Resistor Substitution Box

Commercial substitution boxes uses a lot of switches and a set of fixed resistors. The advantage is that they have a large range of resistance and work at higher voltages.

This device is only good for use in circuits of 12 volts or so but it is much smaller and cheaper. It uses a log taper 2 watt potentiometer (Digikey part# KA1031S28) to produce a range of resistance values from about 50 ohms to 10k ohms.

To protect the potentiometer from self-destructing at the low end of the range when connected to some circuits, a 12 volt lamp is effectively connected in series near the low end of the resistance range. What looks like an unusual way to connect the lamp is to make connections easier (see rollover image above).

After constructing the unit, connect it to an ohm meter. Rotate the knob to produce a specific resistance and mark the legend.