4-bit Relay Logic Counter Begs To Have Its Buttons Pushed

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What’s one to do with some nice little relays of questionable pinout, and prototyping board? How about a quietly clicky 4-bit counter using relay logic with tons of buttons?

The register with LEDs and buttons is on the top board, the incrementer on the bottom board.

[Agatha Mallett] made the counter after finding herself in possession of a quantity of relays burdened by terrible documentation (the datasheet shockingly lacks a pinout, and doesn’t even mention the coil being unidirectional). But since the relays are also small and of decent quality, they were a good candidate for a small relay logic-based project.

The key to the build is implementing D-type flip-flops using relays. This is done by holding the coil voltage of each relay between its set and release voltage levels. A small voltage bump will energize the coil, closing the relay and leaving it closed. Conversely, a small negative spike releases the coil, leaving it open. This forms the basis of the counter, and [Agatha] has a separate write-up all about the details of using relays in this way.