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Last Updated: 2023-06-03 A 2-bit relay adding machine.


Clicky is a small 2-bit adding machine built out of 10 24V relays. Some are 4PDT and some are SPDT. It was designed as a demonstration piece for a group project to show how (telegraph) relays could implement boolean functions. There is no complete schematic for the device but the addition logic is implemented using two 4PDT relays per adder, which is a common schematic available online. The adder, then, takes up only four of the 10 relays. the remaining relays are 2-to-4 or 3-to-8 decoders. The original intent was to add several VFDs to display the input and output numbers (hence the "VFD filaments" switch). All the logic uses a "grounded zero" as opposed to a "floating zero" since this simplifies the adder. When 8th grade me designed the system, I knew about flyback and how you can generate high voltages with a coil. Despite this, the original design included no flyback diodes. To no one's surprise, after just a few minutes of playing around with the circuit, almost all of the LEDs on the front panel had been blown. I threw some "freewheel" diodes across the relay coils and this has fixed the issue.


Front view of Clicky. The control panel has two sets of two switches, along with an LED above each switch to indicate its value. At the top of the control panel are three LEDs to display the addition result. Top view of Clicky. The groups of relays are labeled, A and B are the adders, while 1, 2, and Y are the decoders. Front view of Clicky again, but with it powered on. Shows the result of two plus three equals five.
That's all!
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