The $12 freezer alarm
The cheapest build on this site, and the one that pays for everything else the first time it fires.
What it is
A microcontroller and a waterproof temperature probe taped inside the gasket of a chest freezer, reporting to a box in the house. When the temperature climbs past a setpoint for longer than a few minutes, your phone makes a noise — on your own network, with the internet unplugged.
The numbers
- ESP32 board: $5–8. DS18B20 probe: $3–5. That's the build.
- A full chest freezer of beef, pork, and a season of berries is $600 to $2,000 of food.
- A tripped breaker, a bumped plug, or a lid left ajar takes about 18 to 30 hours to ruin all of it, and all of it happens quietly.
- Alert threshold that works in practice: above 10°F for 30 minutes. Short spikes are just somebody getting ice cream.
Why it's here
Everybody who has lost a freezer remembers exactly what it smelled like. This is the build that converts people — it takes one weekend, costs twelve dollars, and the first time it catches a plug that got bumped by a shop vac, the argument is over.
The automation angle
This *is* the automation. Build this one first and you've already built the skeleton every other sensor on the place will hang on.
The catch
A temperature probe only reports; it doesn't plug the cord back in. And an alarm you can't hear at 3 a.m. isn't an alarm — set this one to the loud tier and leave it there. It is one of the very few alerts that always earns the interruption.
In the guide
Free sample chapter at autofarmation.com — the whole build, start to finish. Autofarmation →