5V Heating Pad - 5 x 15cm

Apply 5VDC and you get 36-40 degrees Celsius (measured with a DHT-22). Needs to much current to use an Arduino pin connection directly, so use a relay.

I've bought lots. For winter drone applications (heating the lipo batteries in-flight, and heating the pilot's hands and other parts!). Be prepared to 'work with them' because they tend to fail easily. Always at the point where the conductive heating wire is bound to power wires. It's poorly done. I've had to do surgery to strengthen this connection. Then, they're fine and highly useful for the cost.