■ WITH TJ BYERS
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist.
Feel free to participate with your questions,
comments, or suggestions.
You can reach me at: TJBYERS@aol.com
The days are short and the
nights are long — a perfect time
to heat up the ol’ soldering iron.
✓Digital pot light fader.
✓DVM high-voltage probe.
✓Golf cart gas gauge.
✓$10 audio mixer.
QI am looking for a ramp
generator, sort of. The ideal
circuit would have a button
which, when pushed would
ramp up the output voltage, then
the circuit would sit at a high output
level until a second button was
pushed which would ramp down the
output voltage and then the circuit
would sit at a low level until the first
button was pressed again. I would
really love it if the time of the ramp
was variable between 0.5 and 15 seconds. And, finally, the output voltage
should be 0 volts at low level and 10
volts at high level. The output current
would not have to be very much,
100 mA max. The circuit would be
used with an old analog theater
dimmer pack. To make it even more
versatile, if the output voltage high and
low points could be programmed to
0-12V or 0-5V, then the circuit could
be used in a variety of audio circuits,
— Tyler Patrick
AWhat you want is a trapezoidal waveform generator.
This can be done using a
digital pot (Figure 1). The first
gate — a 4093 astable oscillator —
■ FIGURE 1
t = RC x 128
Trapezoidal Wave Generator
clocks pulses into a 128-step digital
potentiometer. Two SPST (single-pole,
single-throw) switches determine the
direction of counting, up or down, and
the output voltage, + 5 volts or + 10
volts. Since you have three unused
4093 gates, they can be used to drive
LEDs to indicate the direction of the
waveform, output voltage range, and
other functions. The power supply
is regulated to five volts, giving the
output a range of zero to five volts;
the ramp-up and ramp-down time is
determined by the equation t = RC x
128. The op-amp increases the five-volt
wiper voltage to 10 volts at about 25
mA, which should be enough for most
But you said you need 100
mA of drive. This is easily done by
paralleling two or more op-amps
to increase the output current.
When paralleling op-amps,
though, you have to be careful
that they don’t feed back on each
other and create unstability or
overheating. The standard solution is to use small isolation
resistors feeding a common line,
as shown in Figure 2. There is no
limit to the number of op-amps
you can add to the right.
However, it is better if they are all
the same number — or at least
the same type. Mixing parallel
op-amp types is just asking for