Resistor Substitution Box
For finding ideal potentiometer and limiting resistor
values, I use a resistor substitution box. You can buy
these, but I built one myself in about an hour. It has
5 pots wired in series: 1kΩ, 10kΩ, 100kΩ, 1MΩ, and a
fancy 10-turn 1kΩ pot with a 3-digit dial that I liberated from some old test equipment (substitution:
Mouser part #882-DC22-10-1K, $75).
CAUTION: Unless you’re a trained electrical
engineer, you should only work on circuits that are
battery powered, or that use a very low-amperage
power supply (50mA or so). Poking around inside
high-voltage and/or high-current circuits can be fatal.
Digital audio circuit, battery-powered and fairly
simple Kids’ toys are fair game; they’re cheap and
easy to find, and they usually make lots of different sounds. For this article I used a Multi Voice
Changer megaphone sold under the Toysmith and
128 Make: Volume
Audio Circuit Clock Speed
NOTE: This project is the simplest way to add
a pitch adjustment to audio circuits that have a
pitch resistor. More complicated methods allow
for a wider pitch range and finer tuning.