Flash a freeze sensor from your browser

Before you start

1. Wire the sensor: power to 3.3V, ground to GND, data to GPIO 4, with a 4.7kΩ resistor between data and 3.3V (diagram). That's the same for a DS18B20 probe (red, black, yellow wires) and a DHT22; the firmware detects which one is connected.
2. Optional extras, on these pins (each switch or contact goes between the pin and GND):

   Connect	ESP32	ESP32-S3	ESP32-C3
   DS18B20 or DHT22 data	GPIO 4	GPIO 4	GPIO 4
   I²C sensor SDA / SCL (BME280, SHT31, AHT20)	21 / 22	8 / 9	6 / 7
   Leak / water contact	GPIO 27	GPIO 5	GPIO 5
   Door reed switch	GPIO 26	GPIO 6	GPIO 10
   Battery + through a 1:1 divider (2 × 100kΩ)	GPIO 35	GPIO 7	GPIO 3

   I²C sensors are found automatically; tick leak, door, or battery below so the board knows they're wired.

   Adafruit Feather (HUZZAH32 or ESP32 Feather V2): use the ESP32 column. Probe data (GPIO 4) is the pin labelled A5, the door switch (GPIO 26) is A0, and the leak contact goes on pin 27. The battery divider is already on the board: plug a LiPo into the battery connector and tick battery, no resistors needed. An I²C sensor goes on the Feather's own SDA/SCL pins or its STEMMA QT port; the firmware finds it there at startup (power-cycle after plugging one in). Feathers have no BOOT button on GPIO 0, so the 5-second reset won't work; to change Wi-Fi, send new settings from this page instead.

3. Add a push device under Dashboard → Devices and copy its device key. The free plan includes two devices.

What to buy

Parts that match the pins above. A plain ESP32 is the tested choice and does everything here. The S3 and C3 links are Espressif's own dev boards, which the firmware is built for; when they're sold out, another ESP32-S3-DevKitC-1 or ESP32-C3-DevKitM-1 board (or a clone with the same pinout) works too.

Board (pick one)

ESP32 DevKit (Amazon) · HUZZAH32 Feather (Adafruit) · ESP32 Feather V2 (Adafruit)

Or an S3 / C3 board (stock varies)

ESP32-S3-DevKitC-1 (Adafruit) · ESP32-C3-DevKitM-1 (Adafruit)

Sensor on GPIO 4

Waterproof DS18B20 probe (Adafruit) · 4.7kΩ resistors (Amazon)

Or an I²C air sensor (no resistor needed)

SHT31-D (Adafruit) · BME280 (adds air pressure) (Adafruit) · AHT20 (Adafruit)

Optional extras

Magnetic door switch (Adafruit) · 100kΩ resistors (battery divider) (Adafruit)

Leak sensor: the easiest low-cost option is an LM393 "rain / water sensor" module (sold by many shops): VCC to the board's 3.3V pin (never 5V, so its output stays safe for the ESP32), GND to GND, DO to the leak pin. Turn its dial until a drop of water reads Wet, then check it reads Dry again. Its exposed pads corrode if left wet, so treat it as a starter part. For a longer-lasting install, use two stainless screws or probes about 5 mm apart, one to the leak pin and one to GND. Either way, test with your own water: very clean water or a thin film may not trip it. Sensors whose output goes high when wet, such as Adafruit's water sensor, need a different input and don't work with this firmware.

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After it is live

Readings arrive once a minute. Mount the probe against the coldest pipe, set a freeze threshold of 34–38°F under Dashboard → Alerts, and send a test alert. See where to put the sensor and what temperature to alert at. Prefer to edit the code yourself? The other flashing options cover Arduino, PlatformIO, MicroPython, and ESPHome.