Geiger counter with Z1A or SBM-20 tube

Radiation measurement device (with Raspberry Pi Pico for HV generation)

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Being a noob as to building HV devices and radiation measurement devices in particular this project is dedicated to research. So prototypes will have their flaws, but I'll try to keep a log about findings as well as improvements and this page will continuously be updated.

A lot (virtually all) is copied from this project:

Z1A tube

Detects Beta and Gamma radiation. 

Below copy&paste of the Z1A tube technical data (according to the distributor; as of March 2022, the price was 15€):

> starting voltage 280…320 V
> working voltage 390 +/-10 V
> differential resistance 1.25 %/V
> max. discharge voltage >500 V
> max. radiation 15 R/h
> lifespan at 5 R/h > 150 h
> dimensions (length*diameter): ca. 65*7.5 mm

(note: I have no clear idea what exact physical unit is "R/h"; I presume it is sort of weighted radiant power)

SBM-20 tube

More sensitive. Can also detect only Beta and Gamma radiation. Technical data:

SBM-20_GER1.pdf (

I paid around 30€ including shipment from a seller in Bulgaria (tubes from Ukraine or Russia are not easy to get at the moment, you guess why).


To test the assembly I use Uranium glass that is freely available (for obvious reasons, this matter doesn't radiate intensely).


The code can be quite short if it's just for the HV-generation:

from machine import Pin, PWM
import utime
pwm = PWM(Pin(13))  # pick your GPIO
pwm.freq(1250)  # PWM-frequency (empirical)
pwm.duty_u16(55000)  # duty cycle (empirical), 16bit (0-65535)

For now, a radiation source is either displayed visually (LED) or via clicks (speaker). But as we have a µC connected, there exist more clever ways to process the measurement data.


VBUS = 5V (Pico Pin 40). Diode prevents immediate discharge of the capacitor and tube voltage is building up fast. 

The LED is okay for a first test. Put in a 10 kOhm resistor if you don't use a LED. 


The air wirings behave like an antenna and should be avoided for future prototypes. A ground plane for a PCB design is mandatory.


Gave some insights about the principle of the voltage generation (self-induction) and will be used for ensuing designs. 

Ideas and improvements

  • Counting logic
  • MQTT
  • Speaker instead of LED (done)
  • Opto-coupler
  • PCB for better EMC behaviour
  • ...


Data sheet

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


  • 1 × Raspberry Pi Pico µC
  • 1 × Z1A counting tube Counting tube 390V
  • 1 × MPSA44 Discrete Semiconductors / Transistors, MOSFETs, FETs, IGBTs
  • 1 × BC337 BJT
  • 1 × Resistor 240 Ohm Base resistor

View all 15 components

View all 4 project logs

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biemster wrote 5 days ago 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 5 days ago 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.


  Are you sure? yes | no

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. 

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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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