I recently have built smiple Geiger counter based on cheap STS-5 tube, high voltage power supply from China and ATmega168 with LCD. Currently, I'm facing a problem with counting number of pulses from tube. I connected the output of simple Darlington amplifier to ext_int pin of ATmega168 and it sometimes does not count properly. I get 3-4 pulses instead of 1. I do not own scope or anything so I cannot measure output. I also tried to collect pulses from tube cathode and adding additional filtering capacitor to HV supply output with same results. I do not suspect code to be a cause of incorrect counting, I checked it with basic TTL pulse generator and it works perfectly fine. Can anybody help me with this circuit? Also, my HV power supply is this module: Link
I'd clamp the base of Q1 to +5 and to GND. And bias the Darlington to almost-on, using the 4.7Meg and 1Meg suggested by the editor.
To clamp, simply hang diodes (installed to be reverse-biased) from the base to +5 (thus the diode "bar" to +5v), and from the base to Ground (thus the diode "bar" to the base.
Try it with considerable high resistors. E.g. 4.7 MΩ to +5V and 1 MΩ to GND.