The pdf attached here explains the circuit. The coil does come with it. You just can't see it in the picture.
The enclosure is press fit so it's easy to change the 9V, but will stay firmly shut.
This unit does work very well. It may EVEN detect GHOST activity, OOOOOOO .
I found it interesting that I can detect if someone has a cellphone in their possesion that is powered-up. It will blink if you wave it past a pocket or a purse. It won't detect if the phone is fully powered-down. HHHmmmmmm.......