currently I'm working on a geiger counter project like many other hobbyists. I started to search the internet and I found a lot of schematics and ideas for this project.
My main goal was to build two counters : one base counter on electricity with a led backlighted display not making any noise and a portable one using as less battery-
power as possible but with clicks. There are many schematics on the internet to generate around 400 volts (or more) to power the popular SBM20 tube.
I liked the diagram Indrek used most because it uses very liitle power ( 0.2 mA @ 9 volts!) and you can measure the HV with a normal 10 MOhm multimeter.
On the other part I used broHogan's idea to use the atmega 328 microcontroller to count the pulses.