Help with geiger counter circuit

Thread Starter


Joined Jan 15, 2022

I'm very new, so please forgive me if I'm not descriptive enough. I'm building a geiger counter from here:
It's based around a Wemos D1 mini.
I have my circuit assembled short of the geiger tube (SBM-20 on the circuit diagram). I powered up my circuit and unfortunately my battery polarity was backwards. The solder on the U5 linear regulator immediately melted. After replacing it, I was able to turn it on and programmed my board. I'm supposed to be getting approximately 400v at the positive side of where the SMB-20 connects and I'm only getting a fluctuating voltage between around .5vdc and 1vdc. What could be the issue with my circuit? I've checked the polarity on all the components and they all seem to be correct. The only part I'm unsure about is my 555 timer. It doesn't have the usual notch or indent on it anywhere. It does seem to have a line printed across one end that I've taken to assume is the end with the gnd and and supply voltage

Any help would be appreciated.
GC-20 Schematic.jpg


Joined Aug 21, 2008
Some ideas:

Check the power supply voltage.

Double check orientation of the semiconductors.

If you have a 'scope check for base pulses on Q1.

If R5 is set to too high a value you will not be able to get much of an output because the circuit will quickly go into current limit.

Are you sure you are using a TLC555? A bipolar NE555 may not work well in this circuit.

If all of the above looks ok, then make sure L1 has continuity and it not one of those small radial parts that are intended for small signal use and saturate easily.

Also, Q1, the transistor that boosts the voltage is only rated at 400V, try not to let the collector voltage go higher than that.

Thread Starter


Joined Jan 15, 2022
Thanks for your help.

I definitely have a tcl555
There was some discrepancy on the D2 diode between this schematic and the BOM from the author.
The schematic calls for this (the same as D1):
Mfr.#: RS1JF
1.3V@1A 5μA@600V 250ns Single 1A -55℃~+150℃@(Tj) 600V SMAF Diodes - Fast Recovery Rectifiers ROHS
the BOM lists this part (this is what I used for D2):
Mfr.#: RS2AF
1.3V@2A 5μA@50V 150ns Single 2A -55℃~+150℃@(Tj) 50V SMAF Diodes - Fast Recovery Rectifiers ROHS

I just noticed another discrepancy. This schematic lists part#168110 for the power inductor:
Mfr. Part # FNR6045S153KT
60mA 15mH ±10% SMD Power Inductors
The BOM specifies this:
Mfr. Part # FNR6045S103KT
80mA 10mH ±10% 46.68Ω SMD Power Inductors
Unfortunately I wasn't able to get the above part, I'm using this instead:
Mfr.#: YNR8065-103M
10mH ±20% 22.8Ω SMD,8x8x6.5mm Power Inductors

I don't know enough about this to know what the correct parts should be.

I actually built a second board after posting this and was getting about 60 volts out of it for a short while, and now it's only outputting 1 volt or less. Before I try a third time I'd really like to figure out what went wrong.
I'll look into your suggestions soon. I don't have an oscilloscope unfortunately.


Joined Aug 21, 2008
I don't know where to go from here. I would like to offer the circuit I use that makes a regulated +400V, I know it works because I have been using it for years.

Just one thing, the KSP42 output transistors is delicate. I found a 2N5551 to be much more reliable. Warning: A shock from the capacitors in the multiplier can be very upsetting.

Before making the regulated circuit I was using and NE555 in an open loop power supply.

You can add the multiplier from the regulator circuit to this circuit to get +400V. It uses the more common NE555. The 1 mH choke on the base of the transistor is to provide negative bias to speed turn-off of the transistor, and that choke is not critical and be a small signal type thought the choke in the collector circuit should be good for 100 ma or more without saturating and it should have low resistance. Like the regulated circuit a 2N5551 would be a better choice for the output transistor.

Thread Starter


Joined Jan 15, 2022
D2 must be a high voltage diode - greater than 400V reverse voltage.
Sorry I typed that wrong. my question above was supposed to be be about the D1 diode. Should the D1 also be the 600v diode that's listed on the schematic, or the 50v diode that's specified in the BOM?