Geiger Detector with LED, SCR without low voltage supply schematic This is a very simple circuit for a Geiger counter that runs without a low voltage supply. It was developed as a stationary background radiation monitor for your house. Since it lacks the need for a low voltage supply, the high voltage can easily be made directly from mains voltage by using a Cockroft-Walton voltage multiplier. A Cockroft-Walton cascade provides only very low currents, but this is not a concern, since the maximum current drawn is in the order of 100s of microamps. More on this later.

The Geiger-Müller tube is connected the usual way - 10megs resistor to the high voltage - the other side with a low value resistor to ground. When the tube fires, a small voltage is generated over R2 - enough to fire the SCR T1. Meanwhile C1 was charged over R4. The thyristor T1 has fired and C1 is discharged over R3 and LED1. The LED flashes. When the discharge current goes below the hold current of T1, the SCR blocks again and C1 is charged again. When the tube fires again, the process begins again.

What if you use another tube, that means you have another high voltage ?
Here are the formulae for the calculation:

Legend:
HV ... high voltage
Iv ... LED current

R2 = (R1 * 1.5 / HV) ... approximation(!)

R3 = HV / Iv ... Iv not lower than 10mA, since the SCR has a hold current of 5mA

NOTE: The SCR (Thyristor) TIC106N is only rated up to 800V, but all other SCRs with higher voltage rating have a much higher hold current, are not in the TO-220 package and cost 10-20 times more than the TIC106N. Sidenote: Development of this circuit was performed with a TIC106D, which is only rated up to 400V and it worked too :-)

How to generate the high voltage out of mains voltage:

Theoretically this circuit draws an approx. maximum of 180 microamps, when the capacitor is not charged (the tube fired just before) and the tube is firing again. Since this is a circuit only for visualizing background radiation, it doesn't matter when this subsequent firing is not indicated. So we assume the maximum current occurs when the capacitor is not charged, so we have about 90 microamps. This can be easily provided by a Cockroft-Walton voltage multiplier.

Any voltage with a CW multiplier is possible, since you can use a zener-diode to make any desired voltage.

Troubleshooting

The LED doesn't flash:
Either R2 too low or R3 too high or both

The LED is on all the time:
You did used 10megs for R4, didn't you ?

The LED constantly flashes:
R2 is too high

Developed by cosmicrays.org. Idea to use a SCR by Doug Moore. Thanks Doug! :-)

Last-Modified: Mon, 05 May 2008 16:50:40 GMT

Be very careful when handling high voltages!
cosmicrays.org cannot be held liable for damage of any sort!

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Geiger Detector with LED, SCR without low voltage supply

How to generate the high voltage out of mains voltage:

Be very careful when handling high voltages!cosmicrays.org cannot be held liable for damage of any sort!

Be very careful when handling high voltages!
cosmicrays.org cannot be held liable for damage of any sort!