Many people are afraid of radioactivity and nuclear power plants due to articles in the mainstream media and the constant fear mongering of anti-nuclear organizations. The fact that you can't see, hear, taste or smell nuclear radiation and distrust of the goverment tends to worsen this fear and discomfort with the general population.

Building a radiation meter/monitor yourself lowers the risk of getting tainted/biased/wrong/no data, especially when you really need it. When in the aftermath of the Chernobyl disaster, radioactive clouds reached the Netherlands and Belgium around May 2nd, 1986, no action was taken by the Belgian authorities. Weatherman Armand Pien was even not allowed to mention the radiation from the radioactive cloud on television during his weather forecast. While there were restrictions on selling fresh leafy vegetables and a grazing ban in the Netherlands, it was just sunny weather in Belgium.

The design presented here uses a Russian SBM-20 geiger müller tube which is sensitive to gamma and beta radiation. Electrical impulses from the tubes are counted and converted to a radiation level by an ESP32-devkitC module which sends the data to IOT platforms like ThingSpeak and OpenSenseMap at regular intervals. In addition, the radiation levels are also shown on a LC display.

The high voltage power supply is built around a 7555 based step up converter followed by a high voltage cascade. The output voltage is adjustable between approximately 250 and 550 V which covers several types of GM tubes. By replacing some components, an output voltage up to 1.1 KV is possible. At first we had a flyback converter in mind, but due to the declining popularity of CCFL backlights for LCDs, it becomes increasingly more difficult to find commercially available transformers which are suitable for higher voltages.
As the feedback loop is connected to the output of the cascade, the power dissipated by the feedback resistors, lowers the efficiency of the circuit significantly but this is not a real problem as the unit is not battery powered. Even with the ESP32-devkitC and the LCD backlight LEDs the total average power consumption stays below 1 W.

We designed the PCB for the well known and easily available Russian SBM-20 geiger müller tube. Other tubes like the SI-3BG and the SI-1G can be used as well. It is possible to connect a second external GM tube to the PCB. You can only use tubes with similar operating voltages (plateau voltage range) when using 2 different types simultaneously.

For testing purposes a visual and audible radiation indicator in the form of an LED and a buzzer is available. As the pulses from the GM tubes are very short (microseconds), a pulse stretching network built around a 4039 quad nand gate was added. This could also be done in software but we decided to reserve the computing power of the ESP32 for more important tasks. After all, with higher levels of radiation, the SBM-20 easily generates hundreds of pulses per second.

Unless you live near an uranium mine or a nuclear disaster area, natural background radiation levels are generally quite low with little variation. Monitoring background radiation can become boring after a while. To make things a little bit more exciting, the system can be expanded with extra sensors via the RJ45 jacks. Currently we are thinking about the following sensors for future expansion boards:
 

• Sensirion SPS30 particulate matter sensor (not the cheapest but it has a very long lifetime of 8 years continuous operation)
• VEML6070 UV sensor
• SPEC Sensors 110-406 ozone sensor

As such, the system becomes a "weather station" for environmental parameters.
For indoor use, sensors for temperature, humidity, barometric pressure, VOCs, CO2, ... can also be added, but that is up to the end user.

A final warning: the generated high voltage on the GM tubes is not dangerous and the current is limited by high voltage resistors R30 and R31 to a very low level which you will probably not feel at all. However, the high voltage on other parts of the circuit is potentially dangerous. It is not lethal but a shocking and very painful experience is guaranteed, so be very careful !