I'm not really sure if I've made it correctly but it seems to be working. You see the transformer is not a tranaformer per se, but two separate coils. The first one creates a voltage spike of up to a few hundred volts and sends it to the earth. If if hits a metal object, eddy currents will induce in the object causing it to reflect part of the wave. Then it hits a second coil (the secondary of the example) and induces current in it. The induced voltage spike is taken and compared to the transmited spike. The transmitted spike needs to be slightly delayed, because if the rise/fall time is too fast, the received signal will be slowed down a little and will appear out of phase, so no comparison can be done. The potentiometer supposedly controls the sensitivity level. I don't know it seems to be working
