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 (mikrocontroller.net)

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).

Z1A

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"...