Geiger Counter

Posted by Circuit Labs on Thursday, November 13, 2008

Author: Malte Fischer

April 2006 was the twentieth anniversary of the Chernobyl reactor accident. In the days following the incident winds deposited much of the reactor contents across central Europe, Scandinavia and the UK. A large area surrounding the reactor is still off limits but just how much of the fallout is still lying around on our gardens and farmlands? At the time of the release Iodine 131 was responsible for many cases of thyroid cancer but with a half life of 8.1 days this does not pose much of a threat in the long term. Strontium 90 is more of a problem; it has a half life of 28 years and more than 50 % of the fallout still remains. Radio caesium affects the food chain and can contaminate milk, meat and to a lesser extent crops. Radioactivity is invisible so a detector is needed before we can start to make any measurements; the most common and simplest detector type is the Geiger counter. The design described here uses two NE555 type timer ICs, a small mains transformer and a few other standard components to make a low cost and simple to build Geiger counter. The only fly in the ointment is likely to be locating a Geiger-Mueller (GM) tube; this is not in any way a standard component. The on-line auction site eBay may provide a source of new or used counter tubes or alternatively Google will be useful in identify stockists. A brand new type ZP1300 tube can be purchased from ALRAD ( at a cost of £55.00 plus VAT. The GM tubes are listed under nuclear products and nucleonics.

Circuit diagram - Click to enlarge

The counter tube requires a high voltage in the region of 700 V. IC1 is a NE555 timer configured as an astable multivibrator switching the BC547C transistor which in turn drives the secondary winding of a 6 V mains transformer. An alternating voltage of around 250 V is produced at the primary side which is then multiplied by the classic voltage multiplier configuration consisting of cascaded capacitors and diodes to produce around 700 V DC. The voltage is fed back to the timer input through resistors R3 and R4 to provide some stability of the output voltage.

The counter tube anode is connected to 700 V via a 10 MΩ protection resistor. In normal operation with no radiation detected there will be no current flowing through the tube and gas filling. When a radiation source is brought close to the tube, ionising radioactive rays pass through it and collide with some gas atoms knocking a few electrons out of their shell; this produces a current pulse from the tube terminal which in turn generates a voltage pulse across the 1 MΩ resistor high enough to trigger IC2. The NE555 effectively amplifies the pulse to produce the familiar tick-tick sound of a Geiger counter from the speaker LS1. An external pulse counter can be connected to K1. A copy of an original Philips Geiger tube data sheet is available at ehler/18550.pdf. Philips tubes are now made by Centronic ( and their site contains a useful Geiger selection guide.

 (Elektor Electronics Magazine – 07/2006)

Download this article (#040291-uk.pdf)


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