Got the Geiger Counter finished. The tricky part with building a Geiger Counter is that the Geiger-Müller tube requires a high voltage across it. Whenever a Beta- or Gamma particle enters the tube, it shorts out for a tiny fraction of a second. The high voltage is fed through a large resistor, and when the tube shorts, the particle creates a tiny voltage drop across the resistor.
Home-made geiger counter with tube at the top and HT board at the bottom. The board was printed on plastic foil, and a photo-sensitive print was then exposed and etched to create the circuit board. I know, the soldering leaves a bit to be desired, but I was trying out various component values to make the counter work correctly.
The tricky part with the high voltage was solved using the KU-3294V high voltage generator. This circuit board came out of a broken LCD display that used a fluorescent backlight. With 5 volts in, it generates about 500 volts out in this case. Two 200V Zeners pulls the voltage down to the 400V that is needed for the tube. Furthermore, the detection pulse is formed and fed into the good old SN74121 monostable multivibrator with schmitt-trigger input. This is a brilliant circuit, and makes it really easy to generate nice TTL square pulses on the output when a radioactive particle is encountered. I am not simply hooking up a speaker to get the classic geiger ticking sound – I want to interface this with a computer so that I can make calculations. More about those plans in the next post.