IoT Geiger Counter with ESP32, OLED Display and Thingspeak Channel

Measures radioactive gamma and beta radiation with already quite good resolution at the typical low levels of the background radiation (due to the big STS-6 tube)
Displays the current counts per minute (CPM), estimated dose equivalent rate in micro-Sievert per hour (µS/h) and 10 minutes history with 5 second resolution as bargraph
Low-power mode for use with batteries, OLED display and click sounds on, WiFi off
WiFi mode for thingspeak data upload every minute, see https://thingspeak.com/channels/758223

Feel free to use this project as a base for your own projects AT YOUR OWN RISK!

Hardware

Russian Geiger counter tube STS-6 at 400 V with ~5 MΩ series resistor
High voltage circuit from ArnoR at mikrocontroller.net https://www.mikrocontroller.net/topic/380666, schematic https://www.mikrocontroller.net/attachment/273334/HystereseStepUpTLC555-2.png. Keep the high-voltage capacitor small to avoid dangerous charges! For safety avoid any contact with the high voltage parts!
~400 µs pulse generation using Geiger tube series resistors as voltage divider, high impedance input to darlington transistor and TLC 555 timer for rectangle pulse generation
ESP32 board Wemos Lolin32
OLED 128x64 with controller SH1106 at I2C
Voltage supply from either USB or power supply or 3x 1.5V AAA batteries, circuit works from 3.0 V up to 5.0 V
Pulse input is expected at GPIO 18 (high pulses with at least about 250 µs length)
Switch for WiFi mode is expected at GPIO 4 (low=WiFi mode, high=low-power mode)
OLED I2C bus is expected at GPIO 22 (SCK) and 21 (SDA)

PlatformIO project using Arduino library, ESP-IDF for sleep functions and U8g2 for display output
Low-power mode uses light sleep, a wake-up for each signal pulse change and a wake-up every 1000 ms to update pulse statistics and OLED. This results in about 90% sleep. Could be improved using deep sleep and ULP. However, light sleep is already quite good and much easier.
WiFi mode uses no sleep and simple interrupts for pulse counting. Pulse statistics and OLED are updated every 1000 ms, data is sent to thingspeak every 60 s.
Credentials (WiFi SSID, password, thingspeak channel key) are only declared in credentials.h and must be defined in a credentials.cpp