A metal detector using pulse induction technology.
Inspired by a friend purchasing a metal detector, I built my own based on the circitry in the Minipulse Plus kit. It functions by sending a single short duration pulse to the coil which energises the ground, and then as quickly as the circuitry allows, the coil is used to pick up traces of the collapsing electromagnetic field. This pulse-detection cycle is repeated a thousand times per second.
The most important part of a metal detector is the coil, a compromise of size, depth, and sensitivity to gold. A large coil with many turns of wire is most suitable for finding iron objects at great depth, but a large coil is clumsy to use in the field. A coil with many wire turns has a high inductance, and combined with the interwinding capacitance of the wires acts to "slow" the coil and make it deaf to small gold samples. Many different coil sizes, arrangements, number of turns, and wire diameters were tested to find an optimal coil suited to finding small gold samples at the greatest depth. The "basket weave" arrangement depicted is based on designs discussed on internet message forums. The coil is housed inside two "frisbee" style plastic flying toys, and attached via a non-metallic bolt to a non-metallic carbon fiber shaft.
The circuitry and batteries are housed inside a box as shown, the main PCB in the base, with extra custom built circuit boards under the lid for final audio drive to the speaker, a ground balance circuit, and an absolute value amplifier circuit for the integrated signal.
As seen on the right (displayed at 87.6 Mhz), the box also has a small FM radio transmitter built in for wireless headphone use in the field.