The high voltage output from this stage is regulated to 500 volts needed for our GM tube, *GMT-01 (LND 712) by three zener diodes stacked one on top of the other (D6, D7 and D8). Diodes D7 and D8 are 200V zener diodes and diode D6 is a 100-Volt zener. Together (200 + 200 + 100 = 500), they equal 500 volts. Five hundred volts is the optimum operating voltage for our GM Tube.
The 500-volt regulated output is fed to the anode of the LND 712 GM tube through a current limiting 10 mega-ohm resistor R16. The 10 mega-ohm resistor limits the current through the GM tube and helps quench the avalanched ionization when a radioactive particle is detected.
*To regulate to the 400 volts needed for the GMT-02 Tube, a jumper is placed on P10. THis jumps the 100-volt zener diode at D6. R16 becomes a 2.2 mega-ohm resistor when using the GMT-02.
The cathode of the tube is connected to a 5.1V (D2) Zener diode. The voltage pulse across D2 generated by the detection of radiation, feeds to the base of a 2N3904 NPN transistor.
The NPN transistor clamps the output pulse from the GM tube to Vcc and feeds it to a comparator gate on the LM339. The pulse signal from the gate, pin 14 of the LM339, is a trigger to the 555 Timer through Q4. The timer is set up in monostable mode that stretches out the pulse received on its trigger. The output pulse from the timer flashes the LED and outputs an audible click to the speaker via pin 3.