This post shows the audio circuit portion of the radiation detector in the video, for the simple high voltage power supply portion of the circuit click here

Quick overview:
Q1/Q2 form a one shot that stretches the pulse generated by the Geiger tube output that is coupled by the 1nF 1KV capacitor. The pulse coming from the Geiger tube is very narrow and would not produce a nice sounding click if we just amplified it.

Q3/Q4 form the audio amplifier, the 500 ohm potentiometer connected to the collector of Q4 is for adjusting the volume of the clicks. The 8 Ohm speaker can be any speaker. I used a speaker from a cellular phone ringer and I have used one from an old PC case.

There is an optional meter out that can be connected to an analog meter circuit that will average the counts using a rc circuit (to be discussed later if there is interest).

There is an optional pulse output that will put out a pulse that will pull-down (go from 3V to 0V back to 3V) for every click that can be used for connecting to a micro-controller so that a counts per second display can be used. This output can be utilized if the logic level NMOS and R8 are populated (to be discussed later if there is interest). Also if you do not populate the R8 resistor you can use the fet as a level translator, for instance if you are using a micro-controller that needs 5v logic levels. Connect the fet drain to the micro-controller line with a 5V pull-up resistor and you have an instant level translator.

For a full size image click this image, then click image on the following page:

This is the circuit for the audio board of the Geiger counter in this video