Coin-Cell Wireless Temperature Sensor for Home Assistant
A coin-cell powered wireless temperature sensor node for Home Assistant on a Raspberry Pi, intended for a custom production PCB.
Advanced Project
— This is an advanced project because a production coin-cell wireless sensor requires low-power firmware, RF layout/antenna design, battery-voltage robustness, and reliable Home Assistant protocol integration on a custom PCB.
Assumptions:
- Home Assistant will receive data via a standard home automation radio protocol, most likely Zigbee or Thread, rather than raw WiFi, to preserve coin-cell life.
- The sensor only needs to report temperature, not humidity or occupancy.
- The PCB will be custom-designed with the radio antenna area kept clear and with firmware support for deep sleep and periodic reporting.
- The Raspberry Pi already has the appropriate Home Assistant integration or coordinator hardware for the chosen wireless protocol.
Bill of Materials
Compatibility Notes
- The MCU, temperature sensor, and regulator should all be designed around a 3.3 V rail; the nRF52840-QIAA, ADT7410, and TMP112AIDRLR all support I2C at low voltage, so level shifting should not be needed if the MCU runs at 3.3 V.
- For a coin-cell design, the radio current peaks matter more than average sleep current; make sure the chosen wireless SoC can handle transmit bursts without the regulator dropping out.
- If you use a Zigbee or Thread MCU, Home Assistant will need a compatible coordinator on the Raspberry Pi side; the sensor node itself does not connect directly to WiFi.
- The RF section needs antenna keepout, controlled return paths, and a clean ground reference; do not place the temperature sensor directly next to the radio or regulator heat source.
You'll Also Need
- Coin-cell holder or battery connector
- Decoupling capacitors, pull-up resistors for I2C, and any RF matching components required by the chosen wireless part
- Antenna implementation or a certified antenna module strategy
- Programming/debug connector and test pads
- Enclosure, mounting hardware, and thermal isolation features so the sensor reads ambient air instead of PCB self-heating
- Home Assistant coordinator hardware/software on the Raspberry Pi side if not already present
Estimated BOM Cost: $20-25 (based on live distributor pricing)
Design Considerations
Battery Life
A coin-cell design lives or dies on sleep current and radio duty cycle. If the MCU sleeps at around 1 to 5 uA and wakes once per minute to sample and transmit, average current can stay in the tens of uA range, which is realistic for months to years on a nRF52840-QIAA depending on radio power and packet retries. Avoid always-on polling, long advertising intervals, or frequent retransmissions.
Wireless Protocol Choice
For Home Assistant, Zigbee or Thread is usually a better fit than WiFi for a coin-cell sensor because the radio can stay off most of the time. WiFi-class parts tend to burn too much energy for a small coin cell unless you accept short battery life. If you want the simplest smart-home integration, Zigbee is often the most practical starting point.
Temperature Accuracy
The sensor choice matters less than the mechanical placement. Put the temperature IC at the PCB edge, away from the regulator and radio, and use thermal isolation slots if needed so board self-heating does not bias readings. For room sensing, a precision part like nRF52840-QIAA is useful, but enclosure airflow and placement often dominate real-world error.
RF Layout
Treat the antenna area as a first-class part of the design. Keep copper, traces, and tall components out of the antenna keepout, and follow the reference layout for the chosen wireless part exactly; small mistakes here can destroy range and battery life because the radio has to retransmit more often. If RF layout is new to you, a pre-certified module is often worth the extra cost.
Power Integrity
Coin cells have relatively high internal resistance, so radio transmit bursts can cause voltage dips even when average current is low. Use local bulk capacitance near the radio and MCU, and verify startup and transmit behavior at the end-of-life battery voltage, not just with a fresh cell. Also choose an LDO with very low quiescent current so the regulator does not dominate standby drain.
Firmware Reliability
Use a simple state machine: wake, sample, transmit, confirm, sleep. Add a watchdog and a retry limit so the node does not get stuck awake if the radio stack fails or the coordinator is temporarily unavailable. For production, include a way to report battery voltage and link quality so you can detect failing nodes before they disappear.
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