[aprssig] Homemade Geiger Counter

[Scott Miller]

> As we approach armagedon, I thought it might
> be nice to have a Geiger counter.

http://n1vg.net/geiger/

> Which, if I recall is just an ionization chamber,
> a voltge charge and an audio amp.  Since Radio

Sort of.  You need a high voltage power supply (typ. 500 or 900 volts), and
a way to stretch the pulses from the tube to an audible length.

> Shack does not have GeigerMueller tubes, do I
> remember correctly that you could even use
> a flourescent bulb for the ionization chamber?

Never heard of that... I don't expect they'd be terribly sensitive.  I've
heard you can use CdS cells with an opaque covering to detect gamma rays,
but I've never tried it.  I've got CdS cells and a ~30,000 count/minute
gamma source... I suppose it wouldn't be hard to test.

LND, Inc. sells new GM tubes of all types... see
http://www.lndinc.com/product.htm.  For cheap tubes, watch eBay for surplus
civil defense equipment.  Most of those are over 30 years old, though, and
LND has some nice small tubes.  Last time I checked with LND, they started
at about $75 each.

I've been thinking about putting some more work into my counter firmware and
adding a serial port for PC-based measurement and random number generation,
and maybe offering a kit.  I'm looking for a more efficient power supply
design at the moment.  The current circuit uses a PWM output from the MCU to
drive the primary of a small transformer (CFL type) through a transistor.
The secondary feeds into a voltage multiplier to get the 900 volt supply for
the tube, and a simple shunt regulator controls the output.  After that,
it's just a matter of detecting the pulses with a JFET and driving the MCU
interrupt input - the rest is software.

An EDN Magazine hit my desk yesterday with a fairly simple, efficient design
for a 180-volt bias supply with no transformer and a linear regulator.  I've
been thinking about extending that design to 500 volts, but it'd take a lot
of Schmitt triggers and diodes.  I've been exploring other options for an
inductor-based DC-DC converter.  If anyone's got a good design for a 500
volt, 50 microamp power supply, let me know.

> Just thought it would be nice to have something
> to detect high energy particles around the house
> or on a satellite, or feeding alerts onto APRS?

Somewhere I read a paper about static RAM being used as a radiation
detector.  They just counted single event upsets in the chip and plotted
each event on a map.  It wasn't terribly sensitive, but over time it very
clearly showed an increase in SEUs at higher latitudes and over the South
Atlantic Anomaly.  For practical purposes, Satellite Toolkit will give you a
good estimate of the average radiation encountered by a satellite in a given
orbit, and you can get information on actual conditions elsewhere.  We've
got a Space Weather office a few blocks from here - they put out reports on
such things, but I'm not sure if that's distributed to the public.

Anyway, just keep in mind that a Geiger counter is going to take up a
certain amount of space in your satellite, and introduces all the fun of
high voltage in close proximity to sensitive electronics.

Now, what I think would be cool is a deployable box-on-a-stick with Geiger
counter, toxic gas sensor, GPS, and transmitter.  Next time you've got a
Three Mile Island incident or something, just go hammer the things in the
ground where you need them and turn them on.

Scott
N1VG