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.


  1. Slipster says:

    I’d buy one (or many) of those in a second if produced.

  2. trimmer? says:

    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.

    • Sven says:

      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.

    • KC says:

      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.

  3. I thought he’d bought like $150 of switches, but it turns out you get 10 for $15ish.

    Considering that a factory decade box can cost $200, this is a great build.

  4. sgt says:

    Ordered the parts to make one myself, looks great and easy for a useful piece.

  5. Ian says:

    This project is taken directly out of an old issue of Popular Electronics. I know because I was going to build one.

  6. Zhen says:

    Buy a cheap calculator – stop guessing.

    • George Graves says:

      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.

      • Kaz says:

        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…)

  7. NewCommentor1283 says:

    they still manufacture those ancient decimal thumwheel switches??? the kind used on ancient display-less mini-computers?

    … if only i had a 3D printer…

  8. Macpod says:

    I made one of these towards the start of the year. I made it because I had a need for one.. but since then have found it comes in handy quite often…

    Here’s a video of me making the case for it on my diy cnc mill:

  9. KP says:

    How is it better than this?

    • Anonymous says:

      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.

    • Ren says:

      Interesting that it is listed on *Bay under “Toys and Games”…
      Not trying to get around the surcharge *Bay puts on Business and Industrial products are they?

  10. borgar says:

    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.

  11. Ren says:

    Decade switches are “usually” stack-able, does having the resistors mounted as they are negate this?

  12. Dave says:

    What is the resistance between the terminals when the dials are set to 0?

  13. Almost nothing. About 0.7 to 0.9 ohms….thanks to the gold plated self-cleaning contacts. I’ve had a lot of requests for there – so I’ve built a few and they are for sale”



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