Building A Resistor Substitution Decade Box

[George] built an incredibly tidy resistor substitution decade box. These devices feature a pair of connections and a way to select the resistance between the two of them. In [George’s] case it’s a pair of banana jacks and these eight thumbwheel switches.

What you see above is the side of each thumbwheel switch. These are panel mount devices which show one digit with an up and down button to change the setting. As you can see, the PCB for each provides connections to which a set of resistors can be mounted. This is the difficult part which he goes to great lengths to explain.

At this point he’s got the resistor groups for each digit soldered in place, the next step is to stack the switches next to each other and connect them electrically. From there it’s off to a project box in which they will be mounted.

This project does a great job of explaining the assembly process. If you’re interested in the theory behind a substitution box check out this other project.

24 thoughts on “Building A Resistor Substitution Decade Box

  1. Pardon my ignorance, but how often are these really necessary? I know they’re important but I’m just not sure for what.

    I use multi-turn trimmers of various sizes in test fixtures. In some cases they are connected to switches so I can switch them out of the circuit to measure them with a dvm.

    1. These are nice when built with high precision resistors (at most 1%, better with 0.1%) to be able to select a specific resistance without measuring.

      They are not commonly used outside school labs though, most people who need high precision resistors also need low capacitance and inductance, and these are terrible for that. Also an E96 or even E192 SMD assortment isn’t that expensive for a company.

    2. They’re used when you want to characterize a circuit. For instance, if I want to see how much current a solar cell can source, I would hook it up in parallel with one of these and measure the voltage across the resistor as I lowered the resistance (also called increasing the load) which, by Ohm’s Law, would draw more and more current. Then I’d plot it and figure out the level of compliance of the PV cell.

    1. I found one set of 10 in China for $4 even:
      Would really like to buy but I don’t feel like signing up for Alipay. Wonder if someone else who has an Alipay account and could go in on this together (with PayPal payments)…

      This seems like a VERY cool idea.

      Also came across this one, which would need fewer resistors but more values:

    1. I’ve been using it a lot to help pick LED brightness in an LED display, so that the brighness pleasing to the eye. That’s kinda hard to do with a calculator. ;) Boxes like these are a staple on many electronic benches.

      1. Yep, when I saw this, LED brightness was exactly the sort of thing I had in mind for it. Also to determine the pull-up resistance ranges for some of my USB projects (I’m always substituting random resistors on the USB lines, and I’ve never had a problem, but wondering what I can really get away with…)

    1. It’s more neat, compact, and elegant, plus it has a numerical readout instead of having to do math to figure out what value you’re going to get. Plus it’s an easy little project. Approximately the same price and functionally identical though.

  2. i like this, its neat and compact.

    but those resistors and switches wont be able to handle much current.
    i use decade boxes to test power supply outputs, so it needs to handle dem amps.

    it all comes down to what you need i suppose.

  3. Be careful…I bought a set of thumbwheel switches off of eBay for just this purpose, and it turns out the switches are BCD…so instead of one contact for each number value (a 1 contact, 2 contact, 3 contact, etc.) I only have one per binary digit (a ones contact, a twos contact, a fours and eights). I couldn’t think of a way to make that work.

Leave a Reply

Please be kind and respectful to help make the comments section excellent. (Comment Policy)

This site uses Akismet to reduce spam. Learn how your comment data is processed.