Recently we completed work on Cosmic Array sound and IoT using the new Raspberry Pi Zero W. But the addition of a Raspberry Pi and the audio amplifier caused the detector to trigger more often than it should.   At first, it didn't seem to be an extra load on the 5V regulator. Nor the Geiger-Muller tubes after a check with a radioactive check source and DSO.    

Then I remembered! I cut back on components to simplified the circuit design and cut costs.  One of the components I removed was a 4V7 Zener diode on the output of the 10pf coupling capacitor connecting the Geiger-Muller tube to the 555 monostable oscillator trigger pin 2.   At the time of the redesign, I forgot the real reason for the Zener just assuming it was there for voltage protection between the high voltage and low voltage sections.   

The 555 timer acts as both a Schmitt trigger to shape the negative pulse from Geiger-Muller tube to a +5VTTL and increases the pulse width.  Pin 2 of the 555 is the trigger for the monostable oscillator when it detects ground travelling pulse.  So Pin2 is held at the supply rail voltage by a 47k resistor so when the high impedance negative pulse from the Geiger-Muller tube it triggers.  

The trouble came when both the Raspberry Pi and the audio amplifier where added to the 5V supply rail that supplies the 555 circuits. Although the quiescent state supply rail measured 5V after coincidence is detected, the Pi plays a wave file and then the Audio amplifier causes ripple currents the trigger the other 555 monostable oscillators causing them to trigger randomly resulting in more ripple currents.  

The solution is to hold pin 2 at 4.7V with a Zener diode and the problem goes away.   The new boards are also designed with a higher current regulator further reducing this effect.