My goals were to make a circuit as simple as possible and to use only popular / cheap electronic parts. The device has been tested with very small coil (55mm diameter, about 30 turns of 0.5 DNE) and only 3.0V power supply. It’s my first design of PI metal detector and I’m really happy with the results! The prototype was able to detect little coins (6cm distance) and wires in the wall.
Presented metal detector uses PI method to generate a voltage spike in a search coil connected in parallel with capacitor. Next, ATtiny13 uses an Analog Comparator to measure a decay time to zero of resonant circuit. When a metal object nears the loop it will decrease time it takes for the pulse to decay to zero. The change in the width of resonance time is measured to signal the presents of a metal target.