Electromechanical Digital Relay Clock

*The digital clock to the left is not part of this project. It is only shown for comparison.

This Electromechanical Relay Clock uses 67 relays arranged in 21 flip-flop logic circuits. It is in essence a computer that can add, compare, and clear various “registers” in order to keep track of the time.

It was inspired largely by the electromechanical relay computers from several decades ago as well as the many other hobbyist projects seen today.

In addition to the 67 relays (21 4PDT and 46 DPDT), this circuit also uses a total of 237 diodes, 42 resistors, and well over a hundred feet of wire across nearly 1,400 solder connections. There are absolutely no transistor junctions or integrated circuits here, and the only two capacitors are those used for filtering the supply voltage.

Every component in the circuit is running in-spec according to their datasheets. Still, a  bi-metal circuit breaker and a small fuse insure safe, continuous operation.


The 1pps output comes from a synchronous timing motor/gearbox that references the 60Hz mains frequency and rotates a 3D printed ten-pointed cog at exactly 6RPM. In a sense, this is the computer’s clock, running at a whopping 1 × 10-9 gigahertz.

The motor is a 600 Series Synchron C Mount type. Initial plans called for a synchronous microwave turntable motor of the same speed, however most were later found to  lag behind by a few minutes every hour. This motor on the other hand is designed for use in such applications as analog clocks and chart recorders.


The project was first conceived from the inspiration of the various electromechanical computers throughout history and from intrepid hobbyists alike. Eventually, a couple dozen 4PDT relays were acquired and from that did the project begin.



The clock outputs three binary numbers (red, yellow, green) and one additional bit (blue) to signify AM/PM. To read this clock in decimal, look at any one row and add up only the columns that are illuminated. A numerical guide that values each column is shown below the display area.


Although the circuit is very large, all 21 relay modules are essentially alike. The four logic boards at the bottom however have an additional output relay in place of an LED. A more in-depth writeup of how to read binary is on this project’s blog post: HERE.

Here is a short video highlighting the sheer excitement of this build:

In reality however, I would have to say the many hours of labor involved in this project was more zen then actual labor.


A full schematic was not necessary for this project. Instead a derivative of Simon Winder’s Edge triggered D type flip flop was used and referenced for each and every one of the 21 logic modules. Once one module was completed, all other subsequent ones were simply a copy of the previous.

I highly encourage you to take a look at Simon’s awesome Relay Calculating Engine as it was large part of the inspiration for this project.

Here are two of his videos I watched many times over while designing my project:

Don’t know how to read binary? it’s simple! Visit the blog post to find out and learn more about this project. The electromechanical relay clock project is complete!

7 thoughts on “Electromechanical Digital Relay Clock”

  1. Hi, the finished clock looks great! I love the design. Do you have any schematics for the flip flop modules or is it identical to Simon’s design?

    1. Thanks, The modules are identical to Simon’s design, though for the four “reset” modules I made at the bottom, I substituted another DPDT relay for the led and its resistor. These extra relays break the positive supply rail for their respective unit displays.

      1. Ah right, I am planning on building a relay clock using Simons design but only using SPDT relays as I have a ton of them! I will just need to parallel up the coils to get the number of contacts I need. I was previously going to use another design, but it involved switching the relay coils in series then just accross the supply. I tried it with the relays I have, but it would mean over driving some of the relays and underdriving others. I did try a mock-up of this design but was a little unsure as to how to drive the clear/set contacts. Is this done at the same time as the clock pulse, or have I got the wrong idea?!

        1. I was in she same place with the over/under-driven series relays before finding Simon’s designs. As for the set/clear contacts, at 6:00 in the “Relay logic: Edge triggered D type flip flop” video, he explains that you can make the flip-flop change its state with every clock pulse by connecting Q and -Q to Set and Clear on the same module with a couple series diodes. Another video by Simon elaborates this further and explains the theory and how to make a binary counter. I’m not sure why I didn’t include it in my write-up (I’ll change that now), but I do recall using it equally as much: https://youtu.be/2HqbQfS_12o

          1. Thanks for that, it is much clearer now! The odd thing is, I have watched Simons’ videos multiple times but I must have missed that part!

  2. Wow, so cool. I think I had that same 7 segment digital clock kit years ago when I first started with electronics. Thanks for sharing, maybe someday I will try replicating this!

    1. Awesome! That’s great to hear. If you’re curious, try building a single flip flop module on some bread board in your free time, then add a second flop flop that triggers off the first; now you have a two digit binary counter. That’s basically how this project started.

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