximum "oooooh" and minimum "eeeeeh").

Parts List

Resistors (1/4 watt, 5%):

Potentiometers (screwdriver-adjust trim pots):

Capacitors:

Integrated circuits:

Switches:

THE SMITH-KETTLEWELL UNIVERSAL TONE INDEXER

Abstract

A low-frequency tone generator using the ICL 8038 chip is described. As an outboard unit, this device can be plugged into any tape recorder for the purpose of adding low-frequency tones to mark chapter headings, pages, etc.

Description

Two versions of our tone indexer will be described; one has a built-in microphone, while the other has provision for connecting a remote microphone which can be used to control the tones or to start and stop the tape through the remote jack of the tape recorder. The more complicated unit, intended for use with a dynamic microphone having a remote switch, contains the following bells and whistles:

  1. Two cables emerge from the box; one plugs into the mike input of the tape recorder, and the other plugs into the tape recorder's remote control jack.
  2. A jack to receive the microphone plug is mounted at one end of the tone indexer.
  3. Located one centimeter away from the mike jack is a "remote control jack" through which the tape recorder can be turned on and off in the usual way.
  4. Located one centimeter away and on the other side of the microphone jack is a "remote tone jack" which permits the tones to be controlled by the switch on the microphone.
  5. A "line-input" or "auxiliary" jack is included to permit recording off of high- level sources (such as another tape recorder). High-level material fed into this jack is attenuated so as to be of an appropriate level to feed into the recorder's mike input.

One final feature of my design is that no current is drawn by the tone generator between button pushes. The unit containing a microphone draws very little current (500uA); an on-off switch is nevertheless provided on this unit. No on-off switch need be included for the other version, since the pushbutton serves this function.

It should be mentioned that tone indexers are commercially available. One can be gotten from Science for the Blind, P.O. Box 385, Wayne, PA l9087. The other is very recent; it is a companion to the "Talkman" tape machine, available from BIT, 24l Crescent Street, Waltham, MA 02545.

[Chris Mackey (Mrs. H.V. Mackey, 202 West Grangeville Blvd., Hanford, CA 93230), in various talks on tone indexing, describes a simple system which can be used with tape machines on which separate input level controls allow mixing of a line input with the mike. Into the line input, she inserts a patch cord. A perfectly good 60 Hertz hum is then gotten by touching the tip of the plug at the free end of the patch cord, taking care not to ground yourself via the shield of the plug or by holding onto the mike, machine, etc.

Over-cultivated technologists should note that this is "engineering" in the purest sense of the word; this approach is "ingenious." This tells the true story of the trade; the two words, engineering and ingenious, stem from the common Latin root, "ingenium."]

Circuit Operation

The tone generator consists of an ICL 8038 function generator chip connected in its simplest form. Its sinewave output is fed directly to the mike input via a resistor, rather than being combined in an op-amp mixer. The version intended for use with a dynamic mike actually uses the microphone element as part of its attenuator (to attenuate the tone and present only a fraction of it to the input jack). Condenser microphones, such as that which is contained in the other version, provide no low-impedance path for this primitive system to work. Therefore, the values of resistors chosen are different for the two versions. I could have played around with these values in order to strike some sort of compromise, but I decided that if I made this primitive system do too much, it would do nothing very well.

The frequency of the oscillator is variable over a range of from 25 to 150 Hertz. A PC- mount trim pot (connected as a rheostat) is used for this purpose; a hole was drilled in the box so that this adjustment could be accessed from the outside.

Circuit for the Remote-Controlled Version

The negative side of the battery goes through a normally open pushbutton switch to ground. Across the switch is connected a 1/16 inch remote tone jack; the sleeve of this jack is grounded, while its tip goes to the negative side of the battery. Pin 11 of the ICL 8038 is grounded. Pin 6 of the 8038 goes through 100 ohms to the positive side of the battery; pin 6 is bypassed to ground by 10uF (negative at ground). (The latter components, comprising what looks like a decoupling network, are included to reduce transients when the oscillator is turned on and off.)

Pin 10 of the 8038 goes through 0.1uF to ground. Pins 4 and 5 are tied together and go through 10K, then through a 50K rheostat (pitch control) to pin 6. Pins 7 and 8 are tied together.

The sinewave output, pin 2, goes through 0.luF, then through 330K to the tip of the microphone jack--this jack is a closed- circuit 1/8 inch mini phone jack. The switch contact of this jack goes through 330 ohms to the sleeve, with the sleeve of this jack being grounded. (This 330 ohm resistor on the switch contact is in the circuit when the mike is unplugged, thus providing the oscillator and the line input jack with the necessary voltage divider.)

The tip of the mike jack also goes through 33K to the tip of the high-level input jack. The sleeve of this latter jack is grounded. (This can be an open circuit jack, since the switch contact serves no purpose; it could even be an RCA jack if you wish.)

The cold output terminal (the sleeve of a suitable plug for your tape recorder input) is grounded. The hot output terminal (the tip of the plug) goes directly to the tip of the mike jack. (This plug is usually a 1/8 inch mini phone plug.)

So as to permit starting and stopping of the tape recorder, a remote control jack is provided and connected as follows. The sleeve and switch contact of closed-circuit 1/16 inch jack are tied together and go to +-the sleeve of the remote control plug (this plug is usually 1/16 inch sub-mini phone plug). The tip of the jack goes to the tip of the plug. Sometimes the remote jack in the tape recorder is off ground; its sleeve is sometimes not tied to the sleeve of the mike jack. For this reason, it is unwise (also unnecessary) to ground the sleeve in the Indexer circuit.

Circuit for Self-Contained Version

A Radio Shack 270-092A condenser mike element is used. The shield of its output cable is grounded; the center conductor (mike output) goes through 0.22uF, then through 2.2K to ground. Its power connection (a red wire separate from the shielded cable) goes through an SPST on-off switch to the positive battery terminal.

The negative side of the battery is grounded. The positive battery terminal goes to one side of a normally open pushbutton switch. The other side of the pushbutton goes through 100 ohms, then through 10uF to ground (negative at ground). The junction of this resistor and capacitor goes to pin 6 of the 8038. Pin ll of the 8038 is grounded.

Pins 7 and 8 of the 8038 are tied together. Pin l0 goes through a 0.luF timing capacitor to ground. Pins 4 and 5 are tied together and go through l0K, then through a 50K rheostat (frequency adjust) to pin 6.

Pin 2 goes through 0.luF, then through 680K to the junction of the 2.2K resistor and the coupling capacitor in the mike output circuit. A single mike plug on shielded cable is used for connection to the tape machine. The sleeve of this plug is grounded, while its tip goes to the top of the 2.2K resistor, to the mike's coupling capacitor, and to the 680K dropping resistor.

Mounting Considerations

The version which contains its own microphone has a problem of picking up vibrations as the unit is handled and operated. Though I have not done so in our prototype, it might be wise to rubber mount the microphone. Rubber grommets of sufficient size (an inner diameter of 5/16 inch) are large and thick-fitting them onto such a small box would be quite a problem. Instead, I recommend gluing the microphone into a rubber "washer" (perhaps cut from rubber gasket material) and gluing the assembly into a hole which is larger than the microphone (treating the rubber as a "mounting flange").

The double plugs on remote control microphones often have a spacing of 10mm (1 centimeter) between centers of their prongs. It is handy to note that 10mm is very close to 4/10 inch.

In making holes for these jacks, I use vectorboard with tenth-inch hole spacing as a template. For example, in this instrument we want 3 holes in a row with a space of 4/10 inch between them. I determined the position at which I wanted the microphone jack (the middle one); I drilled a hole through the vectorboard into the box in this position. From this point, I drilled again in hole No. 4 (to the right), and again in hole "minus 4" (to the left). Very little filing was necessary to make these jacks comfortably fit their intended plug.

You can dodge this whole issue by using one of Radio Shack's "Universal Replacement Microphones" on which the two plugs extend from separate short lengths of cable.

Parts List

Remote-Controlled Indexer:

Indexer with Microphone:

THE SMITH-KETTLEWELL SPEAKER PHASE TESTER

Abstract

This battery-operated unit plugs into "auxiliary" inputs of any hi-fi amplifier. It simulates two popular phase- testing signals which are often found on test records. It permits testing of speaker phasing without disconnecting one of the speakers.

I have always had trouble putting my speakers in phase. First of all, quick comparisons are not possible because my speakers are connected with screw terminals. They have to be tested as connected one way; then after a significant period of time during which one is being switched around, a listening test is performed with the phase reversed. With this device, however, a direct "A/B comparison" can be made which will tell you if your speakers are in or out of phase.

The simplest way of making a testing jig is to build a polarity reversing switch into one of the speaker lines. Making such a jig detachable, however, means that you must account for all the different kinds of speaker plugs, binding posts, etc. It seemed to me that a couple of common tests such as those found on test records could be generated using an ICL 8038 function- generator chip. Both the polarity-reversing switch and the eventual speaker phase tester prototype are described here.

Simple Polarity-Reversing Switch

Let us get this out of the way before describing the more complicated device. A double-pole double-throw switch (without center off) is used. Position l of pole A goes to position 2 of pole B. Position 1 of pole B goes to position 2 of pole A. The output terminals of the amplifier are connected: one to position l of pole A and the other to position l of pole B. The arms of the switch drive the speaker.

With the switch in position l, the speaker lead on the arm of pole A is connected to position l of pole A; i.e., the polarity is not reversed. With the switch flicked to position 2, the polarity is reversed.

With a monaural signal feeding both channels of the amplifier, try the two positions of the switch; the speakers are in phase when the monaural material can be clearly localized as being in the center-between the two speakers.

The Fancy Speaker Phase Tester

An 8038 is connected so as to generate two waveforms: One is a 75Hz sinewave. The other is a square pulse with a duty cycle of about 5%. Two phono plugs on a cable emerge from the instrument. A pushbutton switch on the front of the tester determines the phase relationship of the signals available at the plugs. Normally, these signals are in phase; when the switch is depressed, the signals are l80 degrees out of phase.

When the 75Hz tone is used, a difference in loudness can usually be discerned. With the speakers in phase, their motion is such as to be additive; the listener hears the combined power of both signals. With the speakers out of phase, their motion is such as to cancel; a decrease in loudness of the tone should be evident. (This system falls short of being ideal if standing waves in the room are such that the listener finds himself in a node of the soundwave. For this reason, make your cables long enough so that you can move closer to and away from the speakers.)

The pulses are intended to simulate a metronome which, if the speakers are working in phase, can be localized as being in the center between the two speakers. If the speakers are out of phase, the clicking sound cannot be localized; it may sound as if it comes from the sides or from nowhere in particular. Stepping from side to side or operating the balance control will give further evidence of this difference.

Circuit

Pin ll of an ICL 8038 is grounded, as is the negative side of the 9V battery. The positive terminal of the 9V battery goes through an on-off switch to pin 6 of the 8038 and to the VCC line. Pins 7 and 8 of the 8038 are tied together and bypassed to the VCC line by 0.luF.

Pin l0 of the 8038 goes through 0.luF to ground. Pin 4 goes through 36K to one end of a l0K symmetry adjustment pot. Pin 5 goes through 820K, then through 36K to the other end of this pot. The arm of this pot goes to the VCC line. (A switch to be described will short out this 820K resistor.)

Pin 9, the squarewave output, goes through 47K to VCC. For crude shaping of the sine- wave, pin l2 goes through 82K to ground.

Two inverters in cascade provide buffered signals of both phase polarities. An LM358 or other dual op-amp is used. Pin 4 is grounded, while pin 8 goes to VCC. The non- inverting inputs, pins 3 and 5, are tied together and go to a reference voltage. The reference is gotten off a voltage divider; two resistors of 22K are connected in series between VCC and ground. The junction of these resistors is bypassed to ground by luF (negative at ground).

Pin 7, the output of the second inverter, goes through a l00K feedback resistor to pin 6, the inverting input. Pin 6 also goes through l00K to pin l, the output of the previous stage. Between pins l and 2 is connected a feedback resistor of l00K.

Pin 2, the input of the first stage, goes through 100K to the arm of a double-pole double-throw selector switch. Position 2 of this pole goes through 220K to the squarewave output, pin 9 of the 8038. Position l goes to the sinewave output, pin 2 of the 8038. (These connections are made with shielded cable so as to keep the pulses from pin 9 out of the sinewave signal.)

Position l of the second pole goes to pin 5 of the 8038, while the arm of this pole goes to the junction of the 820K and 36K resistors.

Pin l of the op-amp, the output of the first stage, goes through 0.luF to the center conductor of one of the output cables; the shield of this cable is grounded. The center conductor of the other output cable goes to the arm of a single-pole double-throw push- button switch; its shield is left floating so as to avoid ground loops. The normally closed contact goes to the junction of the output capacitor and the center conductor of the first output cable. The normally open contact goes through 0.luF to pin 7 of the op-amp, the output of the second stage.

The sinewave test does not work well if the harmonic content is high. For this reason, the symmetry adjustment was provided to at least enable you to balance both half-cycles. This can be done by turning the treble control of the hi-fi amplifier up and its bass control down--then adjusting the l0K pot for minimum high-frequency buzz. When using the sinewave to test the speaker phase, on the other hand, turning the treble control down and the bass control up will aid in presenting an optimal signal.

Parts List

Resistors (l/4 watt, 5%)

Capacitors

Semiconductors

Switches & Plugs

THE SILKIEST MCW SIGNAL ON THE AIR

Introduction

Here in the Bay Area, there are 2-meter repeater and symplex frequencies on which MCW operation is tolerated and even encouraged. Most of the folks place their microphone next to the speaker in their keying monitor; a few others use a rasty- sounding oscillator made with a 555 timer, or the like, piping this directly into their mike input. Room echo and/or the excessive generation of pairs of sidebands are not my style, and I set about giving WA6NPC a tone with some class.

There are other uses for this device: Tom Fowle, a computer technician at Smith- Kettlewell, uses it as a Morse Code input/ output device for his computer. It makes a high-grade non-fatiguing code practice oscillator for anyone teaching the Code or making Code practice tapes. Our Talking Book readers will note that this device is what I use to put the Morse Code identification tags into the Table of Contents. We have even used this circuit to make a tactile keying monitor for the deaf.

Description

The heart of this unit is an ICL 8038 connected in the conventional way. The only variation is the addition of the keying transistor which shorts out the timing capacitor when the key is up. As a built-in monitor, the device contains an LM386 audio amplifier.

The unit is built in a 2 by 3-l/2 by 6 inch metal utility box, metal being essential if it is to be used around RF. The front panel contains a volume control, a pitch control, an on/off switch which also doubles as the transmit/receive switch, a key jack, and an earphone jack.

The rear panel contains a 4-pin jack by which a patch cord connects the unit to your transceiver; a high-level output jack (which can be patched into a tape recorder) is also located on the rear. (The volume control does not affect the level at these output jacks.) Finally, an internal trim pot sets the level as desired into your transmitter's microphone input circuit.

Embellishments would include another simple open-circuit jack to serve as a low-level output suitable to feed the mike input of tape recorders which do not have "auxiliary." Use of a shorting-type key jack will allow the unit to be used as a general purpose test oscillator, say for checking wow and flutter in tape recorders. A sine/triangle switch would permit you to choose the tone which best suits your mood.

As shown, the available pitch range is from a few hundred cycles to nearly l500 Hz (about a 4-to-l range). Later on, appropriate modifications will be given to provide the tactile user with a range from l50 to about 300 Hz.

Circuit

A double-pole single-throw on-off switch is used. One pole operates the push- to-talk circuit of your transceiver. The other pole of the switch goes between the positive 9V battery lead and the VCC line. The negative side of the battery is grounded. The VCC line is bypassed to ground by l00uF (negative at ground).

Pin ll of the 8038 is grounded, while pin 6 goes to the VCC line. Pins 4 and 5 are tied together and go through l0K to VCC.

Pin l0 goes through a 0.0luF timing capacitor to ground. Pin l0 also goes to the collector of a 2N2222, with the emitter of this transistor being grounded. The 2222 base goes through 33K to VCC. This base also goes to the tip contact of the key jack. The sleeve of this jack (and possibly its switch contact; see "Embellishments") is grounded. The transistor base is also bypassed to ground by the parallel combination of 0.0luF and 0.22uF (the latter being a key-click filter).

Pin 7, VREF, goes through a l00K linear pot (pitch control), then through 33K to VCC. The arm of this pot goes to pin 8, the VCO control pin.

To take some of the harmonics out of the sinewave, pin l2 goes through 82K to ground.

Pin 2, the sine output, or pin 3, the triangle output--as you prefer--goes through 0.luF, then through 220K to the top of the low-level output adjustment (a 2K trim pot). The bottom of this pot is grounded, while its arm goes to the hot mike input of the transceiver. This arm is also bypassed to ground by 0.0luF.

The junction of the above 0.luF and 220K goes to the hot output of the high-level recording jack. The low-level recording jack can either go to the arm of the 2K output- level adjustment, or it can go to the top of this pot so as not to be affected by the level setting. The sleeves of these output jacks are grounded.

The selected output, either pin 2 or pin 3 of the 8038, also goes through 0.luF, then through 220K to the top of the volume control (l0K audio taper). The bottom of this control is grounded.

An LM386 is used for the internal power amplifier; pins 2 and 4 of this chip are grounded, while pin 3, the input, goes to the arm of the volume control. Across pins 2 and 3 is connected a 0.0luF disk capacitor (to keep stray animals out of the amplifier). If you want the full l25mW out of the 386, connect pin 8 through lK, then through l0uF to pin l (positive of the capacitor toward pin l).

Pin 6 of the 386 goes through l0 ohms to VCC; pin 6 is also bypassed to ground by 250uF (negative at ground). Pin 7 is bypassed to ground by 25uF (negative at ground).

Between pins 4 and 5 is 0.22uF. Pin 5 also goes through l00uF (positive toward pin 5) to the tip of the earphone jack. The sleeve of this jack goes to one side of the speaker, as well as going to ground. The other side of the speaker goes to the switch contact of this jack.

I have only used my prototype of this instrument with very low power transceivers, l.5 to 2 watts. Even so, any attempt to use an indoor antenna severely affects the performance of the device. If you have trouble with RF interference, it may be necessary to insert RF chokes in the hot leads of all jacks; then bypass each jack with 0.0luF (located on the jack terminals). Further, the ground lead from the circuit board to the metal chassis should contain an RF choke to completely isolate the circuit from RF present on the box. (Unless low-resistance high-frequency RF chokes are used, the chokes will introduce a prohibitively high resistance in the speaker/earphone system. If chokes are included, eliminate the earphone jack entirely, or wind the output leads in a toroid on a ferrite coil form.)

Our tactile version is identical with two modifications. First, the timing capacitor is increased to 0.047uF; this lowers the frequency to a maximum of about 300 Hertz. Next, the range of the pitch control is restricted; pin 7 goes through the l00K pot, then through a l00K resistor to VCC. The power delivered by the LM386 should be enough to drive a fairly efficient transducer.

Parts List

Resistors (l/4 watt, 5%)

Potentiometers

Capacitors (disk ceramic, 30V)

Capacitors (electrolytic, l0V)

Semiconductors

Jacks & Switches

Miscellaneous

SAVE YOUR BATTERIES WITH A TIMER SWITCH

Abstract

Circuits for giving your battery projects a timer switch are given here. As their switching element, they use recently available "VMOS" power FET's; a survey of these is given at the end of this article.

How many times in the hot pursuit of a DX station on the Clipperton Islands have you reached for your "Auditory Gimmick" only to find that it has been left on for weeks and you cannot now tune your transmitter? No? Well then, how many times have you, in your attempt to capture a rare recording of Nellie Melba singing, "I Didn't Raise My Boy to be in Software," have you reached over to find that your audible VU meter has been left on all night? In any case, the replacement batteries are a needless expense and the inconvenience is untenable.

Nowadays, items such as calculators and the "Speak-and-Spell" toy turn themselves off automatically so as to save their batteries. The internal program of these devices contains a digital counter (probably running off the main clock) which ticks away the minutes until a BIT in the string orders a main switch to open. My first thought was to take this approach using a count-down timer chip such as the XR2242 (discussed in our "Universal Nicad Battery Charger," SKTF, Winter 1981). Besides the switching element, this would necessitate adding a whole new chip to every project--too much work.

On the other end of the spectrum is an approach of extreme simplicity. A power FET can be used as the switch, with its gate being controlled by a simple RC circuit. The gate circuit is arranged so that, as the capacitor charges, the gate gradually loses its forward bias; this will result in the gradual opening of the FET channel. (This scheme was written up in "Popular Electronics," June 1982; the article, "Ultra- Simple VMOS Timer," was co-authored by Forrest Mims and Istvan Mohos.) Its disadvantage is that the switch does not abruptly open; the project just fades away.

I sought the middle ground, a simple circuit with solid on-off characteristics-- one which could be expanded to include automatic resetting if the project stays in use. In my basic circuit, I use a VMOS power FET as a switch, this in conjunction with a "latching transistor." They hold each other on until bias begins to fall away, then they act to turn each other off. A third transistor can be added to dump the charge in the timing capacitor, resetting the timer from activity in the project.

Circuit Operation

In all cases, the battery goes to the source of the FET, while the drain goes to the project. (The negative or positive battery lead can be switched by choosing an appropriate gender for the FET-- the negative side can be switched with an N- channel unit, while a P-channel is required for switching the positive lead.) Being "enhancement-mode" FET's, they will be open (biased at cutoff) when their gate potential is near that of the source; bringing the gate toward the other battery lead will cause the "switch" to close (biasing the FET into saturation).

From the opposite supply line (the one not containing the FET switch) is a transistor which, when turned on, forward-biases the gate of the FET so as to hold it on. Not only does this cause power to be applied to the project, but the FET drain also forward- biases the base of the transistor through a resistor and capacitor in series. In other words, turning on the transistor causes the FET to turn on; the FET in turn holds the transistor on. Eventually, however, a charge is developed on the capacitor in the transistor's base lead, and the base bias begins to fall away. When this happens, the transistor begins to relinquish its hold on the FET gate, and the FET comes out of saturation. But the FET is what has been holding the transistor on. Immediately as the FET opens, the base of the transistor is driven in the direction of reverse bias; its latching action collapses and the gate of the FET goes back to the source, thus cutting off the switch.

The "hang time" (the length of time the circuit stays latched) is about three time- constants of the RC circuit on the base of the latching transistor. With the 47uF capacitor and the 680K resistor shown, your project will stay energized for about 95 seconds. In the refreshable version, where a third transistor dumps the charge on the 47uF capacitor through a 22 ohm resistor, a five millisecond pulse constitutes five time- constants of this latter network, thus resetting the timer.

Circuits

Mirror-image circuits for both P- and N-channel FET's will be given. My favorite uses a P-channel unit in the plus supply line; its refresher circuit requires downward pulses which are usually available from projects containing NE555 oscillators. On the other hand, N-channel units seem to be more readily available, units of this gender being stocked at Radio Shack.

P-Channel Timer Switch for the Positive Supply Line

The source of the FET (P-channel such as the Siliconix VP0300M) goes to the positive side of the battery. The negative side of the battery goes to the -V line of the project. The drain of the FET goes to the VCC line of the project. Between gate and source of the FET is 10K.

The gate of the FET also goes to the collector of an NPN transistor (2N2222), with the emitter of this transistor going to the -V line. The base of this transistor goes through 680K, then through 47uF to VCC and to the FET drain (positive of the capacitor at the drain). Between the base and emitter of the transistor is a diode (1N914, 1N4148, etc.), with its cathode on the base.

The junction of the capacitor and the resistor goes through a normally open push- button "on" switch to the plus battery lead and to the source of the FET. A normally open pushbutton "off" switch goes between the base and the emitter of the 2N2222.

Refresher Circuit for the Above

A medium-power PNP transistor is used (2N4036, something which can handle a peak collector current of nearly half an amp). Its emitter goes to the VCC line and to the positive end of the capacitor. Its collector goes through 22 ohms to the junction of the capacitor and the resistor. Its base goes through 2.2K to a source of downward pulses which, when the project is dormant, stays high. (Such downward pulses, for example, can be found at the output of the 555 or 556 in the Smith- Kettlewell Gimmick and/or the Fowle Gimmique.)

In the case of our VU meter, where the output of the audio oscillator only goes up to 5V and does not rest at VCC, the refresher transistor base can be capacitively coupled to the source of pulses. This base circuit can be modified as follows:

The base of the PNP refresher transistor goes through 10K to its emitter. This base also goes through 1K, then through 0.47uF (positive toward the resistor) to the output of one VU channel.

N-Channel Timer for the Negative Supply Line

The advantage of this version is that the builder has so many VMOS/FET's to choose from. The positive battery terminal goes to the VCC line of the project. The negative battery lead goes to the source of an N- channel FET (VN0300M, VN10KM, etc.). The drain of this FET goes to the project "ground" (its -V line). The FET gate goes through 10K to its source.

This FET gate also goes to the collector of a PNP transistor (2N2907), with the emitter of this transistor going to VCC and to the positive side of the battery. The base of this "latching transistor" goes through 680K, then through 47uF to project ground and to the drain of the FET (negative of the capacitor at ground). The junction of this resistor and capacitor goes through a normally open pushbutton "on" switch to the source of the FET and to the negative battery lead. Between the emitter and the base of the above transistor is a diode, anode on the base. Also between this base and emitter is a normally open pushbutton "off" switch.

Refresher Circuit for the Above

A medium- power NPN transistor is used (2N2219, good for about 1/2 amp peak). Its emitter is grounded, while its collector goes through 22 ohms to the junction of the capacitor and the resistor. The base of this refresher transistor goes through 2.2K to a source of positive-going pulses in the project; when dormant, these pulses should rest at logic zero.

VMOS Power FET's

These are metal-oxide-semiconductor field- effect transistors (MOS/FET's); the "V" in their designation refers to geometry of the channel and its interface with the gate. (For a more complete discussion of their entrails, refer to Siliconix Applications Note AN76-3.) As opposed to junction FET's (JFET's) which conduct until reverse bias on their gate "pinches off" conductivity, these MOS units operate in the "enhancement mode." FET's of the enhancement-mode type are normally cut off until a forward bias (bringing the gate away from the source and toward the drain) causes the channel to conduct. Being devices of the "insulated- gate" variety, the input resistance is extremely high--it can be considered infinite.

To a continuity tester, the gate--with respect to either end of the channel--will look open for either polarity. For whatever reason, the channel looks like a diode to the tester, the FET being used properly with this "diode" back-biased. (The literature never says whether this is an actual diode included for protection of the device, or whether it is the nature of the semiconductor device. However, forward-current specs are given for the diode; they are generally quite high.) The cases and/or tabs of all these FET's are common to their drain, giving you a good way of finding the drain with a tester.

P- and N-channel units bearing the same number (such as the VP0300 and the VN0300) seem not to be "complementary" devices; such specs as their gate-source threshold voltage and their "on-resistance" differ markedly. I have, therefore, shied away from using sister devices to control a dual supply; they would not turn off at the same time, etc.

Siliconix VP0300 P-Channel

(Note: The M suffix denotes a TO237 package; the B suffix denotes a TO39 package.)

General Parameters:

Drain-Source Diode:

Siliconix VN0300 N-Channel

(Units with the M suffix are in a TO237 case; a D suffix denotes a TO220 package.)

General Parameters:

Drain-Source Diode:

Siliconix VN10KM N-Channel

(This device, which comes in a TO237 package, is an N-channel unit in which a Zener diode has been included to protect the gate.)

General Parameters:

Drain-Source Diode:

(No maxima are listed for the Zener currents, although they show a forward breakdown voltage of 15V at 10uA.)

Pin Connections for the Above Siliconix Devices

VN10KM, VN0300M, VP0300M (TO237):

With the pins facing up and the flat side of the package toward you, the three leads are, from left to right: drain, gate, source.

VP0300B (TO39):

With the leads facing up and the tab to the left, the three leads are, from left to right: source, gate, drain.

VN0300D (TO220):

With the mounting surface toward you and the pins facing up, the three leads are: gate, drain, source.

(The above devices are available from Hamilton Avnet, 1175 Bordaux, Sunnyvale, CA 94086.)

International Rectifier
IRFD-123 N-Channel
Radio Shack 276-2073

General Parameters:

Pin Connections:

This is a 4-pin DIP package, pins l and 2 of which are internally tied together; pins l and 2 are the drain. Pin 3 is the gate, and pin 4 is the source.

International Rectifier
IRF511 N-Channel
Radio Shack 276-2072

(Note: This device is no longer listed by International Rectifier.)

General Parameters:

Pin connections:

This device is in a TO220 case. With the mounting surface toward you and the pins facing up, the three leads are, from left to right: gate, drain, source.

SUMMARY OF SKTF SURVEY
By Lesley Brabyn

Surveys were sent out to 352 subscribers of the SKTF in February of l983. So far, ll4 replies have been received and a preliminary analysis of the first 85 has provided the following information:

Twenty-nine subscribers report that they have built a total of 55 devices described in the SKTF. The most popular of these has been the Auditory Gimmick, with l3 made, followed by the Meter Reader and Sonalert Tester, each with 8. Twelve of the 85 had commissioned either a volunteer or paid contractor to make a total of l9 additional SKTF devices, again the most popular being the Auditory Gimmick, with ll fabricated. Many other respondents indicated that they are intending to build projects in the future, but are presently lacking the time to do so.

Fifty-four subscribers had built projects not described in SKTF, but the majority of these (74%) had found information contained in the magazine to be helpful or encouraging in the building of these outside devices. Subscribers listed a total of 40 different devices they would consider buying if they were commercially available at a reasonable price, the most popular being the V.U. Meter, Audible Compass, Little-Go-Beep, and general meter readers, respectively.

A majority of those replying found the articles on soldering useful (84%) and comprehensible (95%). Sixty-seven percent replied that they could now perform certain tasks more easily than they could before reading this material. Subscribers found helpful and/or useful articles on "techniques other than soldering" (point-to-point wiring, wire wrapping, solderless breadboards - 82%), tape splicing and cassette repair (62%), tape editing (49%), component discussions (85%), and instructional materials (66%). Most felt that they could not have met their requirements for information of this type through currently available sources other than SKTF.

Subscribers listed a total of 52 different SKTF articles as being the most enjoyable to read, with Soldering, Digital Electronics, Speechboards, and Gabbing About Gates ranked highest in popularity, respectively. Most readers reported using the information obtained through SKTF in connection with their hobbies, but many also used it in their work as well. An overwhelming 96% found SKTF to be personally encouraging to them, and 92% found it helped to expand their concept of blind people in technical fields. Furthermore, 89% felt that, since reading SKTF, they would be more confident in attempting a task about which a counselor or teacher might say, "It can't be done."

Most subscribers seemed to keep their copies of SKTF to themselves, although some shared them with a few others. A reader living in Japan gave the magazine its widest exposure by translating articles into Japanese and publishing them in a braille technical magazine there. Most readers had "no idea" of what the maximum number of people might be who could benefit from SKTF, and guesses ranged from "a very few" to "250,000." Both Talking Book and Braille readers seemed generally pleased with the quality of reproduction and formatting for these versions. As such a small number of Large Print readers responded to the survey (only 2 out of the 85 currently analyzed), no statement can really be made about this version as yet.

Analysis of the remaining replies is continuing, and an in-depth report will be available soon.

[Editor's Note: Whereas most survey responses are considered substantial if they are 5% or so, you have broken all records in submitting your entries. This Editor thanks you profoundly; I feel as though you have done me a great personal favor.]

EDITOR'S CRYSTAL BALL

If I can rouse myself from the reverie over your kind words of support and the results of the evaluation survey, it is time once again to point the way toward new directions. (Not that I have any illusions about fulfilling all my past promises, but that never stops me--making new ones keeps me on my toes.)

I want to make up some tape recordings of lab activities which beginners could use as step-by-step building instructions. The first such tape will be a recording of three people building identical continuity testers --giving a blow-by-blow description of gathering components, cutting the perforated board, wiring the circuit, mounting the hardware, and testing the completed instrument. Another tape will be a recording of me as I build a more complicated IC project; this will have to include some decision-making about the size of the board, parts layout, etc.

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Now that our Training Program is in full swing and has proven itself, I plan to make live recordings--strictly for encouragement-- of beginners making decisions about which soldering techniques best address their needs, etc. No such tape would be complete without witnessing an experienced builder (like me) try three or four times to make a good solder connection, or without hearing said builder do some damage. Although edited, these latter recordings will show processes being done in real time; the listener will get an idea as to how long it takes to make a solder connection and how long it takes to measure out the location for mounting a control.

Still another tape will contain examples of Yours Truly working with an experienced technical reader, going through a schematic diagram and doing some basic math.

The first tape to be available is the tape of building the continuity testers. Please send me a 90-minute cassette, clearly marked with your address, if you want this recorded. The availability of the others will be announced subsequently.

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I would like to compile information, perhaps in a series of articles, regarding the training of technical readers for personal use--getting people to describe schematic diagrams, read equations, etc. A lot of papers and seminars covering this subject have been given at transcribers' conventions (good reference material), but I have seen nothing written from the viewpoint of students, hobbyists and professionals working on a one-to-one basis with their peers. Those of you who have thoughts on this subject, please pass your ideas along to me.

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The study on our prototype PC-board kits has not been completed, though the preliminary results are encouraging. (Using Thermoform models of the board to illustrate various stages of completion seems viable.) Eventually, if this concept is to become a reality, there are some basic questions to be answered:

If a manufacturer were to supply all the components and accessories for a given project (the cabinet, jacks, controls, hardware, etc.), the eventual product would be tremendously expensive; it would approach the price of a ready-made instrument. (For beginners without a "junkbox," and who are not experienced in ordering components, this would be the best approach.)

On the other hand, the kit could contain only the circuit board and hard-to-get components, thus making it available at a fraction of the cost of the former. This would mean, of course, that the eventual product would be appropriate for more experienced builders. I would like your feedback as to which approach should be taken initially; which would you buy if it were available.

Our first prototype was of the basic "null- type" meter reader ("Basic Analog Meter Reader," SKTF, Spring l98l). This design having been completed, this may be the first kit produced. Upon completion of our study, I would like to design a kit for the dog whistle beacon ("Little-Go-Beep II," SKTF Summer l982). It will be necessary to know how many of each we should get made, and in which form (the complete collection of parts, or only the circuit board). Once again, I would like to hear from you.

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Our subscribership is in need of another boost. For whatever reason, our readership is down from the previous year by 20%. We are taking steps to get notices in various magazines and into the hands of education and rehab personnel, but it is my experience that the most effective advertising is word-of- mouth, as done by you. Please, help us grow stronger and more secure by "talking us up" within your organizations, and on the air. (If a compatriot resists, twist his telegraph key.)

To simplify our mailing lists and bookkeeping, we have been converting subscriptions to coincide with the calendar year; eventually everyone will renew his subscription in January. Except for a few, your subscription for the year of l984 is now due. I don't want to lose anybody, so please keep this in mind. (Those who need to renew will get a reminder notice in January.)

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Your response to the survey, along with what we have accomplished in the magazine this year, catapults me into the New Year with a light heart and good spirits. I wish the same, combined with prosperity, for all of you.