That's what I built for our Halloween party last year. We had a guided tour, where the resident mad scientist took people around in a small group. When they got to the toxic waste dump, they were "checked out" with the fake geiger counter...and wouldn't you know it? Some of them were already exposed before they came. Tsk, tsk.
It turns out that a low-key, realistic effect like this will creep people out much more than a lot of traditional stuff (I speak from experience.) A skeleton popping out of the ground? Eh. A zombie munching on an arm? Cute. I've been exposed to radiation?! AAAHHHHHH!
After searching the web, it seems that no one has created a live geiger counter sound effect generator. You can find short sound clips of geiger counters, but they're only a few seconds long and they sound like a recording, plus if you used one it would repeat the same pattern over and over. I wanted a hand-held device that would generate the sound effects live.
This is an extremely accurate simulation of the real thing. I've shown it to people who work with radiation in their jobs, and they've all said, "Yup, that's what it sounds like."
Check out this video:
There are two parts to making this - the electronics and the geiger counter prop itself.
Electronics Sound Effects Overview
This is powered by an Arduino. When you turn it on, it makes a slow, "background radiation" clicking sound effect. Press the button and the sound effect rapidly ramps up to an alarming rate. Release the button and the clicking goes back to normal. There's also an optional LED that acts as a power light and flickers with the clicking, but the unnerved Trick-Or-Treaters didn't seem to notice it.
The Arduino generates a randomized clicking on one of its pins. This is sent to a small LM386 amplifier which boosts the volume, and powers a mini 8 ohm speaker. I tried connecting the Arduino directly to the speaker, but even with a capacitor, you can barely hear the clicks, so I added the LM386 circuit.
I've found a pre-made mini audio amp that is better than the LM386, and is really cheap. See the last step.