Pseudonymous maker “unsurv” has designed a keychain accent with a distinction: it is a battery-free thermometer, harvesting sufficient vitality to take a studying once you scan its near-field communication (NFC) tag along with your smartphone.

“You’re looking at a passive NFC thermometer primarily based on the [Microchip] ATtiny1626 and the [Texas Instruments] RF430CL330H dynamic NFC tag,” unsurv writes of the venture. “To measure the ambient temperature simply maintain your cellphone to the system. Energy comes from the magnetic discipline the reader creates and is siphoned by way of two diodes seen on the backside. It creates about 2.7V for the ATtiny and thermometer to work with.”

The precise temperature readings are taken by a Texas Devices TMP117 thermometer, which usually presents 0.1-degree accuracy — however that, given the constraints of the venture and the short-term energy provide, is used right here in one-shot mode. “With my Fluke 54 II Thermometer I can reproduce measurements very exactly,” unsurv notes of the end result, “after giving the system a while to accommodate (2-3 minutes).”

Studying a temperature sensor with a microcontroller is not often a tough venture, however once you’re harvesting vitality utilizing a self-designed NFC antenna it poses new and fascinating challenges. An preliminary try to make use of the ATtiny3227 with an analog temperature sensor failed resulting from an absence of steady reference voltage, which led unsurv to modify to the “moderately costly however very correct” TMP117 with its built-in analog to digital converter. “[I] wanted to make use of some unofficial brownout detection voltages to make it work,” the maker provides.

“Android is healthier at detecting the system that iOS,” unsurv says of the compact board’s real-world utilization testing, “which wants some particular angling. Programming is finished by way of testpoints and pogo pins on the again of the PCB.”

Extra info on the venture is obtainable in unsurv’s Reddit publish, with KiCad venture information and Arduino supply code revealed to GitHub below an unspecified license; unsurv can be promoting assembled variations of the board on Tindie for $35.