After a fair amount of designing and redesigning, I decided to make my geiger counter look like an old-timey cathedral radio with a detachable wand to check various sundries for radiation. It was a bit more ambitious of a project than I realized at first but I think the results were well worth the effort.
Step 1: Materials
Nixie tubes, drivers, and power supply
-electronic components to build a power supply for said Geiger tube
-various electronics tools, such as a multimeter, alligator clips, soldering iron, etc.
-an oscilloscope is helpful
12 V wall wart/power supply
multiposition selection switch + knob
curly phone cable + 2 jacks
clear tube to use as a wand for the geiger tube
-router and router bit
Money for laser cutting
Step 2: Get the Nixie Tubes Going
Ogi Lumen has an arduino library written for controlling the tubes which you can download from their website. This makes testing your assembly easy enough and you can have some fun with the nixie tubes before you get everything else working too.
Step 3: Get the Geiger Tube Working
Quick tip, I found that fuse clips, if bent out a little, were just right to hold the geiger tube and make electrical connection to its ends.
Testing the voltage can be a little tricky. Most multimeters don't have enough output impedance to be able to read the voltage from this circuit without dropping the voltage substantially. I put ten 10 megaohm resistors in series with the multimeter to act as a voltage divider, and then multiplied the voltage I read by the appropriate amount. It's not the most precise way of doing things, but it gets the job done.
Once the high voltage is going, you'll want to test that it's actually counting as well. An oscilloscope comes in pretty handy here, but you may need a radiation source of some kind so that you get a high enough count rate for the trigger.
I've drawn up a schematic in gEDA and added it. A couple of notes: the three 0.01 uF capacitors in the voltage doubler section and the 47 pF capacitor should have a voltage rating greater than 500 volts and probably about 1000 volts. Also, the MPSA42 transistor was chosen for its high voltage rating; a regular old 2N3904 definitely won't cut it there.
Step 4: Put All the Electronics Together
The output from the geiger tube goes to pin 2 on the Arduino. There is an interrupt attached to this pin, which picks up a falling edge and triggers that as a count. I added a little piezo so that I would get a nice click for every count.
It's nice to have everything connected and working together, but it's no time to rest on your laurels, there's plenty more to do.
Step 5: Move the Geiger Circuit to Its Wand
The wand itself was a plastic tube that came full of bay leaves or oregano or some such. I liked the clear tube so I made a few modifications to suit my purposes. There's some slots cut in the end, over the geiger tube itself, in order to increase the acceptance. You'd be surprised how much radiation a few millimeters of plastic will absorb. There's also a slot cut into the lid so that the perfboard can poke through and the jack can mount onto that.
I used one of those curly phone cords to carry the signal in to the Arduino. Pay attention to the wiring of this cable; when I put everything back together it took me quite a while to figure out that I had the lines from the phone cord mixed up. Also, those cords have tiny plastic clips, make sure to get the right size jacks for them. 4P4C RJ-11 is the appropriate designation I think, but it's been a while since I bought those parts.
Step 6: The Enclosure
Step 7: The Laser Cut Parts
Ponoko takes vector graphics files; I made mine in Inkscape. If you're not familiar with Inkscape, I highly recommend it. There's a handful of good tutorials out there that will teach you to use it. There are two files for this, one is for the light colored wood (bamboo) and one is the dark wood (walnut veneered mdf). The light files aren't entirely up to date; a couple holes need to be added for the switch, the knob, and the jack. Also, the dark inset pieces on the design are a little too big to fit without some adjustment. I may get around to amending it one of these days.
I added the design files I used as a zip file. They are a little hard to read since the laser cutter needs a very thin line of a specific color. You can "select all" and increase the line size to get a better look at it, but be sure to check ponoko.com for the latest specifications before submitting them for cutting.
It takes a few weeks to have everything laser cut and shipped to you, so you could probably send off for these first and complete all the electronics while you're waiting.
Step 8: Assembling the Structure
First, I glued the walnut inserts to the bamboo front. The I glued the walnut frame to the front piece. I glued the struts to the middle support next, then the struts to the front assembly, and then I glued the back on. As a backing for the central design, I found an interesting piece of perforated copper sheet. I cut it to size with tin snips and glued a few tabs in to hold it in place.
Step 9: Gluing Down the Veneer
What I ended up doing was clamping everything down and gluing just a little each day. I worked around the edge, moving the clamps each time and gluing a few more inches. It took about a week total.
After it was glued down, I just trimmed the excess with an exacto knife.
Step 10: The Base
Step 11: Put It All Together
The knob on the front is to select modes. It just passes a high signal to a pin on the arduino depending on its position. There's a mode that just increments for each count, and a mode that estimates the counts per minute. I intend to add another mode or two eventually, since there's some space left on the selector.
The switch routes power from the input to all the parts. Input power is from a 12 VDC wall wart. The nixie supply takes 12 V and the rest of the electronics, which runs on 5 V, draws little enough current that the Arduino's linear regulator can handle it.
It's somewhat of a rat's nest inside after everything is connected together, but it works!
Step 12: Detect Radiation
Here's a video showing it just counting background radiation:
and here's a short demonstration: