by Khaleel123 @ instructables.com:
This sensor is very sensitive and can pick up electrical signals from anything that uses power. It displays the amount of interference it picks up by sequentially lighting 4 led’s one after the other. I have tested it all around the house and its been kinda fun to see how different things compare. It can also be used to follow house electrical wiring inside the walls, yes its that sensitive.
Attiny45 EMF Sensor – [Link]