RadMon Plus Geiger Kit counts without GM tube

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3 years 7 months ago - 3 years 7 months ago #3916 by zurich
For testing purposes I have connected a 5 Megaohm resistor instead of the tube.
And look, the kit keeps on going and count.
How can that be?
Is that the same with you?
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Last edit: 3 years 7 months ago by zurich.

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3 years 7 months ago #3920 by pylon
What else did you expect?

The tube normally isolates the high voltage. If radiation passes the tube it activates kind of tunnel for the electrons, so the tube gets conductive for a short time. This activates an interrupt on the controlling microprocessor. As the connection is made over a capacitor (C6 in your case) that generates an interrupt from time to time if you connect the tube pins by a high resistance.

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3 years 7 months ago #3921 by zurich
Interrupt with tube, ok? Without tube and with resistor nok!

If there is connect the tube pins by a high resistance, there is a constant voltage across the resistor.
C6 is connected to an input of the mcu.
Why is there an interrupt without a tube and what intervals do you think?

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3 years 7 months ago #3922 by pylon
Why do you expect the circuit to work with a 5MΩ resistor instead of the tube?

The high voltage generator doesn't provide a continuous high voltage signal and some spikes in this signal may trigger the transistor that will trigger the interrupt (I found only the schematics of the Geiger Kit Plus but I guess the radmon kit is similar). The circuit doesn't work correctly if you replace the tube by a resistor.

Did you measure the high voltage by scope? Do you have a constant voltage? You cannot measure that with a multimeter and expect to see alternating values if the signal has a frequency of over 3kHz.

What do you want to show by connecting the resistor instead of the tube?

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3 years 7 months ago - 3 years 7 months ago #3925 by mw0uzo

For testing purposes I have connected a 5 Megaohm resistor instead of the tube.
And look, the kit keeps on going and count.
How can that be?
Is that the same with you?


I took a look at the circuit diagram to see what might happen.
So with no tube connected, you should get 0 counts?

With a 5M resistor in place of the tube, C7 will slowly charge and when the voltage across R9 >0.7V, Q2 will switch on and the interrupt pin will be held low. Not a short pulse duration.
My guess, is that with it held low for a long time, a case not found in typical operation, the firmware interrupt count routine does not function quite correctly (maybe a timer overflows or something) and some spurious counts are registered.
John the designer is on the forums, he'll see this post at some point and will be able to tell you what is happening. Personally I would not worry about it, as the software in the uC had been developed looking for a wide range of fast pulses, not a stuck interrupt line.
Last edit: 3 years 7 months ago by mw0uzo.

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3 years 6 months ago #3936 by zurich
Why am I asking about the behavior? My GK does not work anymore.
Now I try to find the mistake. Is it the tube, is it the electronics?
How can you test the circuit without a tube?
That's why I would like to compare with your results.

Meanwhile I know more. Now you can test the circuit without a tube!
You can measure the voltage at the cathode of D2 against ground with a DVM or a scope.

Without a tube, there are no interrupts but you have to use the right resistor.
If you look with a scope, the voltage is calm and constant, there are definitely no interrupts!

The error is not measurable but in the end it was the diode D2.
Because of the defective diode the internal resistor was too big and the tube did not count anymore.

Thank you pylon for explaining how the tube works but usually interrupts are definitely not with the right resistor instead of tube.
And thank you mw0uzo for your explanation.

And if john reads that, respect you are the best ;-)

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