geiger counter circuit

  
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I bought one CI-22BG tube and one CI-3BG tube for total of 16G. Because I didn`t have anything radioactive to test my Geiger counter, I also ordered a piece of radioactive Fiesta dinnerware from ebay. Red Fiestware used to have uranium oxide in its glaze that was radioactive enough to be detected with a Geiger counter. Geiger tube needs a high voltage, my tubes need 400V, to function.
geiger counter circuit - schematic

We need also a microcontroller to count the events and maybe a LCD to display the output. Also no Geiger counter is complete without a buzzer. I decided to also have USB port now that I included a microcontroller. The device can be powered by USB or external voltage source. Circuit on the upper left corner, which was copied from this Stack Exchange answer, is responsible for choosing the right power supply. It will use USB if it`s connected, otherwise it`ll connect the external power supply. IC1 is step up converter that will increase supply voltage to 7V, or pass it through if it`s already over 7V. This will allow the device to work with even 2V external voltage. IC3 is 5V low dropout regulator, that will regulate 7V to 5V for microcontroller. At the center there`s the high voltage supply that will output 400V from 7V input. It runs in a closed loop with microcontroller adjusting the duty cycle to get a stable output voltage. Normally Geiger tube will pass only a small leakage current, but when ionizing radiation hits the Geiger tube some of the gas inside the tube is ionized which will allow higher current through the tube, 4. 7M anode resistor R10 limits this current to a safe value. This current will raise voltage at the base of T4 turning it on which pulls the detect net low. Then the microcontroller interrupts at the pin change and counts one event. High voltage supply schematic. R18, R19 and C13 are for feedback to...



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