Build a sensitive metal detector for a fraction of the cost of a commercial one.
Who hasn’t dreamed about owning their own metal detector and searching for buried treasure, or at least a few dropped coins? In this project, we will build a metal detector based on a dual oscillator circuit. One oscillator is fixed, and the other varies depending upon the proximity of metal objects. The beat frequency between these two oscillator frequencies is in the audible range, and as the detector passes over metal objects, you will hear a shift in this beat frequency. Different metal types will cause a positive or negative shift, raising or lowering the audible frequency.
This "heterodyning" principle is used in many applications – radio, scientific measurements, even the Theremin!
Check out more Weekend Projects.