Introduction: How to Make a Fake Geiger Counter

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What sound effect can enhance a mad scientist's lab at Halloween?  The ticking of a geiger counter!  And imagine...What if you had a fake geiger counter you could wave over anything, and whenever you wanted, set off furious clicking? 

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.

UPDATE, 5/22/2013:
I've found a pre-made mini audio amp that is better than the LM386, and is really cheap.  See the last step.

Step 1: Electronics Schematics

This is quick to assemble on a breadboard.  Note that you should click on the schematic and view the original size, so you can read all the notes clearly.

Electronics parts:

- Arduino.  I used the "Diavolino" from Evil Mad Science:
http://evilmadscience.com/productsmenu/tinykitlist/180 
If you use this you'll also need an FTDI cable to program it, see their site for details.  Not only is it inexpensive but the Evil Mad Science people have lots of other cool kits.  Check out their Larson Scanner!
- Battery box.  The Diavolino can use one that holds 3 AA batteries and this lasts much longer than a 9V.

- LM386 Low Voltage Audio Power Amplifier.  This is a classic chip, available at Radio Shack, Jameco, etc.
- 220nf capacitor
- 220uf electrolytic capacitor
- 8 ohm speaker.  I had a few surplus speakers, but none of them were good enough.  I finally used a quality one that's loud, Jameco Part no. 135589.

- Pushbutton, momentary on.  One of these: http://www.radioshack.com/product/index.jsp?productId=2062539
- 2.2k resistor

- LED.  I used red.  This is optional but adds a nice touch.
- 330 ohm resistor

- On/off switch.  This is spliced into one line from the battery box to the Arduino.

After testing the circuit, I built the LM386 amp on a small Radio Shack project board, and included the connections for the button as well.  This was to save space in the project box.

Step 2: Program the Arduino

A little background and credit where credit is due...

This project started when "joshua17ss2" on Halloweenforum.com asked if there was a circuit to make geiger counter clicking sounds.  After some discussion, "Hooked on Scares" (who knows his stuff and has his own controller board at http://www.ohmmygadgets.com/) suggested some code.  I discussed this with a friend of mine, who is a network software engineer, and he got intrigued by the problem.  After a couple of weeks of coding in his off time, we had the result I've got here.  Of course, when he did it, he said, "What's this Arduino stuff?" and coded it in straight C for the Atmega chip.  After Halloween, I spent time over a few nights wrapping my head around what he'd done and porting the code to the Arduino environment, which turned out to only be a few changes. 

If you want to run this on an Atmega chip without the Arduino bootloader, you can find the changes needed in the comments.

So, program the Arduino with this code, hook it up to the LM386 amp, speaker, and the button, and you'll be good to go.

Attachments

Step 3: Assemble It

These days, a geiger counter is a small square box.  But we wanted something a bit more interesting (and threatening), so I made this look like one of the older models with a handle.  Do a Google image search for "geiger counter" if you want examples.

Parts needed:

- Project box.  The one I had on hand is 5 1/2" long, 3 1/4" wide and 2 1/2" high.  Made by Serpac, I got several at Jameco.
- 1/2" PVC parts.  From the hardware store.  There are two end caps, two 90 degree elbows, and some straight parts.
- Lid from a can of spray paint, used for the sensor area (the speaker is hidden here).

This was pretty simple, I got it all done in an afternoon.  We were in a rush to complete a bunch of stuff for Halloween, so this isn't as neat as it could be (you can see hot glue here and there, and the holes in the sensor aren't as straight as I'd like, and they could've been cleaned up a bit), but it was used at night and no one noticed or cared.

Assemble the handle from the PVC tubing.  Drill holes in both end caps, one to mount the button and the other for the wire to come out inside the box.  Solder long wires onto the button.  After spray-painting the handle black, thread the wires through the hole in the cap, screw the button in place, put the cap on the end of the handle, then thread the wires through the rest of the handle.

Drill a hole in the project box for the PVC tube, push the end of the handle through the hole, thread the wires through the other end cap, and use the second end cap to hold the handle in place.  Hot glue around the base of the handle where it meets the project box will keep it from slipping.

For the sensor, cut the lid of the spray paint can down so it's shorter (this is 3/4" tall), then drill a bunch of holes in it in a pattern.  I also spray painted it silver.  When you figure out where the sensor will mount, drill a hole for the speaker wire behind it.  Hot glue the speaker in place in the lid, thread the wires from the speaker through the hole in the body, and hot glue the sensor assembly in place.

Drill holes for the on/off switch and the LED.  Wire them up and mount them to the lid.

Finally, tape the battery box into the bottom, connect up all the wires to the Arduino and the LM386 amp, and close it up.

Step 4: Here's the Result

Step 5: Update, 5/22/2013

A friend let me know about this very cheap, very tiny, audio amplifier board.  It's based on the PAM8403 chip and can easily be found if you search for "PAM8403 Super Mini Digital Amplifier Board" or "Mini Digital Power Amplifier Board 3W+3W DC AMP".  It costs less than $5.00 and can be found as low as $3.25.

You can use it instead of the LM386 board, and it will take less time to make, and will be louder.  The top photo is a closeup, and you can see how small it is really is next to an USQuarter on the breadboard.

To hook it up, power goes to 5+ and GND in the center (the GND next to L+), the speaker goes to R+ and R- on one end, and then the sound from the Arduino connects to RIN on the other end of the board.  The board supports stereo (and is pretty good at it!) but for this use, we only need one channel.  I found no difference in this application if I connected the GND next to the RIN to the ground of the circuit.

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