This simple LED collar pulses to your heartbeat and is small and portable.

From the project creator Sidney San Martín:

I used a Fanstel BT832 bluetooth module with an nRF52832 microcontroller, a strip of WS2812B LEDs cut from a bigger reel, a 3V CR123A battery, and a Polar heart rate sensor strapped to my chest.

The project is powered directly from the battery. WS2812 LEDs are supposed to need a minimum of 3.3V, but I tried connecting the circuit to a power supply and slowly dropping the voltage until I stopped around 2.1V because everything was still working (just a little bit dimmer). The bluetooth module, LED strip, battery holder and some soldered wires make up the whole project. I laid it out on a few strips of Kapton tape and added a huge blob of hot glue to hold everything in place.

I only had a couple of days for the whole project and spent most of it programming, so there’s no easy way to take the collar on and off — I had a friend hot glue the other end of the strip in place while I was wearing it! Lots of room for improvement.

I programmed the project in C with the nRF5 SDK. When it powers on, it looks for Bluetooth LE heart rate sensors and connects to the first one it finds, then starts pulsing the LEDs based on the heart rate from the sensor.

Project code: github.com/s4y/heart_collar

Check out Sidney’s other work and drop him a line at his neat website s4y.us !