This is my simple and rather amateurish heated bracelet built around an attiny.

The heating pad

First of all, I used a simple resistive heater “e-textile” as a heat source. It is driven by a PWM signal that switches a transistor.

The heater is close to the skin, then on top of it is a thicker insulating piece of textile and on top of that is the power source (a LiPo battery) and the control circuitry.

This is wrapped in a second layer of insulating textile (and then put in a “sleeve”).

Embarrassingly enough, the powerdraw was causing the attiny to brown out, so a cap had to be added at the last moment.

The status LED and control button were attached with snap buttons.

And here it is all put together. It works neatly - you snap it closed to turn it on; press and hold the control button to cycle the intensity; the frequency of the smoothly pulsing purple LED reports that intensity. I should still work on my sewing skills though.