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+---
+title: HAT Geiger Counter
+date-published: 2026-04-22
+type: sensor
+standard: global
+board: esp32
+project-url: https://github.com/homeautomationthings/HAT-Geiger
+made-for-esphome: True
+difficulty: 1
+---
+
+## Product Images
+
+![HAT Geiger Counter mounted outdoors](device-outdoor.png "Installed outdoors")
+![HAT Geiger Counter internals](device-internals.png "Internal components")
+
+## Overview
+
+The HAT Geiger Counter is an outdoor-rated ionizing radiation monitor built on the ESP32-C3.
+It measures radiation in CPM and µSv/hr using a J321 Geiger tube, integrates natively with
+Home Assistant via ESPHome, and optionally uploads readings to [radmon.org](https://radmon.org)
+for public radiation monitoring.
+
+The device ships pre-flashed with ESPHome firmware and can be adopted directly from the
+ESPHome dashboard or provisioned via BLE (ESPHome app) or USB-C (improv_serial).
+
+## Features
+
+- Measures ionizing radiation in **CPM** and **µSv/hr** using a J321 Geiger tube
+- Native **Home Assistant** integration via ESPHome native API
+- **Outdoor rated** — designed for permanent external installation
+- Optional radiation upload to **radmon.org** every 65 seconds
+- Wi-Fi provisioning via **BLE** (ESPHome app) and **USB-C** (improv_serial)
+- ESPHome **dashboard adoption** support
+- Fallback Wi-Fi hotspot and captive portal
+- Safe mode recovery via boot button
+- Power via **USB-C** or **6–36V DC** screw terminal
+
+## Hardware
+
+| Component    | Details                        |
+|--------------|--------------------------------|
+| MCU          | ESP32-C3 (ESP32-C3-WROOM-02U) |
+| Geiger tube  | J321                           |
+| Power input  | USB-C or 6–36V DC screw terminal |
+
+## Getting Started
+
+1. Power the device via USB-C or the screw terminal (6–36V DC)
+2. Provision Wi-Fi using the ESPHome app (BLE) or a USB-C cable (improv_serial)
+3. Adopt the device in Home Assistant via the ESPHome integration
+4. Optionally configure your radmon.org username and password from Home Assistant