Overview

This is a decade resistor counter, a mechanical rotary decade switch module that lets you quickly select precise resistance values using an array of dials, each representing one digit of resistance. This particular unit is modular, with each digit being its own switch block. They stack together to create a multi digit decade resistor.

Each switch connects resistors in series, allowing you to dial in a specific resistance. These were commonly used in test equipment, calibration, and lab work before digital electronics became mainstream. There’s something satisfying about the mechanical click and seeing the number wheels line up!

How It Works

Each module contains a rotary switch and a set of contacts arranged in a circle. When you turn the wheel, a wiper moves across these contacts, switching in a different resistor for each position (0–9).

The modules shown here use a simple PCB to connect each contact to an external pin. The resistors are mounted externally, which makes it easy to swap values or repair them if needed.

Resistor Arrangement

The resistors are connected to the module pins on the back. The image below shows a diagram of the contact arrangement and the path through each resistor as you turn the dial. Build each layer one at a time. The below for example, using 9x 1 ohn resistors.

When 0 is selected, between common and 0 will be a short.

When 1 is selected, there is a 1 ohm resistor between common and 0 connections.

Again, when 2 is selected, 2x 1 ohm resistors are daisy chained. And so on through to 9.

Your next section would be with 9x 10R resistors then 100R and so on until you have enough sets to achieve your goal. All daisy chained together through the common and 0 connections.

Stacked Assembly

Multiple modules are stacked together to create a multi-decade resistor, as seen here:

Closeup: Resistor Soldering

The resistors are hand-soldered to each module. Here’s a closer look at the back, showing the wiring:

There’s a real charm to these mechanical switches. No complex tech here, just solid reliable hardware design.

There are a few minor niggles with this. You'll notice that the counters + and - would have been much better the other way around, with + at the top. Also, its only as accurate as the resistors that you buy. I went through a load of resistors with a meter to try and find the best ones out of my collections.

To find these pushwheels, search for

"22mm x 8mm 0-9 digits decimal code pushwheels thumbwheel switches"

DONT BUT THE BCD ONES !