NOTICE

ORDERS AFTER DEC 18th WILL BE SENT OUT DEC 31st

new version is here and available. Features include:

• 0.8 mm board thickness
• two 0603 size Caps replace the 1206 100 uF Capacitor which allows for a slimmer design.
• different thermometer package size for better adhesion to the board

What is it?

nfc_temp is a passive NFC tag based on the ATtiny series from Microchip and the dynamic NFC IC RF430CL330H + TMP117 thermometer from Texas Instruments. It measures ambient temperature with a twist; It does not have and independent power source, but is able to be powered by a NFC reader like your phone.

Hold it near the tag and the PCB will siphon some energy from the reader field to power itself and read the temperature from the 0.1 °C accuracy TMP117 thermometer. The ATtiny will compute the temperature value and send it to the NFC IC so the reader can read it in time before the time window for a single read closes.

Note that the PCB needs some time to provide an accurate reading. I advice to let it sit on a table for some at least 2 mins.

Why did you make it?

I worked on a low wireless footprint device to track surveillance cameras you have come into contact with. For this I worked closely with the RF430CL330 dynamic NFC. The inspiration to create a passive device came from one of the example use cases. Because of size constraints I needed to rework the TI approach and used an ATtiny series microcontroller combined with a digital thermometer. With these changes you can even reprogram it via the Arduino IDE given you have made some modifications to program with Microchips UPDI single wire signaling protocol.

What makes it special?

• passive design approach, have your phone at hand? read the temperature
• small form factor, similar to a popular 2FA Keyfob, ideal for your analog keychain
• completely open source build stack (KiCad, FreeCAD, Arduino IDE)
• design + production made in Germany

Notes

• Creating the live demo was difficult since the LED strips at the top are a pretty big heat source so the temperature could not really settle in on the temp_sense. I did however see very accurate results for example after letting my keys sit in a room overnight.