“The idea of this circuit is to hack PIC oscillator circuit, by replacing the crystal by a coil : the frequency of the oscillator then depends on presence of metal near the coil, just like in a classic metal detector. To detect changes of main oscillator frequency, we use the PIC watchdog as internal time reference. By comparing both oscillators frequencies, we can know if a piece of metal is near the coil, and then light a LED. It shares the same idea as PicoBat, the ultrasonic bat detector.”