A walk-in cooler that drifts warm overnight ruins whatever is inside it, and the grower usually finds out at the worst possible time: the next morning, with the product already gone. This config is the smallest thing that stands watch: a Bluetooth thermometer, a few-dollar board, and a place for the reading to land, so a slow drift becomes an alert instead of a loss.
The need.
What you want to know: is the walk-in staying cold, and can I prove it?
Commercial refrigeration has its own thermostat and its own alarms, and those stay in place; this config does not replace them. What it adds is a second, independent witness: something that watches continuously and keeps a record that survives even if the network, a hub, or a laptop in the office goes down. For an operation with compliance requirements, that record is the point. For everyone else, it is the difference between finding out at 6 a.m. and finding out three days later.
The solution.
When you are done, you will have a temperature record that never sleeps: a reading from inside the cooler, taken continuously, landing in a place you own. That is what this build delivers, and it is all this build delivers; the sensor gathers, nothing more. The dashboard that shows the week at a glance and the text that says "the door is open" come from your endpoint, Home Assistant or any endpoint you choose, reading the record this build keeps feeding it. The page for the endpoint side is Connectivity; this page builds the witness.
The config: the parts.
| Part | Job | Cost |
|---|---|---|
| A BLE temperature sensor | Reads the cooler. A Govee or similar Bluetooth thermometer. | $10–20 |
| A small board (ESP32 family) | Runs the BLE Sensor Listener sketch; hears the sensor and pushes the reading on. | $5–10 |
| A USB power supply | Keeps the board running. A phone charger works. | $5–10 |
| A data USB cable | To flash the board and, if left plugged in, to power it. | $3–8 |
Every row links to the full comparison in the Hardware catalog. Already own a Bluetooth thermometer or a spare ESP32? Use it; the sketch does not care where the parts came from.
The sketch and the build.
One sketch runs this whole config: the BLE Sensor Listener. Flash it from your browser (no software to install), place the Bluetooth sensor in the cooler where the product actually sits, not by the door, and point the listener at the endpoint you chose below. No wiring, no soldering, and the board never touches the sensor; it only listens for the reading the sensor is already broadcasting.
This is the no-solder path. Prefer to wire a probe straight into the cooler instead of listening for a Bluetooth sensor? The Hardware Build Configs library has a wired version of this same watch (a waterproof probe run through a small sealed hole, ESPHome firmware), along with a cellular variant for a cooler with no reliable Wi-Fi at all. Same need, same shape, a different way to get there.
Where the reading goes.
The listener pushes to whatever endpoint you point it at: your own server, a cloud service, or Open Agriculture Technology's live demonstration while you are testing. Connectivity compares the options side by side, and the data standard is the packet it sends. Because the board pushes straight to that endpoint, the reading keeps arriving even if a hub on the same network goes down; the cooler is the thing you actually care about watching, and this config watches it independent of everything else running at the farm.
Same need, other paths.
This is the no-solder answer. The same question has two more, depending on your site:
Both live in the Hardware Build Configs library; search the build's name there.
Variations.
More than one cooler. One listener can hear several nearby Bluetooth sensors at once; give each a clear name during setup and one endpoint can show all of them.
A door sensor. Add a BLE or Zigbee door contact in the same cooler; a door left open is the single most common cause of a warm cooler, and it is worth catching before the temperature moves at all.
A moving truck. A cooler on wheels cannot rely on the farm's Wi-Fi. See the "Walk-in freezer fail alarm" config in Hardware Build Configs for the cellular-connected version of this same watch, or browse Cellular for the parts on their own.
Frequently asked questions.
What is the cheapest way to monitor a walk-in cooler?
A Bluetooth temperature sensor (around $10 to $20) and a few-dollar ESP32 board running the BLE Sensor Listener sketch. The board hears the sensor and pushes the reading to an endpoint you choose, so you get an alert if the cooler drifts warm, for roughly $25 to $50 total and about an hour of setup.
Does a cold-chain monitor need Home Assistant?
No. This config pushes straight from the sensor to an endpoint you choose, with no hub in between. Home Assistant is a fine place to send the reading if you already run one, but the config works completely without it, which matters if the point is a record that survives a hub going down.
How do I get an alert if my cooler gets too warm?
Point the BLE Sensor Listener at an endpoint that supports alerting, your own server, a cloud service, or a system like Home Assistant, and set a threshold there. The sketch's job is only to get the reading to that endpoint reliably; the alert rule lives at the endpoint you chose.
Can a DIY temperature logger be used for food safety records?
It can capture and store the readings, which are the raw material of a compliance record, but whether those records satisfy a specific rule depends on the sampling frequency, retention period, and calibration documentation the regulation requires. Those requirements are met at the endpoint that stores the data, not by the sensor itself, so check the applicable rule first and configure logging and retention to match it.