1. Prototypes

Current status as of May 16, 2022

Geiger counter LCD version.

(video with subtitles)

Status as of May 1, 2022

Geiger counter Speaker version.

(video with subtitles)

2. G-M Tubes

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 should be counted. Below the starting voltage, no pulses should be counted at all.

SBM-20

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

SBM-20_GER1.pdf (mikrocontroller.net)

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).

Z1A

Far less sensitive than SBM-20 (or any other tube available), but okay for first tests and easily available in Germany. Can detect only Beta and Gamma radiation. 

Below copy&paste some of the tube's technical data (according to the distributor www.pollin.de; as of March 2022, the price was 15€):

• starting voltage 280…320 V 
• working voltage 390 +/-10 V 
• dimensions (length*diameter): ca. 65*7.5 mm

3. Emitters

To test the viability of the assembly I use small pieces of Uranium glass that can freely be purchased (for obvious reasons this matter doesn't radiate intensely).

Natural radiation is detected about 20 cpm with the most recent prototype and SBM-20 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(13))  # pick your GPIO
pwm.freq(1250)  # PWM-frequency in Hz (empirical)
pwm.duty_u16(55000)  # duty cycle (empirical), 16bit (0-65535)

C code would be a little longer here, but by and large it's equally easy.

As to first prototypes, a radiation source nearby is either displayed visually in a simple manner (5mm LED) and/or via clicks (speaker), realised thru an "outer electronic path" (see paragraph "Schematic"). 

But as we have a µC connected there exist more clever ways to handle measurement data. Triggering an IRQ for that matter is probably the most pragmatic way for a pulse (gas discharge, i.e. counting event) has a duration of about 0.3 ms (in my github repository you'll find complete working examples).

The Pico SDK also provides libraries for the most common types of displays (OLED, LCD).

5. Schematic

VBUS = +5V (Pico Pin 40). Using the 3.3V from Pin 36 would be the 2nd option here.  

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"). 

An LED is good for a first test. I suggest putting in a 10 kOhm resistor in the "outer path" when you want to do (connect) something more fancy (but consider connecting 3.3V to this path then).

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 to blame for this).

6. Simulation

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

7. EMC 

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

8. Misc

The background photo of this project page shows a former Uranium mine in Příbram, Czech Republic.