So... I've decided to use another Geiger Tube. Instead of using the SI-3BG, which isn't very sensitive, I'll use the classical СБМ-20 (SBM-20), it'll generate more counts, but will need a bigger enclosure.

The first enclosure (which will hold the radioactive sample, shielding plus Geiger tube) is a small plastic box I bought on eBay for about 5 bucks. The tube fits inside nicely!

I'm still working on the 400V power supply. I've already made a first try which includes the clicker (copied directly from Brohogan's design), but I'll chop off that part in the next version, andjust keep the power supply and the microcontroller interface...

That circuit board should be ready by next week!