Geiger circuit working

Having completed the oscilloscope kit, and with my car away at the garage for its MoT, Ive had time to finish the PCB for the Geiger counter.

This is now complete and working. Some slight alterations will be needed when I receive the case for it to go into, as the LED and loudspeaker will need positioning on the case, and proper battery connections, with a power switch, will be needed. I have also still to decide on how best to give a 'pulse out' connection for external equipment.

The case will need holes for the LED and the loudspeaker, and also for the HV ON neon indicator. It will also need a slot cutting along the tube side to allow the most radiation to enter. The boxes are on their way from China, so I guess I will get those when I get them!

Ive also now made the first use of the DSO138 oscilloscope in anger! Using it to look at the oscillator waveform and the cathode pulses from the Geiger-Müller tube.

Monitoring the flyback converter oscillator

I can see from this little gadget that the oscillator is pulsing low every 40ms, but with some seemingly random positive spikes. This equates to 25Hz, which seems rather low (I'd expected several kHz), but its not a worry, so long as its generating the HV supply!

A radiation event pulse

Having finally mastered setting the trigger threshold value (right hand triangle on the display), I can now get the 'scope to trigger on each pulse at the G-M tube cathode. With the cathode to ground resistor I have, each pulse is about 1.5V and decays away over a period of around 200uS - this equates well to the expected 190uS dead time of the tube.

So this little £10 oscilloscope is actually quite a handy little device! Its just a shame about the horrid bright white column of duff pixels!

Update - Some Geiger counter circuit measurements...

Pulse height - approx 1.6V; Pulse duration - approx 200uS; Standby current draw - 1.24mA; High count-rate (Radium dial at 5mm from tube) current draw - approx 3.1mA; Background count - 18-20cpm.

I'd like to get the standby current down below 1mA if possible. Thats something to look at in future.