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item3source code (github)
PCB (top)
PCB (bottom)

it was designed using through hole components, on a single layer PCB board. It features the Atmel 328p microcontroller, as the central processing unit, that runs the firmware code. There's a slot to mount a SBM-20 Geiger tube, a connector for the Ethernet module and one for the Nokia 5110 LCD screen. Both the LCD and the Ethernet adapter can be removed, allowing you to configure the final device: make that a portable dosimeter, a monitoring station or both. A speaker provides audible signals, including clicks and alarm, and a push button permits user interaction with the software. The bottom part of the PCB can be used to mount a battery holder for two alkaline batteries in series, or the unit can be powered using the DC connector, via a LM317 regulator. The entire board runs on 3V, and the high voltage inverter boosts that up to 380V (configurable in software up to 500V). The inverter uses a ferrite core transformer, connected to a switching transistor controlled via PWM signal by the microcontroller. An ADC port reads the high voltage level via a resistive divider, and adjusts the duty cycle so that the output voltage matches the value configured in the software. Therefore this is an efficient regulated high voltage supply.

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PocketMagic DIY Dosimeter Geiger Counter

(original here)