Build a DIY Geiger Counter with a Pancake Tube and ESP32

A Geiger–Muller (GM) tube-based radiation detector is a genuinely useful piece of test equipment for anyone who works with thoriated tungsten welding electrodes, vintage orange Fiestaware, camping lantern mantles, certain older smoke detectors, or antique clock/watch dials with radium paint — all common sources of low-level radioactivity that a hobbyist might reasonably encounter. This build pairs a pancake-style GM tube with an ESP32 to log counts per minute, convert to an approximate dose rate, and optionally push readings to a web dashboard over WiFi. It's also simply a fun, satisfying electronics build that touches high-voltage circuit design, pulse counting, and radiation physics fundamentals.

How It Works

A pancake GM tube is a sealed low-pressure gas tube with a thin mica window that lets in alpha, beta, and gamma radiation. Each ionizing particle that enters triggers a brief avalanche discharge across the tube, which needs roughly 400–500V DC to sustain (pancake tubes typically run lower voltage than the tall pencil-style tubes, which often need 900V+ — check your specific tube's datasheet before applying any voltage). Each discharge produces a short voltage pulse that a simple pulse-shaping circuit converts into a clean digital pulse the ESP32 can count on an interrupt pin. Counts per minute (CPM) is the raw measurement; converting to a dose rate (usually μSv/h) requires a tube-specific conversion factor supplied by the tube manufacturer, since sensitivity varies significantly between tube models and radiation type.

Circuit Overview

StageFunction High-voltage boost moduleSteps up 5V or battery voltage to the 400-500V the tube requires — use a purpose-built GM tube HV module rather than improvising one, since this stage has real shock-hazard voltage present Current-limiting resistorA high-value resistor (typically 5-10MΩ) in series with the tube's anode limits avalanche current and protects the tube from damage Pulse-shaping / coupling circuitCouples the tube's brief high-voltage pulse through a small capacitor to a low-voltage transistor or op-amp stage that produces a clean 3.3V logic pulse ESP32 interrupt inputCounts pulses on a GPIO configured for a falling or rising edge interrupt; software debounces and accumulates counts over a rolling window Display / loggingA small OLED or LCD for local CPM/dose readout, plus optional WiFi push to a web dashboard, MQTT broker, or Home Assistant

Assembly Notes

1. Mount the pancake tube in a 3D printed or laser-cut holder that protects the fragile mica window while leaving it exposed to air — do not cover the window with anything solid, since alpha particles are stopped by even a sheet of paper.
2. Build or wire the high-voltage module in a fully enclosed section of the case, physically separated from any part a user might touch during normal operation.
3. Keep the high-voltage anode lead as short as practical and route it away from the ESP32 and any low-voltage signal wiring to minimize noise pickup.
4. Flash firmware that debounces pulses in software (a minimum dead-time window, typically a few hundred microseconds) to avoid double-counting a single avalanche event.
5. Calibrate CPM-to-dose conversion using your specific tube's datasheet conversion factor — do not use a generic online conversion factor, since it varies meaningfully between tube models.

Safety

This build involves a genuine high-voltage circuit (400V+) that can deliver a painful, though not typically lethal, shock if touched directly — treat the HV section with the same respect as any mains-adjacent circuit: fully enclose it, use heat-shrink or conformal coating on exposed solder joints, and never probe it live with bare hands. On the radiation side, a properly functioning Geiger counter is a detection and measurement tool, not a shield — it tells you a source is present, it does not protect you from it. If you're using this to check a suspected source (old radium dial, thoriated electrode, etc.), keep exposure time brief, keep distance where practical, and do not store found radioactive items in living spaces; many local hazardous waste programs will accept them for proper disposal.

Once built, this becomes a handy piece of shop test equipment — a quick way to check whether that box of vintage camera lenses (many older lenses used thoriated glass) or estate-sale Fiestaware is actually hot, and a genuinely educational project in high-voltage design and pulse electronics along the way.