Seven Decade Programmable Resistor – A Low Cost Solution

I have spent a lot of time over the years prototyping electronic circuits and the amount of resistors that have ended up in the trash because they are so cheap you don’t bother to keep them tidy or organised once you take them out of their organised storage – you know the story. One potential solution to this is a programmable resistance box but the problem with these things are they are bulky and expensive and do not lend themselves well to breadboard prototyping. The cost of construction means they are typically the reserve of high-precision resistance boxes. I have a CROPICO RBB6E resistance box in my lab which I open up to have a look inside, its really well made, mostly by hand too, far too nice to abuse in prototyping….

I looked around at what is available but did not find a solution that met my own requirements so I decided to design something simple myself. I also wanted to make a simple project to get manufactured by machine which apart from other things requires reasonable volume, and I thought this project would be useful enough to others that I should get some made and make them available.

Various Pictures

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Project Discussions

Project discussion thread on EEVBlog forums

Other Resources I mention in the Video

Cheap Resistance Substitution Box by @vtl on the EEVBlog
Cheap Resistance Substitution Box – Hack-a-day

If you do want one of these I have these for sale on “tindie.com”, the item listing is here:

https://www.tindie.com/products/gerrysweeney/seven-decade-programmable-resistor-1r-9999999r-1-500mw-gerrysweeneycom/

I have also listed these on e-bay, you can search for “gerrysweeney.com” which will find the listing.

As ever, if you have any comments, suggestions or feedback, please use the form at bottom of this page

Thanks for watching

This content is published under the Attribution-Noncommercial-Share Alike 3.0 Unported license.

33 thoughts on “Seven Decade Programmable Resistor – A Low Cost Solution

  1. Really cool little project that addresses fundamental problems in prototyping!
    I was excited by your thinking and, as one does, got to thinking.
    What if you made each decade on a single board but with a right angled 2 pin header as a terminal at the bottom, then a small “arduino type breadbord” could be used as a back plane >- multiple resistances in a small package!

    All the best Steve

    • Hi Steve,

      Thanks for your comments, thats an interesting idea, I am not sure how useful I would find that as you would be limited to the step of that decade but I see where you are going.

      Gerry

      • Very fresh idea. I can see it being used for an LED brightness matching rig. I’d buy a few of them right away but I dumped my PayPal account a month ago because of the bank account linking requirement. ‘Working on an alternative.

        Caps decade box next? (kidding) Keep up the good work.

        BTW, those current sense resistors are hard to find in the US.

        Mike

        • Hi Mike,

          Thanks for the feedback, much appreciated. I already have one of my pals using it for exactly that purpose, matching LED brightness, it’s very handy for that. Its a simple idea but people have generally warmed to it, many more than I expected have sold already.

          Those current sense resistors are hard to find here in the UK too, I only found them because someone posted a comment and pointed me at the distributor in Europe that has them, but they are not cheap either, about £7 each all told.

          Gerry

    • Dear Jürgen,

      I tried to download the case, but it was too big to fit through the small internet wires coming into my house.

      I’m just making a dumb joke, of course. That’s great that you designed the case; thank you! If anyone offers to sell these cases, I hope you’ll let us know here.

  2. Hello Gerry,
    I received my 2 programmable resistors today — thanks for such quick shipping to USA.
    This design is truly an ingenious blend of functionality, simplicity and economy — it’s really great.
    I know you’ve had a lot of suggestions, but if you don’t mind 2 more cents, my only input is that a low-cost enclosure might protect it and make it bit more durable on my messy bench. I wouldn’t even say that you should provide the enclosure, but I wonder if it could fit a standard plastic case similar to Dave’s MicroCurrent, in which the PCB becomes the top of the box. Well, regardless, I’m very happy with them; thank you very much!

    • HI John,

      Thank you for the feedback, much appreciated, I am glad you like the device. I have asked a question of an injection moulder but I suspect that it will be cost prohibitive. Jurgen posted here and made a case using a 3D printer, he has made the design available (link in the comments), you can have these printed using one of the on line 3D printers if that helps.

      Gerry

  3. Hi Gerry,

    No I wouldn’t suggest a custom box. I was wondering if one of those off-the-shelf black or blue plastic project boxes would be the right size. But no worry; now I’m looking forward to the capacitors!

    • Hi John,

      When I sized the board I did look around quite hard for an appropriately proportioned standard case that I could use but I could not find anything in the end. I am still looking into options, I am interested in learning a little bit about 3D design and plastic injection moulding – then all I have to do is find a company that can create a low-cost injection mould – these things cost $1000’s to make from what I understand.

      Gerry

  4. Just a quick comment
    Our chinese “hobbyists” already appreciated your great , yet simple idea
    [[link removed]]

    • Hi Alex,

      Indeed, I seen this yesterday when someone pointed it out. Its nice that I came up with an idea thats worth the effort of copying :) I removed the link to lower the link back traffic but if anyone wants to see a clone of my programmable resistor board you can search e-bay for item ref: 161074502921

      Gerry

  5. Hi Gerry. I tried to buy some from Tindie but New Zealand is not accepted as a destination. Mailing something to NZ must be pretty much the same as to Australia. As a Ten-Pound-Pom I must insist all the colonies be treated equally. Regards

    • Hi Nick, of course no problem, I just need to respond to the Tindi request to add postage for NZ but I only have mobile access until tomorrow. I will see if I can do it now. Gerry

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  10. Lovely idea – well done.
    For a custom case, have you thought about vacuum moulding? The lines are not as crisp as an injection moulded case, but the entry costs should be lower. My daughter got to make a small case in school for an electronics project that way and it seemed robust enough for work bench use.

    • Hi Kevin,

      Not really given the vacuum moulding consideration, I will look into it, I wonder how much of an alternative this would be, I thought VM was suitable for thinner materials. Thanks for the suggestion

      Gerry

      • I think you are right, the thinner end of manufacturing for sure – but a couple of minutes on e-bay found vacuum sheet up to 2mm thick.
        I don’t visualise measurements well so I grabbed a few things off my desk for comparison, the case of an old usb stick 1.5mm thick – the Ferrero rocher box I keep screws in 2mm.
        I think with careful choice of shape it would suit the size of enclosure you are looking at – if you can find somewhere that will let you do a test run you could probably test the theory quite cheaply.
        Of course, the look is somewhat more ‘organic’ than injection moulding or 3d printing so it might not be aesthetically where you want to be.
        K

  11. Hi, can you tell me if you will have anymore of the seven decade resistors for sale on eBay soon please or is there somewhere else I can buy in the UK. Thanks

  12. An interesting chip is the AD5246 that is programmable from the circuit it’s being used in, in theory. Programmable resistors as they call them, also store the value programmed previously, so they can be used for prototyping. In practice, you’d put them on small board with header for programming signal and voltage. Essentially like a DAC but with the pull-up resistors tied instead to an external VIn pin so that you aren’t restricted to a specific internal voltage reference. Yes, it’s analog and unpolarized so you can use it anywhere that a normal resistor array and MOSFET-based switches would go.

    • Yeah I have seen these before, not used one but electrically they have limited application because of their reference to ground – you cant beat an actual passive resistor for prototyping :) Thanks for the suggestion and comments though.

      Gerry

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