Python Radio 61: Build a Metal Detector

Just a $5 Raspberry Pi Pico 2 and some wire.

Photo by author.

The schematic is simple:

The physics is fairly simple as well.

We put current into the tank circuit (the coil and capacitor). This generates a magnetic field around the coil. Then we turn off the current, and read the voltage in the tank circuit as the magnetic field collapses, inducing a current in the coil.

If there is metal near the coil, the collapsing field induces eddy currents in the conductive metal. This robs the coil of energy. We see the voltage drop more quickly.

Think of the circuit as a bell. We hit it with a hammer, and listen to the sound ring down.

We need a small-value resistor to protect GPIO 16 from the high voltage generated as the magnetic field collapses. 30 ohms is probably enough to protect the pin while giving us enough power to produce a good magnetic field. I used 38.5 because it was close enough and handy.

I wound the coil around a 5-gallon plastic bucket from the hardware store. This gives a large enough coil to make finding metal less tedious.