Resistor Substitution Box

[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.

16 thoughts on “Resistor Substitution Box

  1. It’s a nice idea, but those things are a bitch to calibrate. You need as many significant digits of precision in your components as you have stages.

    A 1% 1M resistor will be somewhere within 10k of a megohm. The device’s 1M setting will actually be somewhere between 1,010,000 and 990,000 ohms. Even if we assume that all the other resistors are perfect, there’s only a 10% chance that the 1M setting will be within 1000 ohms of the 999,000 ohm setting, and a small chance that it could be lower.

    To make the thing work right, your 1M setting has to tie into a chain of resistors that actually come within tolerances for the smallest step you want to make. *Then* you have to start worrying about the resistance of the connections.

    1. No, it’s still a good design. To tighten the tolerance for each stage you want to add, you simply add a 1M variable resistor at the end of the chain, make your measurements with a good multimeter as you switch in each stage, then mark the readings on the VR dial. Voila–a properly calibrated resistor substitution box using a cheap decimal thumbwheel switch.

  2. Addendum..

    Having RTFA, he discusses that, and points out that he isn’t trying for full-range precision. As such, it’s a nice way to switch between different values quickly.

    Anyone looking to build a similar one should remember that you’ll only get a couple significant digits of accuracy out of 1% resistors.

  3. is it just me but I don’t see why this would be a hard thing to calibrate. If the problem happens to be that the resistance of one 1M resistor is not the same as that of another 1M resistor then surely the simple answer is to buy a bag of 100 or so and test them until you find one that has a finer tolerance. (I do that anyway, I have bags of odd value resistors for fine tuning projects – mainly for resonance purposes).

    @spiralbrain: I would love to see this sort of thing in chip format maybe with an array of them on one chip to send out analogue patterns of resistance all from one stand alone chip (:

  4. These switches are great for testing Nixie tubes too. 3-bit SCSI ID push switches make a good replacement for bulky rotary switches, and I’ve found good quantities of them in surplus stores.

  5. Hmm, a chip that can digitally alter it’s resistance. That’s a neat idea! Something like a digital potentiometer. If only someone could build one…
    Oh wait, they’re around for years.

  6. I have one of the thumbwheel versions of this switch here, 5 digits.

    Yes they are a pain to calibrate, but not if you have 1% tolerance resistors already.
    Just select a suitably large buffer resistor to go in parallel with each stage to fine tune the value so that in the middle of the range it is spot on.

  7. …and 1-9 Ohm resistors, “longer pieces of wire.”

    yeah, you can do that, but you also have to know that just using lengths of wire can lead to inductance/capacitance effects that may not be intended.

  8. “Spiralbrain says:
    July 22, 2011 at 9:31 pm

    In the near future we might have an electronic version of these. Imagine buying blank resistances and programming them before putting them in a circuit.”
    Buy an ADN2850 and you’re good to go for 1024 possible values. :) I’m trying to figure out if there’s a large-value one to cascade to get the full 6-digits of a 1Ohm-1MOhm device. The neat thing is that in theory they can make them with permanent drop-in values. You program them with the 3 pins on top, solder the 2 pins into your existing resistor’s holes, and get a nice test device. If clever, you can make a second board that has some of the programmable resistors networked to an USB microcontroller for testing out different sets of values. If I was making it, I’d put 3-pin cables with SIP sockets on it to make it easier on the developer. Of course, in practice, most of the electronics I work with don’t even care about the difference between a 110-Ohm and 170-Ohm resistor. :P

    One gotcha is that you can’t use such a board for RF work or the like. The feedback between leads would be horrible.

  9. Oh, and it’s trivial to make a fusible link like PROMs for different resistors and then use a laser to calibrate the individual tracks at the time of manufacturing. A more efficient method is to have some kind of highly accurate digital VOM and then ramp down/up the resistance real-time until you have the oddball value you need. A super cheap method used for years is to just take a regular carbon resistor and file a small amount away to increase it, then seal it. This is almost exactly how resistors are sometimes made. Of course, it’s a fire trap so a no-no on power supplies and high-current amps! heh

  10. I’m from Belgium and read the posts on this site for a while now searching for a neat project to teach my son . What some of you forget is that this site is like a online hackerspace , you just have to think and create . Giving comment is easy , building stuff like this is for people that have a real hackermind and that is having fun in what you do .

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.