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 Geiger_schematic

  • \$\begingroup\$ When Q2 is off, how many volts will COUNTER be? Consider DC operating point. When Q2 is on, what is COUNTER? The forward voltage of a yellow LED is approximately 2.2v, which is not providing a clean digital output to COUNTER. \$\endgroup\$
    – rdtsc
    Apr 21 '20 at 11:56
  • \$\begingroup\$ There is no base bias for Q1 and events will feed it negative going pulses, possibly approaching -200V. How exactly is this amplifier intended to work and where did you find this design? \$\endgroup\$ Apr 21 '20 at 12:19

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.

  • \$\begingroup\$ I tried that and it's not working. It seems like there is some kind of weird oscillations when radiation pass through tube. But I really don't know why. \$\endgroup\$
    – WIwanowicz
    May 7 '20 at 11:30

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