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Geiger Counter

Geiger Counter

Humans are only able to sense ionizing radiation indirectly, meaning it's not detected until one starts loosing hair and teeth. Fortunately there are ways to measure radiation directly which is how humans can stay safe around ionizing radiation. The heart of such a device is an evacuated tube, charged with a voltage so high the tube is just below breakdown. When an ionizing particle hits the tube it will pass through it, and as the name suggests, ionize particles along the way. Even if a particle only ionizes a few gas molecules the tube will avalanche, or arc over internally for a brief moment. This avalanche can be detected by an electronic circuit and emitted as a tick, flash or sent further to a digital counter.

FHZ74.  SBM21

Geiger Mueller tubes. Tiny SBM21 at the right, detection range 0.04 - 0.4 r/H. FHZ74 on the left, 0 - 50 r/H.

The circuit presented below consists of a high voltage supply, and means of converting tube avalanches to audible ticks. A small, regulated flyback supplies the high voltage needed. Each Geiger Mueller tube will have it's own "plateau" which is the working voltage range. Voltages higher up the plateau increase radiation sensitivity, voltages lower down do the opposite. I chose 420V since it lays within the plateau of most tubes.


By changing the zener diode values one can adjust the output votlage to whatever is needed by the tube in question. For the main transformer a disposable camera transformer is required, these vary little from camera to camera and all will have the necessary windings. I assume they have a common pin out, but if not experiment until you get the circuit oscillating. The approximate inductance of the windings can be determined by the winding size in the schematic (I know, very crude), so if you have an L/C meter handy finding the pin out shouldn't be too hard. The ingenious flyback supply isn't my design, I just ripped it off from here. Once the tube is energized and a particle hits it, it will short out. This causes a negative potential at the diode side of the 100pF capacitor which turns on the PNP Darlington transistor combo (the two BD140s). Once on they flash the ultra-bright LED and chirp the buzzer. The voltage at diode UF4007 is approx 5-6V. A real darlington transistor could substitute the two BD140s, I only used them because I had some available.

Inside of boxed-up GM coutner Finnished Geiger counter

Update: Leslie kindly donated some antique radium watch hands so I could test my counter. With the SBM21 tube the counter goes mad when the watch hands are held close, and at a few cm distance the count rate is over 100 counts per minute. So the circuit is proven to be functional.

Ra-226 Watch hands

Here's a Youtube video:

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Disclaimer: I do not take responsibility for any injury, death, hurt ego, or other forms of personal damage which may result from recreating these experiments. Projects are merely presented as a source of inspiration, and should only be conducted by responsible individuals, or under the supervision of responsible individuals. It is your own life, so proceed at your own risk! All projects are for noncommercial use only.

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