Planning to build a Geiger Counter I bought two Geiger probes, an SBT10A and an SBM20, I also bought a Theremino Geiger Adapter that produces the required voltage for the probe to operate and detects ionizations events that happen in the probe.
Then I built a first prototype with an Arduino and a breadboard; with this I managed to make some first measurements, estimating the radioactivity of the low sodium salt by measuring radiations emitted by the radioisotope (K40) that it contains (Potassium is naturally found in a blend of three isotopes, one of which is radioactive); I also discovered that my flooring is slightly radioactive. After having tested the probe, I soldered all the components on a perfboard (a DC-DC converter step down, to reduce the voltage of the battery, 12V, to 5V, required by the LCD; the high voltage generator for the probe; the probe; a buzzer; an LCD 20×4 HD44780 compatible; an Atmega328; and some buttons). The board can be programmed using the ICSP connector, which is normally used by the buzzer that needs to be unconnected from the board for programming.
The system is powered by a lead-acid 12V 1.2Ah rechargeable battery; voltage is then reduced to 5V by the switching regulator. For now only the SBT10A probe is connected (the largest of the two I have) but I am planning to add the SBM20 as an external probe.
The SBT10A has a good sensitivity and detects also alpha radiation.
I also tried to detect the natural background radiation levels in my room and measured 120-130CPM 1m above the floor.
I also noticed that the probe is subject to interference from low frequency electromagnetic emissions (50Hz): by turning on a light bulb measured values increase noticeably, even though there is no source of ionizing radiation nearby.
I also measured interesting values with the low sodium salt, because of the potassium it contains.
I will soon release the code and schematics of my Geiger counter.
I also built a pocket sized version of the counter using the SBM20 probe. It is about 10 times less sensitive that the other and detects only beta and gamma radiation. It is powered by a 9V Ni-MH rechargeable battery that lasts for about 24h of operation. I will soon release all the details also about the pocket-sized version.
UPDATE 2/2/2017: The source code of the pocket-sized version (called Geigerino) and its schematics are available on GitHub: