and the tranformer for audio out is not needed if a piezo beeper is used.
Anyone knows a cheaper Geiger tube ? or an alternative way to detect radiation ?
yes, take a look at this schematic (from sparkfun): http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Sensors/Radiation/Geiger_Counter-v21.pdf
They used audio transformer to generate high voltage, not for audio output....Alternative is semiconductor , "elektor" has published a series of articles, stating last year