Geiger counter

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

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The 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 (may be critical with higher counting rates).

I use the Raspberry Pi Pico µC 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 WiFi capabilities per se); this feature was not really necessary for me anyway.

1. Prototypes

Status as of November 04, 2022

With an LCD and a buzzer (on-board LED GP25 also used).

2nd revision PCB (still preliminary, 3rd revision in progress). The coil causes the humming you hear throughout the video.

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 both 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, apparently they are not so easy to obtain at the moment).


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 viability of the assembly I use 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).

4. Program development

A MicroPython program can be quite short if it's just about the PWM- and HV-generation, respectively:

from machine import Pin, PWM
pwm = PWM(Pin(16))
pwm.freq(1250)  # PWM-frequency in Hz (empirical)
pwm.duty_u16(55000)  # duty cycle (empirical), 16bit (0-65535)

However, I used MicroPython only for early ad-hoc tests and switched soon to C.

As we have the Pico µC connected there are many 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) and/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 (conveniently, the Pico SDK makes common types of displays, e.g. an LCD, fairly easy to use).

5. Schematic

Diode prevents immediate discharge of the capacitor and tube voltage is building up quickly. The coil voltage peaks into the double, then triple digits every time the transistor cuts off (see paragraph 6, "Simulation"). 

Upper limit of the PWM frequency is a few kHz: around 2 kHz the HV is starting to drop considerably (I got the hint that the diode is mainly to blame for this).

The original minimalist configuration is now (November 22) endowed with an experimental control loop for the tube HV.

Note 1: MPSA42 changed to MPSA44, see comment section

Note 2: In lieu of the 10k resistor connected to 3.3V, the PAD (internal) pull-up resistor could (or is preferable to ??) be used

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 behave like an antenna and must be avoided for more advanced prototypes. Ground planes for PCBs should be used, but have to be properly designed (creeping current may be a problem).

8. IoT (Internet of Things)

As mentioned in the description, connecting the device to the internet goes beyond the project scope for the "classical"...

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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 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

View all 21 components

  • HV control loop

    Florian Wilhelm Dirnberger11/15/2022 at 06:47 0 comments

    First experimental SW and HW for a HV control loop (see also previous project log).

    Pay attention to the humming sound. It isn't continously anymore, for the capacitor gets charged in short bursts.

    Elevator pitch: Less power consumption.

  • HV measurement

    Florian Wilhelm Dirnberger11/12/2022 at 13:34 2 comments

    Simple HV measure arrangement: consisting of four 82V Zener diodes, one 10 MOhm and one 2.2 MOhm resistor (series connection). These are parallel to the tube Anode and GND.

    The Voltmeter is parallel to the 2.2 MOhm resistor.

    On the picture you see a voltage of some 11V, what means the HV amounts in this case to:

    HVtube = 11 + (11/2.2)*10 + 4*82 = 390V 

    (actually it is slightly higher since the Voltmeter has an internal resistance in the 10 MOhm range what affects the measured value)

    Varying the PWM parameters in the SW would lead to higher and lower voltages, respectively.

    The assembly described above could be the basis for a later control loop design.

    Keeping two things in mind here though:

    • voltage tolerance of Zener diodes is poor
    • as with all semiconductors, the electrical characteristics change with temperature

View all 2 project logs

Enjoy this project?



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?

  Are you sure? yes | no

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!

  Are you sure? yes | no

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.

  Are you sure? yes | no

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.

  Are you sure? yes | no

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

  Are you sure? yes | no

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). 

  Are you sure? yes | no

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