Arduino Geiger Counter

I have found somewhere the following scheme for an arduino based geiger counter. (The arduino main purpose is to count the pulses from the tube) As far as I can understand it, the transistor would amplify the small current coming from the tube and the arduino would then detect a low logic level when a ionizing radiation is passing. My doubt is about the capacitor: why is it there? I have tried to build the circuit simulating the passage of radiation by pressing a button. I tried to remove the cap from the circuit too, and yet the detection is noticeably more accurate with the capacitor in. I guess it is because the capacitor is like a filter for "false pulses".Can someone please explain me the role C1?

• I would guess the capacitor lengthens the low pulse because it has to charge back up after each pulse. Aug 1, 2018 at 23:07
• if you want to sumulate radiation by pressing a button connect a full size microswitch with NO connected to C and NC connected to A, leave Common unconnected. Aug 2, 2018 at 6:48

It's there to stretch the pulses into something the arduino can see.

AVR microcontrollers can't reliably detect pulses shorter than one clock period.

the capscitor in combination with the pull-up resistor will stetch the pulses out to ballpark 100 microseconds which should be easily detectable.

C1 acts as an integrator. The current out of the geiger tube is amplified by Q1, then the collector current is integrated on C1. The current is a pretty sharp pulse of short duration so it will look like a voltage step. The voltage step then exponentially decays back up to the baseline (~5V) with the time constant C1*Rpullup (~103 ns).

The capacitor works as an output voltage stabilizer: a random short "firing" of the transistor doesn't discharge the capacitor much, and hence doesn't register as pulse.

• Why do you call it "voltage stabilizer" if the effect you describe usually called "filter". Or low-pass filter, to be precise. I understand that the two could be the same in some context, but somehow this does not feel to be the case. Aug 1, 2018 at 23:26
• Well, it's a filter, but but very much in the sense you'd usually use it. You could call it a noise suppressor; it builds a pulse energy threshold in conjunction with the gpio voltage threshold. There's many names you can call it by Aug 1, 2018 at 23:30
• what a wonderfully non-linear circuit Aug 2, 2018 at 4:20
• well yeah, GM tubes have negative resistance. the circuits fine if you want to count individual events. it;s not designed to drive a pointer. Aug 2, 2018 at 6:44