This is a newer version. The high voltage is much more reliable. (and got its own website) A serial communication (I2C) has been added as well. The voltage for the geiger tube is displayed on the lcd, so adjustement can easily be repeated, when the tube should be changed. The tube is connected via an BNC connector, so different types may be exchanged quickly. Just five wire-bridges are necessary to allow for a single sided pcb. Power consumption is about 200 mA at 12 V DC. As this device is intended for environment monitoring, the nagging loudspeaker has been replaced by a flashing led. The design uses an Atmega8.

The firmware was developped with Codevision. It uses the Interrupt 1 (PD3) to count the pulses. The timebase is formed by Timer 0 which overflows each 25 ms. Depending on the tube used, you may want to used different counting intervals to get a more or less meaningful number. We use two variables to hold the counts. One is used for the actual counts, whereas the second holds the counts of the full (last) timescale (e.g. 1 minute up to 30 minutes).

The device is that flexible, that different tubes may be used. On one hand, the high voltage is continously measured and displayed, so quick adjustement is easy - on the other hand, the tubes are mounted on a bnc plug, which allows an easy exchange. Different tubes have different dose rate ranges (sensitivities). We use a ZP-1300 and a ZP-1320 because they could easily be ordered from Distrelec/Schuricht.

Name	α	β	γ	Threshold	Background	Dead Time	Dose Rate Range	Cost
ZP-1300		•	•	500 V	1 cpm	11 µs	10-1 - 104	68 €
ZP-1310		•	•	500 V	2 cpm	15 µs	10-2 - 103	?
ZP-1320		•	•	500 V	12 cpm	45 µs	10-3 - 102	100 €

Overview over some easily obtaineable tubes.

When you start with this "hobby" i.e. build your first counter, it is motivating to see a lot of pulses, beeing assured that your circuit (and software) works fine. Often you start with a "bargain" tube, which produces only very few pulses (e.g. 1 per minute). This may be frustrating when comparing with other designs from the net, where high count-rates are presented. With the Geiger Selection Guide (see Downloads) you may now guess from the background radiation which tube other homebrewers used :-)

The lcd displays "CPM" which is counts per minute. It is the number of atoms in a given quantity of radioactive material that are detected to have decayed in one minute.
This number can be converted into µS/hr or any onther unit with the help of a calibration curve which is specific to every tube and material used. As we use the Geiger Counter for environmental monitoring, we are interested in overall behaviour and therefore a calibration - which anyway is just a linear scaling - makes no sense. In case you have only cesium in your lab then a calibration with a cesium standard mayst be considered.

Overlay of approx. 100 pulses shows good reproducibility. The pulse length is ~ 100 µs. Bottom trace is at the output of the comparator. Tube used is a ZP-1300 (500 V).

After having assembled this thing, downloaded the firmware, the question arises : How can one test it ? Yes, shortening the bnc socket will cause a count, so you know the microprocessor does count. Waiting very long time will also count (background radiation). But what material is easy to get and still emitting enough radiation to make an impressive youtube video ???

For a very first test we borrowed some TIG welding electrodes. Some of them are radioactive because they contain thorium (~ 2%). Says MightyOhm and my colleague from the workshop. That's why they have a red head. Sample : Pack of 10 pcs.

Tube in use	Bias [V]	Background [cpm]	With Sample [cpm]	Leff [mm]
ZP-1300	501	0.85	38.716	3.2
ZP-713	500	6.35	169.76	27.9

Overview over measured decays per minute. Counting interval : 60 minutes.

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