ESP32-Geiger Counter

Avalanche32: ESP32-S3 Geiger Counter

A handheld, battery-powered Geiger counter built around the ESP32-S3-WROOM-1 and a Soviet SBM-20 Geiger–Müller tube. It has a firmware-regulated 400 V supply, an audible click for each count, a 2.42" OLED display, and a single USB-C port for both charging and programming.

Status: Rev A, in progress. The perfboard prototype is working: it regulates the tube supply at 370–400 V, counts background radiation, and clearly detects real radioactive sources (see Measurements). The Rev A schematic and PCB layout are complete, and the boards and parts have been ordered. Assembly and testing are next.

Front	Back

Hardware Overview

High-Voltage Supply

• Topology: discontinuous-mode boost converter. The ESP32 drives a UCC27517 gate driver, which switches an STN1HNK60 600 V MOSFET into a 10 mH inductor, with a US1M fast-recovery rectifier.
• Regulation: closed-loop hysteretic control. A 3 × 100 MΩ + 680 kΩ divider (with a 2.2 nF filter cap) feeds the ESP32 ADC, and the firmware gates the boost pulses to hold the setpoint.
• Protection: a 450 V Zener clamp (3 × 150 V in series) on the output, plus a firmware overvoltage shutdown.
• Filtering: a 20 nF / 1 kV reservoir, then a 1 MΩ / 10 nF RC ripple filter, then a 5.1 MΩ anode resistor into the tube.
• Layout: the HV section sits in its own isolated area with a "400V High Voltage" marking on the silkscreen.

GPIO Map (Rev A PCB)

Test points are provided for 3.3 V, GND, PULSE and HV_SENSE.

PCB

Measurements

These readings are from the perfboard prototype, with each sample placed directly against the SBM-20 tube. Dose rates use the firmware's conversion factor of 0.0057 µSv/h per CPM (SBM-20, Cs-137 basis), so they are indicative only.

Contact readings on glazed ceramics are mostly beta from the uranium decay chain, so the µSv/h values overstate the actual dose.

Known Issue: Tube Re-Firing

The current SBM-20 is degraded and sometimes fires a burst of 100–1000+ pulses instead of a single one. The firmware works around this with a burst filter (BURST_FILTER) that merges each burst into one count, at the cost of undercounting at very high rates. A replacement tube is on the way.

Firmware

Firmware/HV_Test/HV_Test.ino is a bench test for the HV supply and counting chain, written for the Arduino ESP32 core. It provides:

• Hysteretic HV regulation (370 V setpoint, 5 V hysteresis) with a 460 V overvoltage shutdown.
• An interrupt-driven pulse counter with the optional burst filter (2.5 ms window).
• A 60-second rolling CPM and an estimated µSv/h.
• A 4 kHz piezo click for each count.
• Serial output at 115200 baud showing HV, boost on-time, total and raw counts, CPM and µSv/h. To change the boost on-time, send a number from 5 to 120 (µs).

Note: this sketch targets the perfboard prototype, which uses a classic ESP32 dev board. Its pin assignments (IO4, IO35, IO27, IO25) do not match the Rev A PCB. Update the constants at the top of the file using the GPIO map before running it on the PCB.

Repository Layout

To regenerate the images (requires KiCad 9+ and, for PNG layer plots, pip install pymupdf):

python scripts/generate_images.py

Roadmap

• HV boost designed and simulated in LTspice
• Perfboard HV prototype with regulation verified at 370–400 V
• SBM-20 background count test (15–25 CPM indoors)
• Source tests: uranium glass, radium-dial watch and uranium-glazed Fiestaware detected (up to ~670× background)
• Bench-test firmware: hysteretic HV control, rolling CPM, µSv/h estimate, piezo click
• Full schematic: power, charger, ESP32-S3, HV supply, detector and display
• Rev A PCB layout and fabrication outputs
• Rev A boards and parts ordered
• Rev A assembly and bring-up
• Application firmware: OLED UI, menus, dose alarm, logging, battery monitoring

Tools

KiCad · LTspice · Arduino (ESP32 core) · C++

Safety

This device generates ~400 V DC. The stored energy is small, but do not touch the HV section while the unit is powered. Before handling the board, let the capacitors discharge: wait for the HV reading to fall to 0 V.

Store radioactive test samples (radium dials, uranium glass, uranium-glazed ceramics) sealed and away from living spaces. Never open radium-dial watches or gauges, and never eat or drink from uranium-glazed dishware.