[Vincent] on the EEVblog forums had an idea for an inexpensive resistor substitution decade box.
The build uses cheap decimal thumbwheel switches he bought on eBay. Each switch is wired up with resistors for each digit, and each switch is wired up in series. The result is a small, easy to read resistor box with a range of 1 Ω to 10 MΩ.
This isn’t a new idea; using encoders like this has been done before with a BCD capacitance substitution box. We saw some references to a cheap programmable current source (layout available here) that uses BCD switches and an LM317 regulator. While this type of build has been around forever, these projects are becoming economical because of the inexpensive decimal or BCD switches are available from China.
For [Vincent]’s project, we’re wondering if it would be better to have a pre-set 10 Ω box as the least significant digit. Of course, this would mean re-doing the project but it would increase the maximum resistance and get around the very small resistance in the smallest digit. Quite often, we’ve called Zero Ohm resistors “wire” and 1-9 Ohm resistors, “longer pieces of wire.” If you design for the E series, you’re never exact anyway.