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🧊 Freezer-too-warm alarm
“How can I get an alert if my freezer is left open and warms up?”
A waterproof temperature probe sits inside the freezer. The board stays outside, where it's warm and dry, and warns your phone if the temperature stays too high for too long.
What to buy
| Part | Why this one | Rough cost |
|---|---|---|
| ESP32 or ESP32-C3 board | WiFi for the alert. | $4–7 |
| Waterproof DS18B20 probe (metal tip, long cable) | Reads −55 to +125 °C over a single data wire. | $2–4 |
| 4.7 kΩ resistor | The DS18B20 needs this “pull-up” between data and 3.3V. Some probe kits include it. | $0.10 |
| USB power | The board lives outside the freezer. | you likely have one |
Prices are rough ranges, not quotes. Check the current price and stock before you order.
What to install
ESPHome supports the DS18B20 directly. Or use a short Arduino sketch with the OneWire and DallasTemperature libraries. Send the alert with ntfy or Home Assistant.
A good starting rule: a freezer runs at about −18 °C, so alert if it stays above −10 °C for 15 minutes.
Build it in three steps
- Read it on your desk. Wire the probe: power, ground and data (usually red, black and yellow; check your probe's listing), with the resistor between data and 3.3V. Check it reads room temperature, then close to 0 °C in a glass of ice water.
- Put the probe in. Run the thin cable under the door seal near the hinge, and check the door still closes tightly.
- Add the rule and the alert. Alert once when it crosses the line, again every 30 minutes if it stays warm, and once when it's back to normal.
What catches people out
- Defrost cycles and door openings. Many freezers warm up briefly on their own. That's why the rule waits 15 minutes.
- Air changes fast, food slowly. For fewer false alarms, tape the tip to a frozen item or put it in a small container of water, so it follows the food rather than the air.
- Copies. Many cheap DS18B20s are clones. Most work, but test in ice water: it should read close to 0 °C.
- Keep the board out. Electronics and batteries don't like −18 °C, and water condenses on them when they warm up.
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