Working Relay BCD Decoder



Been a while since I posted on this subject. I have had the PCB fabricated and assembled one BCD controller. It tool a long time to solder in all the parts, but eventually it got built. After some initial testing of the PCB I found that I had messed up ONE circuit, the one that controls the output enable relay. I had wired it up and found later after looking at the schematic that I have back flow of voltage though it. And about the only fix was to use diodes or just pull the relay and add a jumper. The latter was the way to go as it’s really not needed.

I wired in a box with some 120v hot dog shaped lamps and made a short video of the board in action. In watching the ammeter on the power supply it draws in the 3 amp range worse case. The BCD switches and LED’s all over the board make it a fun thing to play with.

Now just need to find the time to build a few more of these up, and figure out how I’m going to drive it.

Here’s the short video on the decode board –


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7 Responses to Working Relay BCD Decoder

  1. Chris Elzey May 12, 2016 at 7:04 am #

    So cool. What kind of relays are those? Ive been thinking about starting a project like this one for a while, using relays for combinational logic.

    • Sandy Ganz June 22, 2016 at 8:48 am #


      I picked up a huge amount of Omron GR2’s spdt relays off of ebay. They worked out well with the simple logic for the decoders. The problem now is figuring out how to make the counter circuits up. I’m not good at sequential logic but found a few simple circuits that could work. Would likely need DPDT relays for that, but I also found that they are available in bulk off of ebay from a good number of sources.

      Sorry for the late reply I must have missed the email notification that I got a post.


  2. devicemodder November 8, 2016 at 11:12 am #

    Got a schematic for the bcd decoder?

    • EZADMIN January 25, 2017 at 4:34 pm #

      I do have it, is you email good on the post I can send what I have as a pdf or something similar.


      • devicemodder January 25, 2017 at 4:46 pm #

        The email is good. Pdf is fine. Do you want my email or can you see it when I leave a comment?

        • EZADMIN January 26, 2017 at 11:42 am #

          It will be a bit before I can get up into the garage and fire up the machine with the schematic on it. Bug me if you don’t see it by the end of this weekend. I do have your email as part of the post so just a matter of getting to the machine. Their is one flaw on the circuit, I can’t remember if I removed it, but it was not logic related, but related to the Lamp Enable that was supposed to control all outputs. I made a dumb mistake and it would work, so I just added one jumper and left off the one affected relay. Otherwise it seems to be OK. I can also see if I still have the Logic Friday file that I used to generate the gate logic from the truth tables. It’s really cool to mess with.


          • devicemodder January 26, 2017 at 11:43 am #

            Thanks and no rush to get the schematic.

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