 full the current flow stops, and the charger stops, which turns the red light green. Battery voltage can be constantly seen on display because I connected the battery to the Arduino’s analog-to-digital converter. This is the main difference to original diy-geigercounter kit.
After I got the charging and the power supply working a new problem came up. I could not get the Arduino detected when I connected the USB to the computer. I changed all my wiring and connectors, even bought new Arduino boards. It was infuriating. Nothing helped and whenever the Arduino was powered by an external source and USB was connected, the computer would just show the “Unrecognized device”-popup. If I removed the Arduino from the board, and connected it separately, it worked. Only after de-soldering the board twice, and trying three different Arduinos I found out that fault was in Arduino itself! All boards, clones and genuine, have same thing. FTDI FT232 USB to Serial bridges datasheet notes that the “TEST” pin must be connected to ground, or the USB will not work. Well, someone designing the original Arduino had not noticed that, and the “TEST” pin was floating freely, not connected to anything. If there was a charge that would pull the signal up, the USB would not work. After connecting that signal pin to ground everything started to work. So much work wasted when fault was already there in bought factory-made parts. Still… I’m happy that it was so simple to fix and I could get the whole device to work properly.
After there was no problems with other circuitry, I continued to the high voltage supply. Geiger-Müller tubes need high voltage to work, because ionization and avalanche would not happen with lower voltages. The tube I used needs approximately 400V to work. Good thing is that GM tubes are not that picky, because they work as long as the voltage is inside the plateau region. It can have an impact on the results received, but just marginally. Getting the voltage right is difficult as it cannot be measured with ordinary multimeter. The 10Mohm impedance of the meter is too low, and it lowers the voltage down. I get only about 80V while the tube is working when a meter is not connected. I just started without a voltage meter, and turned the voltage up until I started to get results. Then I tweaked it just little bit up and left it there. Later it proved to actually be on the high side, as I got 0.2µSv/h when the nominal background radiation level was 0.109µSv/h measured by STUK (they have background radiation measurement network on every major city and village here in Finland).
Building switch mode power supplies is always a bit tricky. They work on higher frequencies where stray capacitances and magnetic fields start to affect. I bought some parts from a Finnish electronics webshop, and soldered them in, and got high voltage on the first try! It just wasn’t steady. The voltage was fluctuating up and down, and every time I tried to close the case the meter would go crazy. Even without the tube connected it would show huge amount of counts and piezo speaker would ring constantly. Yet again I desoldered everything.
For the third revision of the high voltage supply I changed some parts. The potentiometer I used previously had an overly high value, 50Kohm when the range for high voltage is not even 100ohm. The adjustment range with that one was so narrow that even breathing towards to it could change its value. I replaced it with 2Kohm 20 turn one that could be adjusted much more precisely. Good thing that I had those lying around. Also, the inductor that induces high voltage spikes was replaced to one that had a better ferrite core. Lastly, I changed the diode, not because it was not working, but because I somehow misplaced it during my build process. I still haven’t found it, and I believe it may had have something to do with The Borrowers.
This version worked and I got my second counts (I had previously tested the GM tube using 500V from my insulation tester). It was the first time that the meter could work on its own, at least.
At this 