Today I have built a BFO (Beat Frequency Oscillator) metal detector. It uses two modules to drive the coils. Each coil is 3.5 inches in diameter and 20 turns. When you combine two of them together and the output through two transistors, a resistor, and a capacitor you create a beat frequency between the two coils and the beat frequency can be heard despite the fact that the coils are going at 430kHz. Say for instance one coil is going at 430kHz and the other is going at 425kHz what you hear is the difference. In this instance you would hear 5kHz. After connecting a 9 volt battery for the power there will be a little bit of tuning that needs to be done. The two coils need to be pushed together until they overlap and don’t make any audible noise. They will still be making noise, it’s just not a noise we can hear. Then you want to pull the coils apart until you are on the edge of where they are producing an oscillation. There will be noise. After that you want to take another coil and place it partially over one on the coils to fine tune it. Move the third coil until the sound stops. Now you can take a quarter or another piece of metal and move it over the first coil. It will make a sound. You now have a working BFO metal detector.