Melexis thermometer MLX90614ESF-ACF-000-TU
Wire. 24 gauge. (20 to 26 gauge is fine) Solid or stranded.
Drill press or hand drill
The Arduino is the head and heart of the project. Everything connects to it one way or another. Oddly, it is not the most expensive thing in this project. Arduinos are ordinary microprocessors with an extraordinary community.
This project was intended to be very modular. Everything can be unplugged and swapped for a different version. Instead of being stuck with a pair of cheap headphones you may plug in your favorite pair. If you decide to build this one on the cheap and upgrade to a better thermometer later you just solder up the brand new thermometer and plug it in.
Arduino and Melexis temperature sensor as hooked up by bildr.
Note: I did not use this model of Arduino and my temperature sensor connects through a plug/socket/cord. The reason for this was size. Building a circuit the same was as bildr will still work.
The most expensive component is the Melexis infrared thermometer. The first prototype featured a much less expensive model, also made by Melexis. This thermometer was chosen because other people had successfully used an Arduino to interface with it and because the entire line uses the same interface. If you can talk to one, you can talk to all the others which use the same voltage.
The thermometer was put on a repurposed iPhone headset wire. This was done for a very specific reason. The cords on these headsets tends to be limp and flexible which is desirable. Since this can be worn on the head, having a stiff wire trying to bend at the same time is a bad idea. Four conductors are needed so a stereo headphone with microphone is perfect and easy to find cheaply on eBay.
The thermometer has a few components soldered to the leads in back. There are two pull-up resistors which are necessary for the communication with an Arduino and a capacitor to filter out some noise. These were sealed under ordinary black shrink tube after reinforcing the wires with hot glue. It felt like a shame to put a $26 thermometer on a $1 cord but it seemed like the best solution.
The Arduino and thermometer are the most expensive and complex parts of this project. Due to the modularity of this project, the rest is compartively simple.
There are five ports on the ESPeri.IRBud. The first is the 3.5mm socket for the thermometer. It doesn't need to be switched since there must always be a thermometer plugged in or it will not function properly.
Pinout of the TRRS 3.5mm socket
Wires inside the scrapped iPhone headphones