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メーカー品番 11050
TEMP SENSOR WTRPRF DS18B20 11050
SparkFun Electronics
DIY Wireless Freezer Monitor: Local Temp Monitoring with ESPHome
2026-06-01 | By Nate_Larson
License: Attribution Temperature Wifi ESP32 XIAO
Introduction
If you’ve ever had a refrigerator or freezer unexpectedly quit on you, you know the panic of discovering an appliance full of spoiled food. While some modern appliances include temperature alarms, they often demand privacy trade-offs or only offer basic audible alerts that require you to be nearby. Existing DIY solutions have their own challenges: wireless sensors struggle to transmit through freezer insulation like a Faraday cage, while hardwired sensors typically mean drilling holes or sealing gaps that compromise your appliance’s integrity.
This project walks through how to build a connected, wireless freezer monitor that makes installation easy, with no permanent modifications to the appliance, no batteries to worry about, and virtually eliminates any air leaks due to sensor cables. Plus, it ties into your personal Home Assistant server, so all your data stays local and yours.
This project assumes you already have a Home Assistant server running with ESPHome installed.
Required parts:
Seeed XIAO ESP32-C3 (or other ESP32 dev board)
Required materials
Required tools:
Step 1
Measure out 18-24 inches of the flat flex cable and cut to length with the scissors.
Step 2
The flat flex cable consists of a substrate with flat conductors on it and an insulating coating over the conductors. When looking at the cable from the end, you should be able to determine which side of the cable the conductors are on, as they will be slightly raised.
Orient the cable on a sturdy work surface with the conductors on top, and carefully use the edge of a sharp knife, held perpendicular to the conductor, to scrape away about 3/16” of the insulation off both ends of each conductor, as shown in the following images. Don’t try to remove all the insulation in one go; just keep scraping away small amounts until the copper is exposed.
Step 3
The DS18B20 requires a 4.7K ohm pull-up resistor on the data line. Connect the resistor between the 3.3V pin and the D10 pin, and position the resistor to lie flat, being careful not to short the leads to the shield or other pins or components on the board. Cut the resistor leads flush with the bottom of the board, but do not solder the resistor yet.
Step 4
Place the XIAO board with the pull-up resistor onto the Flat Flex cable so that the bare portion of the conductors align with the XIAO’s GND, 3.3V, and D10 solder pads, as shown below.
With everything in place, solder the XIAO to the cable while simultaneously soldering the resistor into place. Solder these connections quickly, with a hot iron, as allowing the heat to creep into the cable can melt the plastic substrate, deforming or damaging the cable. Applying a bit of paste flux before soldering will help you make good solder joints quickly, helping to prevent heat damage to the cable.
Step 5
Prepare the other end of the flat flex cable by applying solder to the exposed copper – again, flux is your friend here.
Trim the sensor’s cable to an appropriate length for your application (I typically leave about 12 inches). Strip and tin the leads, then match them to the connections so GND connects to GND, 3.3V to VCC, and the data line to pin D10 of the XIAO. Solder everything in place. Using flux and pre-tinning will make the connections clean and reliable, which sets you up for success in the next step: testing!
While these connections are not ideal due to not pre-tinning the wires and not using flux, resulting in some melting of the flat cable, it still worked
Step 6
Attach the antenna to the XIAO board, and use a USB cable to connect the XIAO to your PC. Navigate to your ESPHome Builder dashboard, click the “New Device” button, and select the “New Device Setup” option.
Give your new device a name.
Click “Connect” and select the port your XIAO ESP32 is connected to, and wait for the initial firmware upload to complete.
Now, click edit to customize the configuration for this device and add the one_wire BUS config. Looking at the XIAO pinout diagram, we can see that pin D10, which we are using for the DS18B20 temperature sensor, is GPIO10.
Click “Install” to compile the new firmware and push it wirelessly to your XIAO. When the code finishes uploading, watch the logs in the window to find the address of your sensor, which you will need to copy to add to the configuration. If you get a message that no one_wire devices were found, double-check your connections are correct, and your solder joints are good.
Click “Edit” to modify the configuration for your device again, adding the sensor configuration, with the address you found in the logs, and name the sensor as you see fit. I also added a filter so if the temperature doesn’t change by a significant amount, it won’t update the server – this helps reduce the number of unnecessary datapoints the device transmits and the server logs.
Click install to compile and flash the updated config and watch the logs to see your temperature data start flowing in. With this, we now have a working sensor, so the next steps will be to get it ready for installation.
Step 7
Fold the flat flex cable near where it is soldered to the XIAO board so the cable is parallel to the USB connection, but comes away on the antenna end, as shown here.
Place the magnet into the corner of a plastic bag (such as the one your DigiKey parts arrived in!) and cut off the corner – this will insulate the magnet from the XIAO board so it doesn’t short any of the contacts.
Cut a 2-inch-long piece of the heat shrink tubing, slide it over the XIAO board so the USB connector is just inside the tubing, then slide the magnet (in its plastic sleeve) into the heat shrink on the bottom side of the XIAO board, and position the antenna so the heat shrink will keep it in place and prevent it from catching on anything.
With everything in place, carefully shrink the tubing with a heat gun. I recommend starting at the end with the USB connector to ensure it remains accessible and work your way to the other end of the heat shrink until it is snug around the assembly.
With this end complete, we need to address the open connections where the sensor cable connects to the flat cable. In a well-ventilated area, coat the connections with some liquid electrical tape and allow it to dry. Once dry, ensure the connections are fully coated and sealed against shorts or moisture intrusion.
Installation
Installing the device is straightforward. The magnet securely holds the board to the outside of your fridge or freezer. At the same time, the thin, flat cable passes through the door seal with virtually no air leakage – a major advantage over traditional wired sensors. Position the sensor probe inside your appliance, then secure it with a small cable tie to keep it in place. Connect the XIAO via a USB cable to a nearby 5V power supply. Within moments, you’ll see live temperature data flowing into Home Assistant. From there, you can set up automations and alerts to notify you if the temperature drifts outside your desired range, giving you peace of mind that your food is staying safely cold.
Conclusion
You’ve now built a practical, wireless freezer monitoring system that solves the real-world problems that plague DIY cold storage monitoring. No batteries to replace, no holes drilled through your appliance, no reliance on manufacturer cloud services; just clean, local data flowing into Home Assistant on your own terms.
From here, the possibilities expand. Consider adding a second sensor for your refrigerator or other temperature-critical appliances (you can have multiple one-wire sensors on a single GPIO pin!). You could add a door sensor to your build to know if the door was left open, and even integrate the data into more complex automations, like triggering notifications only during specific hours or combining freezer alerts with other home automation routines. The ESPHome platform makes it easy to customize and extend your setup as your needs evolve.
Most importantly, you now have reliable monitoring protecting your food and your peace of mind. That unexpected thaw in the middle of the night? You’ll know about it before the damage occurs. That’s the real win.
Have questions or comments? Continue the conversation on TechForum, DigiKey's online community and technical resource.
