Described here is a simple and easy to build Geiger Counter. The Geiger tube used here requires a HT voltage of 360Vdc to 420Vdc, this is provided by a HT generator consisting of a 555 configured as a astable and a small 100V line transformer. The 555 grounds the hot end of T1 secondary at a rate of 1KHz, the primary produces 400Vdc (after voltage doubling by D1 and D2 ) as a result. R2 is used for crude current limiting of the switching waveform.
When the Geiger tube becomes ionised due to a Gamma particle entering the tube, a very brief discharge is produced from the anode to cathode. This results in a very short pulse being produced across R1. This pulse is fed to another 555 configured as a monostable, here the original pulse is effectively lengthened to a pulse that is now long enough to actually be heard by the ear. Tr2 is a simple pulse averaging circuit to drive a meter from the pulses generated across R1. A buffered output from R1 could be used to feed an event counter ( counts per minute ), although I have not bothered with this facility. The meter was very roughly calibrated by feeding a squarewave pulse generator into Tr2 base resistor ( the pulses were differentiated via a 1nF capacitor before entering Tr2 base so it only triggered on the rising edge of the waveform ) and marking the meter scale accordingly, i.e. a frequency of 10Hz represents a reading of 10 counts per second etc. The diode across the meter affords some protection to the movement should it go off scale, however, if this were to happen I doubt if you would be worried about the meter! The circuit has been designed to run off four AA cells ( 6V ) as these are cheap from the local pound store. Current consumption is approximately 100mA. For the interconnecting cable between the PCB and the Geiger tube I used some good quality balanced microphone cable, the screen of the cable is grounded at the PCB end only and serves as a electrostic screen.The insulation on the conductors is sufficient for this application. The tube is fitted into a piece of plastic overflow pipe in order to protect it. The Geiger tube used here was obtained from a Russian seller's shop on Ebay, the tube cost approximately £10 and is new ( unused old stock ). This particular tube is sensitive to Gamma radiation.
To test the Geiger counter a source of Gamma radiation is needed, I used an old ionisation type smoke alarm. The smoke sensor in these has a low level source of Gamma radiation, you will need to hold the Geiger tube very close to the dismantled smoke sensor. Next time we are camping in Cornwall I shall try it out on some of the large granite rocks out of curiosity.
Note : use the LM555CN version of the chip for this application.
Barry Zarucki M0DGQ
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