Geiger counter

Radiation measurement device w/ Raspberry Pi Pico and STS-5 tube

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The basic technical concept ("boost converter") for this microprocessor controlled radiation measurement device is copied from biemster's project.

Albeit having the virtue of simplicity there are some conceptual drawbacks, for there is no control loop for the tube HV (that will be addressed here).

I use the Raspberry Pi Pico MCU for prototypes, so connecting the device to the Internet will briefly be discussed yet is beyond the project scope (for the "classical" Pico has no IoT capabilities per se); this feature was not really necessary for me anyway.

1. Prototypes

Status as of December 16, 2022

With a commonplace 16x2 LCD and a buzzer (on-board LED GP25 also used). LCDs are not well-suited for battery operation though (those with backlight), but they are comparatively cheap.

PCB Rev 3.1 w/ HV control loop (sort of final). The (chopped) humming is caused by the coil.

2. G-M tubes

In this project I use(d) two different G-M tube models: STS-5 and Z1A.

For registering pulses by the counting system proper working conditions have to be established. That means in practical terms generating a tube voltage within the Plateau area.

Beyond the "Knee" all pulses are (or should be) counted. Below the starting voltage, no pulses are counted at all.

SBM-20 (or STS-5, Cyrillic CTC-5)

As far as I know old USSR stock. Can detect only Beta and Gamma radiation (for Alpha you need expensive special design tubes). Technical data:

SBM-20_GER1.pdf (

For the tubes you are seeing on the pictures I paid around 30€, including shipment (bought one from a seller in Bulgaria, and one from an online store in Lithuania; one can also try to get cheaper ones in Ukraine, they are not so easy to obtain at the moment for apparent reasons).


Origin unclear. Far less sensitive than SBM-20 (and probably most other counting tubes because of its small size), but okay for first tests and easily available. Likewise, can detect solely Beta and Gamma radiation.

3. Emitters

To test the general viability of the assembly I used small pieces of Uranium glass that can be bought in online shops (for obvious reasons this matter doesn't radiate intensely).

Natural radiation is detected about 20 CPM with the most recent prototypes and STS-5 tube (in 49°46' N, 11°12' E), but there's (inherent) randomness in the measured data.

4. Program development

C was the programming language of my choice. 

Necessary for the HV-generation is a PWM with e.g. a frequency of 1 kHz and a duty cycle of 60% (output here on GP3, see next paragraph).

As we have the Pico MCU connected there are several ways to handle measurement data. Triggering IRQs is probably the most pragmatic way for a pulse (gas discharge, i.e. counting event) has a duration of about 0.3 ms.

A radiation source nearby can either be shown visually in a simple manner (flashing LEDs) or via clicks (buzzer), which are connected to one or more GPIOs. 

For a more sophisticated display of data an off-the-shelf subassembly can be used. The Pico SDK makes common types of displays, e.g. a 16x2 LCD or a 0.96'' OLED, fairly easy to use - keeping in mind here that an LCD/OLED connected to the I2C bus may or may not have its own pull-up resistors.

5. Schematic

Diode D1 prevents immediate discharge of capacitor C1 and tube voltage is building up quickly. The coil voltage peaks into the double, then triple digits every time the transistor cuts off (see next paragraph). 

Upper limit of the PWM frequency is a few kHz: around 2 kHz the HV is starting to drop considerably (see "Note 3").

The original minimalist configuration is now (PCB Rev. 3.x) endowed with a control loop for the tube HV.

Note 1: T2 MPSA42 changed to T2 MPSA44, see comment section

Note 2: values for R1, R5, C2, C3 can be varied

Note 3: tests suggest that UF4007 is superior to 1N4007 within the context of this design (UF4007 better suited for higher frequencies)

6. Simulation

Though a boost converter is neither a new concept nor very original (element values can be copied from other projects), it may be worthwhile to play around with a simulation (screenshots of the tool "MapleSim" below).

Note the exponential function build-up of the tube voltage.

7. EMC 

Air wirings act like antennae and must be avoided for more advanced prototypes. Ground planes for PCBs are arguably advisable (minimizing EMI/RFI emissions), but they have to be properly designed (creeping...

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Drawing 9V battery holder

Adobe Portable Document Format - 77.25 kB - 11/24/2022 at 13:21


Buzzer module (active).pdf

Data sheet Buzzer.

Adobe Portable Document Format - 526.54 kB - 06/25/2022 at 18:52



Data sheet Diode. Reverse Recovery Time: ca. 2µs

Adobe Portable Document Format - 158.71 kB - 05/02/2022 at 05:18



Data sheet HV transistor

Adobe Portable Document Format - 121.79 kB - 04/14/2022 at 13:48



Data sheet transistor

Adobe Portable Document Format - 183.00 kB - 03/04/2022 at 06:30


  • 1 × Raspberry Pi Pico MCU/µC
  • 1 × MPSA44 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 1 × BC337 BJT; similar type possible
  • 2 × Resistor 240 Ohm Or higher, value not so critical
  • 1 × Resistor 100 kOhm

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tormozedison wrote 05/24/2022 at 19:22 point

The counter on some of your photos is not SBM-20, but STS-5, an older version with similar specifications.

  Are you sure? yes | no

Florian Wilhelm Dirnberger wrote 05/25/2022 at 04:06 point

Ah, you are apparently right. Then the seller from ebay kinda cheated me :-D. I will add a note on this project page.

  Are you sure? yes | no

tormozedison wrote 05/25/2022 at 06:26 point

I won't call it cheating, STS-5 is more vintage and therefore rare.

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Nazwa wrote 04/17/2022 at 17:22 point

this will be working with radioactive @home?

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biemster wrote 04/07/2022 at 12:43 point

Nice! I've been looking to revive the project you've based this on, so if you find improvements over that previous schematic please add that to the logs!

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Florian Wilhelm Dirnberger wrote 04/07/2022 at 12:49 point

Hi :) hope your project gets en route a little boost in attention as well.

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Florian Wilhelm Dirnberger wrote 04/04/2022 at 15:22 point

Thanks Alan for your comprehensive answer. You spotted a flaw in the design that wasn't really obvious to me. I'll try out different transistors in future prototypes.

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agp.cooper wrote 03/31/2022 at 04:36 point

Does not work that way. Your particular transistor is breaking down at 400v.

Providing the power is low, it should not hurt the transistor.

Better if you used a voltage doubler or a transformer.


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Florian Wilhelm Dirnberger wrote 04/02/2022 at 17:25 point

What exactly is the problem with the µC/transistor combination for I can effortlessly diminish or increase voltage by changing duty cycle and frequency via SW.

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agp.cooper wrote 04/04/2022 at 12:32 point

Hi Florian,

I had a closer look at the CE breakdown voltage and I see it is rated at 1mA. This means the transistor parameter is for avalanche breakdown mode, so yes the zero current (peak) breakdown would be higher, as you have found.

So what is the problem? Basically you are operating the transistor in an area of its operating envelop that has not been defined.

Often a MOSFET is used in this application. One reason is that the breakdown voltage is better defined and the maximum "inductive power" the transistor can absorb is stated. For an IRF470 it is 500v and 30 mJ. So providing you dissipate the power there is problem operating in avalanche breakdown.

So at some point above the 300v the MPAS42 transistor will go into avalanche breakdown mode. Your circuit will need to limit the power to some unknown amount.

That is why I don't like this type of circuit.

If you use a diode double then avalanche  breakdown will not be a problem. 

Regards Alan

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agp.cooper wrote 03/26/2022 at 10:34 point

I doubt you will get 400v to 500v from this circuit as required for a geiger tube to operate.

The breakdown voltage of the MPSA42 is only 300v. 

Circuit simulators do not always model transistor breakdown voltage. 

  Are you sure? yes | no

Florian Wilhelm Dirnberger wrote 03/26/2022 at 10:55 point

Yes you are correct. MPSA44 would be better but wasn't available for my first prototype. Next prototype will have different transistor.

Edit 03.04.: Carried out a further measurement. MPSA42 works even with >400V since that voltage is actually (i. e. steady-state) present on the cathode of the diode, not on the anode. I am gauging some 400V, what is the voltage's under limit in any case (oscilloscope with probe has 10 MOhm impedance). 

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