Hi,
After 4 years of continuous use mounted in a project box with a red board and electric imp, my Sparkfun Geiger counter has stopped sending data.
I took it out of the project box and connected it with a USB cable, but I get nothing over the comm port. I also don’t see any flashes in the green LED to indicate clicks.
I have the schematic. I can see that it’s an oscillator feeding a voltage multiplier circuit. but I’m not sure what voltages to expect in the high voltage area or even where best to start checking things.
Does anyone have any suggestions on trouble-shooting?
Thanks,
Gaston
Hi Gaston.
It’s possible air may have leaked into the Geiger tube, that would prevent the tube from detecting alpha and beta particles. If it’s a bad tube, the only thing you can do is replace it. Unfortunately we don’t carry Geiger tubes anymore, but you can sometimes find them surplus on eBay.
Voltages for the tube are usually between 200 and 400 volts.
Thank you, Chris. I’ll measure the voltage. That’s helpful.
I don’t have a high voltage probe or high impedance voltmeter. From the comments in the Random Numbers with a Geiger counter tutorial, it looks like a 10 Meg ohm meter should read above 100V, as a 10Meg voltmeter is a voltage divider with the 10Meg resistors in the circuit.
I just wanted to do a sanity check before pronouncing it dead. Many thanks.
Gaston
Hi,
The high voltage measured zero, actually around 0.5 V. The voltage multiplier appears to be okay. The diodes and caps seem to be good. But I don’t think the oscillator is working.
Working backwards, it looks like VCC is being held low by something shorting to ground. I get a good +5v on the one side of the HV DPDT Power Switch, but when I turn the switch on, VCC is only brought up to about 0.3 to 0.5v. (I was hoping the fuse F1 was blown, but I would expect +5v at the switch itself if I was that lucky)
Looking at the schematic I think one of the oscillator transistors may be shorted. I would check Q1 first, but that’s an surface mount component, and I really don’t have the components or tools to remove and replace SMD’s.
At this point I wonder if I should just salvage the Geiger tube and some of the PTH components. If someone has any suggestions or advice, I’d be grateful.
Thanks,
Gaston
It’s likely the transistor Q1 has failed shorted, it would be getting hot when you turn Vcc on. You can remove it and see if the short on 5V goes away.
If Q1 tests OK then check Q3 or maybe capacitor C2 has gone open. I would just replace the transistor. It’s not too bad to remove a SOT-23 part.
Thank you.
For the record, I think Q1 did fail shorted. It was getting hot, and it failed when I tested its junctions with an ohmmeter.
Rather than replacing it, I went ahead and harvested the Geiger Tube, 1:1 Audio Xformer and 1kv Caps. I hope to use them in a future project.