Update October 21, 2012: This design is obsolete. It will work, but it contains several mistakes. See the newer design: V3

Because the recent events in Fukushima, Japan, brought a lot of concern on radiation levels, by popular demand, here is a simple Geiger Muller counter using the Russian tube SI-22G and a single transistor regulated inverter that puts out 400V, as required. The complete setup has been assembled in only 5 minutes, so this is a very simple detector, but can prove useful because of the excellent sensitivity:

The radiation is indicated by a led and a small speaker. Each pulse (flashing led and the clicking sound) represents a radiation quantum passing through the tube.
To exemplify I've used a small source of radiation, the Eii-43-100 Spark gap radioactive tube, containing less than 0.9uC of Cs137, emitting aprox. 1uSv/h of gamma radiation. The detector reacts promptly to the presence of the Spark Gap tube, the frequency of clicks increases, much over the background level.
Circuit diagram:

This design can be used with a large variety of Geiger tubes, but the output voltage and the limiting resistor might need to be adjusted accordingly. For the SI-22G, the parameters are:
Recommended Operating Voltage (volts) 390
Initial voltage (volts) 285-335
Plateau length (volts) at least 100
Maximum Plateau Slope (%/1 volts) 1.25
Gamma Sensitivity Co60 (cps/mR/hr) 540 (sbm20 has 78)!
Inherent counter background (cps) 1.3
Tube Capacitance (pf) 10
Resistance (meg ohm) 9-13
Maximum Length (mm) 220
Maximum Diameter (mm) 19

To get an idea on a correlation between the clicks and the real radiation dose, I've placed the excellent Radex RD1706 dosimeter close to the homemade geiger setup and the radioactive spark gap:

The radiation level detected corresponds to 0.82uSv/h. Here are some pictures:

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