A builder has put together a quietly clicky 4-bit relay logic counter that uses relay-based D-type flip-flops and a button-and-LED register for both counting and direct bit control—built after tracking down relays with poor documentation.
When you’re handed a box of relays with “terrible documentation,” the easy option is to set them aside. [Agatha Mallett] did the opposite. She built a 4-bit relay logic counter that doesn’t just keep count—it begs to be prodded by hand, button after button, with every click doing real work.
The project starts with a practical problem: the relays she acquired were small. of decent quality. and maddeningly hard to use. The datasheet, she says, lacks a pinout, and it doesn’t even clarify whether the coil is unidirectional. That’s the kind of missing detail that can turn a weekend prototyping job into a string of guesswork. But instead of walking away, she treated the relays as a challenge.
At the heart of the counter is a way to implement D-type flip-flops using relays. The method hinges on controlling the coil voltage so it sits between two thresholds: set and release. A small voltage bump energizes the coil, closing the relay and leaving it closed. A small negative spike does the opposite, releasing the coil and leaving it open. That behavior—closed stays closed unless nudged the right way—is what the counter relies on.
The build is split across two boards: the register with LEDs and buttons is on the top board. while the incrementer lives on the bottom board. The end result is a machine you can interact with directly. It includes indicator LEDs. buttons to increment or clear the current count. and buttons that let you set or clear individual bits—so you’re not limited to just watching the count climb. You can reach in and change the state.
Getting the logic to behave reliably took more tuning than the idea suggests. Holding relay coils “between” set and release voltages sounds simple. but it’s a balancing act—described as a figurative knife-edge. Not every relay is perfectly identical. so [Agatha] had to tweak resistors and capacitors. piece by piece. until the behavior settled into something consistent.
There’s also a short video on the project page showing the counter going through its paces—exactly the kind of follow-through a relay logic project like this needs, because the real proof is in the timing, the steadiness, and the way the clicks line up with the buttons you push.
[Jess] gets thanked for the tip, but the main story is harder to miss: sometimes the best electronics aren’t built despite messy parts—they’re built because someone decided the parts deserved better.
relay logic 4-bit counter D-type flip-flop prototyping electronics buttons LEDs coil voltage
So it’s just a counter but with relays?? Why not use a normal chip.
The headline says “clicking” and I immediately thought it was gonna be like a bomb or something lol. But hey, if it works and the relays aren’t labeled, that’s kinda impressive? Still sounds risky.
I don’t get the “between two thresholds” part. Like, how are you just sitting a relay coil in the middle without it freaking out? I feel like that’s how you burn stuff out faster. also 4-bit just means 4 buttons right?
I love when people refuse to throw away messy parts, but also… relay logic is like the least chill way to do anything. Those “negative spikes” sound like a thing I’d accidentally do wrong and then wonder why it never clicks right. Glad it has LEDs though because without that I’d just be pressing random buttons forever.