Update October 21, 2012: Design V2 is obsolete and has been superseded by V3. It will work, but it contains several mistakes not consistent with Geiger Tube theory.
This geiger counter circuit features a regulated high voltage supply, with adjustable output. It can run on a single AA battery (1.5V). It is a clicker meaning that will indicate using an audio signal each detection event: a radiation particle passing through the detector tube will ionize the gases inside and trigger a very small discharge, that will be signaled by the electronic circuit composing the clicker. The video shows several geiger tubes tested with this device:
BG-08, G21H, SI22G, SBM19, SI14G
Circuit diagram v2.0:
SCH File (ExpressSCH): geiger_clicker_2
You can view the previous article on DIY geiger counters here.