eter. 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 point I had a fully working Geiger counter which could measure radiation rate and data log values to a computer. I did a short logging test run to see if background radiation would match to STUKs measurements. Of course, it depends very much on the place, as ground has all kind of isotopes of elements, and man-made fallout. As you can see, the average value is a bit high, it should be nearer 0.1µSv/h. I made a slightly longer data logging period to get some more accurate data.
For 12h measurement period I got some weird results. What’s with the sudden peak at the end? The average is two times too high as well. I turned high voltage adjustment a little bit down and values came right down to what was expected. I did not thought that it would affect the reading so much. That rising at the end, could it be due to the sunrise? It starts to rise after clock 7. Maybe it was too sensitive when excessive voltage was applied.
After recording of 24h of data with a lower voltage, I got this. Just what I was expecting. Normal spread that averages to similar values that official levels were at that time. I emailed to my lukio’s (Finnish school) physics teacher and asked if I could compare my readings to the Geiger dose meter of the school.
I got a very similar readings from a uranium-rich ore sample. Notice, how my meter is connected to my Android phone with a USB-OTG cable. Radiation can’t be measured absolutely, so I’m pretty happy with the readings I got.
Closing words. This has been an interesting project that has shown me some important things. I could have done it even better, from build view to project managing. I could have planned it better before starting to make the parts. Doing so, I would have avoided some of the mistakes and the redoing of some parts. Maybe I should have used a real PCB. Also , some would say that making my own firmware and circuit design would have been more meritorious, and some other would have wanted to see more work done together by the hacker and maker community. As an open source project I