About three years ago, TimMJN of the LMNC Discourse made a module out of a cheap Geiger counter. Whenever the counter detects a particle the module produces a time-decaying control voltage. Nuclear decay, get it? Kind of a poor man’s version of Nervous Squirrel’s uranium-driven Random Voltage Generator .

Recently I decided I wanted to build a module like Tim’s too. I mean, I used to work in nuclear and particle physics research, I felt like I kind of really should.

I bought the Geiger counter on AliExpress . It arrived pretty quickly, but was initially nonfunctional. Fortunately TimMJN’s Git repo provided a link to some (not great, but better than nothing) documentation. So when I saw there was no jumper on the J4 jacks, I checked the schematic and found that jumper is how the high voltage connects to the Geiger tube. Seems kind of important.

So I put a jumper there. Still no joy. I took a look at the instructions for calibrating the voltage. It said to do it before installing the tube and to “Connect positive Tube Clip to pins (J4) with a jumper wire”. I wasn’t at all sure what that meant. So I said what the heck, left the tube in place, and just checked the voltage at J2. It was about 50.8 V, below the 57 V they talked about, so I adjusted the trimmer until I was reading 57. And it started clicking.

Not very frequently — averaging maybe once or twice a minute, I’d say. But that’s about the rate TimMJN was getting too.

So I went ahead and built a version of TimMJN’s circuit. I did modify it — I added a trigger output. That required a third op amp, so I replaced the MCP6002 with an MCP6004. Then, rather than letting the fourth op amp go unused, I put it to work driving the LED for a more accurate indicator of what the envelope output is doing.

I built it on one of my protoboards , more or less improvising the layout — it could be cleaner, but it’s okay. As for the Geiger counter board, the one thing I modified was I switched out the “vertical” 3-position pin header for a “horizontal” one. (In quotes, because the board is vertical in the module, so the “horizontal” header ends up actually being vertical.)

I designed a front panel, which I’m going to get fabricated, but first I verified it by making a 3D printed version. My panel has a window in it so you can see the Geiger tube. I also printed a bracket to secure both the Geiger counter board and the protoboard to the front panel.

Oblique back view of a synth module consisting of a Geiger counter board and a protoboard attached to a 3D printed bracket, which is attached to a 3Dprinted front panel. Two jacks, an LED, and a board mount pot with its legs straightened are connected via wires. It's lying on a workbench and the power LED is illuminated.

Nuclear Decay module (with temporary panel) on the bench

And yes, I did have to resort to panel mounting a board mount potentiometer because that was what I had on hand.

But it all works!

Just for laughs I tried holding a bag of potassium chloride near the tube, but the clicks didn’t get dramatically more frequent. Anyone got any Fiestaware ? Depleted uranium frizzens ? Nervous Squirrel’s uranium ore is a little beyond my budget. Anyway, it just needs the fabbed front panel and it’ll be ready for the case. Which is in the basement, and if the clicking down there is much faster, I’ll have some words for the people who did our radon test.