This Energy-Harvesting Keychain Accessory Takes Temperatures at the Tap of a Smartphone
Built around a Microchip ATtiny1626 and a dynamic NFC tag, this temperature sensor needs no wires nor batteries to work.
Pseudonymous maker "unsurv" has designed a keychain accessory that's actually a battery-free thermometer, harvesting enough energy to take a reading when you scan its near-field communication (NFC) tag with your smartphone.
"You are looking at a passive NFC thermometer based on the [Microchip] ATtiny1626 and the [Texas Instruments] RF430CL330H dynamic NFC tag," unsurv writes of the project. "To measure the ambient temperature just hold your phone to the device. Power comes from the magnetic field the reader creates and is siphoned via two diodes visible at the bottom. It creates about 2.7V for the ATtiny and thermometer to work with."
The actual temperature readings are taken by a Texas Instruments TMP117 thermometer, which typically offers 0.1-degree accuracy — but that, given the constraints of the project and the short-term power supply, is used here in one-shot mode. "With my Fluke 54 II Thermometer I can reproduce measurements very precisely," unsurv notes of the result, "after giving the device some time to accommodate (2-3 mins)."
Reading a temperature sensor with a microcontroller isn't usually a difficult project, but when you're harvesting energy using a self-designed NFC antenna it poses new and interesting challenges. An initial attempt to use the ATtiny3227 with an analog temperature sensor failed due to a lack of stable reference voltage, which led unsurv to switch to the "rather expensive but very accurate" TMP117 with its built-in analog to digital converter. "[I] needed to use some unofficial brownout detection voltages to make it work," the maker adds.
"Android is better at detecting the device that iOS," unsurv says of the compact board's real-world usage testing, "which needs some specific angling. Programming is done via testpoints and pogo pins on the back of the PCB."
More information on the project is available in unsurv's Reddit post, with KiCad project files and Arduino source code published to GitHub under an unspecified license; unsurv is also selling assembled versions of the board on Tindie for $35.