Sunday, September 23, 2018

Binary Resistance Decade Box

I built this project in June of 2013 and posted it on Instructables


Binary Resistance Decade Box

I've been an electronics hobbyist for many years, so I've used and even built my share of resistance decade boxes. Each one consisted of rotary switches with labels identifying a different power of 10 for each switch. You "dialed up" the desired resistance by turning the appropriate knobs to add up to the target resistance value.

Well, I'm also a computer geek, so I got the crazy idea to build a decade box using DIP switches (instead of rotary switches) and binary values (instead of decimal values). Each switch represents a power of 2 and the resulting resistance equals the combined value of the "ON" switches.

Since binary DIP switches are more difficult to read on site than rotary switches, I decided to include two sets of binding posts; one set to attach to an ohmmeter (to verify the selected resistance value) and one set to put the resistance in-circuit. A DPDT switch lets you toggle between them. Also, since the combined analog resistor values tend to vary from the perfect digital values you want, I added a 25 ohm POT for fine tuning.

This project uses two 8-channel DIP switches, which provide the following binary values: 1, 2, 4, 8, 16, 32, 64, 128, 256, 512, 1,024, 2,048, 4,096, 8,192, 16,384, and 32,768. With it, you can combine switches to create any value between 0 and 65,535 ohms.

As is customary with binary numbers, I started with with the least significant digit (lowest ohm value) on the rightmost switch and increased in power-of-two increments to the maximum value on the left.

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