Assessed as Utility
Freezer Temperature Alarm
Assessed From Published Builds
A temperature probe inside the freezer, reporting to Home Assistant, which raises an alarm when the compartment stays too warm for a sustained period. The probe goes through the door seal; the electronics stay outside in room air.
Before you order anything
- Hardest part
- Network and Home Assistant Configuration. A running Home Assistant instance, an ESPHome device adoption, and one automation with a sustained-window trigger rather than an instant one.
- Point of no return
- None in the build. The probe passes through the existing door seal and everything is reversible by unplugging it.
- Waterproof DS18B20 temperature probe
- Adafruit showed out of stock on 11 August 2026. The UK part ships with the 4.7k pull-up resistor fitted; the Adafruit part does not, so a US build needs one separately.
- Reliability bar
- It has to alarm when nobody is watching, including when the mains has dropped and taken the router with it. A freezer alarm that fails silently is worse than no alarm, because you stopped checking the freezer.
What it costs
| Item | US, dollars | UK, pounds |
|---|---|---|
| Seeed XIAO ESP32-C3, headers pre-soldered | $4.99 | £5.80 |
| Waterproof DS18B20 temperature probe | $9.95Out of stock | £3.50 |
| Build | $14.94 | £9.30 |
| SONOFF SNZB-02LD Zigbee probe thermometer | $22.95 | Not checked |
- If it does not work
- The parts and an evening. Nothing is cut, drilled or glued, and the freezer is not modified. The real cost of failure is not the hardware: it is a freezer of food lost to an alarm that was silently offline.
- Expected cost
- Not yet available. It is the build cost plus the failure rate times the cost of failure, and the failure rate comes from builder reports. Nobody has reported on this one.
What it asks of you
Network and Home Assistant ConfigurationNeeded
- How much
- A running Home Assistant instance, an ESPHome device adoption, and one automation with a sustained-window trigger rather than an instant one.
- If you get it wrong
- An automation triggering on the instant reading alarms every time the door opens, and gets muted within a week. A muted alarm is the same as no alarm.
Escape hatch
None. Without Home Assistant or an equivalent, there is nothing to raise the alarm, and the commercial units use their own app instead.
SolderingNot needed
- How much
- None, if the board is bought with headers pre-soldered and the probe is bought with its pull-up resistor fitted. One published build solders a flat flex cable and a discrete resistor instead.
- If you get it wrong
- A cold joint on the one-wire data line reads as no sensor found rather than as a bad reading, which is at least an honest failure.
Escape hatch
The Pi Hut sells the XIAO ESP32-C3 with headers pre-soldered for GBP 0.70 more than the loose-header version, and its DS18B20 probe ships with the 4.7k pull-up already fitted. Together those remove soldering from this build entirely.
Firmware and Command LineNeeded
- How much
- One ESPHome flash over USB, then a YAML file of about fifteen lines.
- If you get it wrong
- Re-flash and try again. Nothing is lost but the evening.
Escape hatch
ESPHome's web installer flashes from a browser, so no command line is strictly required.
Tools
Not owning a machine does not make this impossible. It changes what it costs.
- soldering iron and multimeter
- Substitutable. Buy the pre-soldered board variant and the probe with the resistor already fitted. Most makerspaces have one, and many public libraries now do too.
If it goes wrong
- Reversibility
- Free to Retry
- What is at risk
- Nothing
- Do this first
- Leave it running on the bench against a known thermometer for a day before trusting it, and set the automation to alarm on stale data before you rely on the temperature reading at all.
Is it worth doing?
- Verdict
- Pays Back
- On what ground
- Costs more than buying
- Still right for
- Anyone already running Home Assistant who wants the reading beside everything else. Whether it does the job depends on the open question below, which builders have not yet answered.
The product does
- Runs on its own battery over Zigbee, so it keeps reporting through a mains failure that takes out the router.
- Carries a local display, so the reading is visible without the hub.
Freezer Temperature Alarm
A waterproof probe sits inside the freezer, its cable passing through the existing door seal. The board it connects to stays outside in room air, reports to Home Assistant over WiFi, and an automation raises an alarm when the compartment stays above a threshold for a sustained period rather than the moment a reading blips.
Before you order anything
Everything else on this page is secondary to one constraint, which two of the three sources state independently and which is the most common way this project fails: nothing but the probe goes inside the freezer. An insulated cabinet attenuates 2.4GHz badly, and the cold flattens coin cells. Sealed all-in-one sensors placed in the compartment stop reporting, and they stop reporting silently.
The second constraint is the automation, not the hardware. Trigger on an instant reading and the alarm fires every time somebody opens the door for the peas; a week later it is muted, and a muted alarm and no alarm are the same thing.
What it costs
Building is cheaper. Around USD 14.94 or GBP 9.30 against USD 22.95 for the SONOFF probe unit, and both assume you already run the hub each depends on. That is what the verdict above says, and it is all a price comparison can establish.
The open question
A utility build is judged on whether it works when nobody is watching. A mains-powered ESP32 on WiFi stops reporting when the power drops, which is one of the two events the alarm exists to catch. The other is a door left ajar, which it catches well.
Whether that first case matters in practice is not something this page can settle from published builds. Nobody writes up the night their freezer alarm was offline. So it is asked rather than answered, and the question is on the page:
Does the mains-powered version actually miss the failure it exists to catch, and when it did, did you lose anything?
If you have built one, the report form answers it. A report saying the power went out, the alarm was silent, and the freezer survived anyway settles it as clearly as one saying it did not. Enough of either and the verdict gets reviewed by a person, with the reason recorded.
What we can say without waiting: adding a second automation that alarms on absence of data, not only on temperature, costs nothing and closes the gap most of the way.
Notes on the sources
The three published builds disagree about difficulty, and the disagreement is informative rather than contradictory. Buying the probe ready-made instead of building the cable removes the soldering, and buying the board with headers fitted removes the rest of it. In the UK that escape hatch costs seventy pence.
Who already has this, and how they differ
| Source | Weight | Licence | What is different about it |
|---|---|---|---|
| DigiKey: DIY Wireless Freezer Monitor with ESPHome | Complete Published Build, Unknown Provenance | Creative Commons Attribution, per the article header | The only source that solves mounting properly: a magnet in heat-shrink, and a flat flex cable thin enough to pass the door seal without holding it open. Also the only one that names the real radio problem, that freezer insulation behaves like a Faraday cage. Polls with a change filter rather than using an interrupt. Requires the most soldering of the three. |
| JPK.io: Local Fridge and Freezer Temperature Monitoring | Independent Build, Bought Hardware | Unstated | Contradicts DigiKey on difficulty, calling the wiring trivial and jumper-wire level, because it uses a ready-made waterproof probe rather than building the cable. Supplies the thing no build guide does: the automation must trigger on a sustained window, around thirty minutes, or door openings mute it. Also names the failure pattern seen repeatedly on r/homeassistant, sealed sensors placed inside the cold box, and points at the SONOFF probe unit as the honest alternative. |
| Random Nerd Tutorials: ESP32 with Multiple DS18B20 Sensors | Complete Published Build, Unknown Provenance | Unstated | Not a freezer build at all, and included because it is the reference every freezer build silently depends on for the 4.7k pull-up and for reading sensor addresses off the bus. Covers multiple probes on one bus, which is what turns this into a fridge-and-freezer build for no extra board. |
Where they disagree · Use-Case Mismatch
One says the build needs careful soldering of a flat flex cable, the other says the wiring is trivial. Both are right about different builds: DigiKey makes a permanent magnet-mounted unit with a cable thin enough not to hold the door seal open, JPK uses an off-the-shelf waterproof probe and accepts a fatter cable. The disagreement is about the mounting, not about the electronics, and a reader choosing the second one removes soldering from this project entirely.
What builders reported
No reports yet. This assessment is a theory, built by comparing what has already been published. It stays one until people who built the thing say otherwise.
Open question · reliability
Does the mains-powered version actually miss the failure it exists to catch, and when it did, did you lose anything?
Builder reports of the outcome, and the six-month question in particular. A report saying the power went out, the alarm was silent, and the freezer survived anyway answers this as clearly as one saying it did not.
Built this? What actually happened?
No account, no email required.
Safety
No Special Hazard
Low voltage, USB powered, no mains work, nothing structural, no modification to the appliance. Placeholder only: the class has not been set by a person, and this entry must not publish until it has been.