BCD Decoder Schematic

The other posts show the bits and logic schematics for relay implementation of typical gate level logic. This is the main schematic of the BCD decoder. It does have a bug in that the circuit that controls all of the output relays (EN’) will not work. Due to not thinking about other relay’s giving the ability to be connected all to the same ground line, it causes some odd behavior without diodes or a bunch more relays. I didn’t really need that feature and the PCB’s were made so to work around it the EN’ switch, and related are removed and the relay that all of the octal output relays are simply grounded by a jumper.

So here are the octal output buffer (with bug as noted) and the main schematic that wires up all the gates. Note that the way that the schematic was done was to create each type of the single gate and re-use them on the main schematic. Altium takes care of the part numbering and related wiring. Keeps the schematic less complex as all the gates are just cut and paste into the schematic and wired up like modules.

Relay Octal Output Buffer

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Above is the octal output buffer. In the schematic Relay K1’s common should be jumped  to GND. The rest of the components including the K1 Relay should be removed from the circuit.  If you think about what can happen when K1 is energized you can see that it’s possible for a ‘sneek circuit’ from other relays causing odd states and feedback into the logic. The solution would be additional relays or insertion of diodes. Diodes are out since the track of this project was to NOT use any semiconductors as logic. Diodes to suppress spikes are added to keep relay contacts  alive longer but could have been omitted.


Relay BCD Decoder

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Above is the BCD decoder module, remember the EN’ switch and related are not needed as they don’t work.


That’s the project. Some folks asked why I didn’t use DPDT relays or Diodes for decoding. On the relay front I picked up a TON of SPDT Omron relays off of eBay and that’s the direction I went with. Diodes other then spike suppression was out of the question as that would eliminate the use of many many relays and what’s the point then :).  Now to get some help with the relay based counter circuits.


Happy Motoring!

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2 Responses to BCD Decoder Schematic

  1. Sandy August 4, 2017 at 8:33 pm #

    This is a link to a simple 4 bit counter. Will require 3PDT relays but looks like a low parts count design and simple.

    A 4-Bit Counter Using Relays – Department of Electrical and Computer …

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