i'am in the process of designing a metal detector for my textile automation project. i have used the principle of inductive proximity sensors for the same. after having designed the colpitts oscillator, i could see the damping of the oscillator output sine wave when metal is brought close to the coil of the tank circuit. could anyone suggest an appropriate circuit for sensing this damping of the output amplitude. i have attached the circuit for metal sensing.

 

Perhaps you could just tag a peak detector (diode to cap/resistor to ground, sense at junction). An analog to digital converter could sense this.... but that is very dependent on how much the peak changes when a target is in the field.

Basically you need to do some experiments to see what signals you need to sense. It is not impossible that all you need is a simple comparator replacing the micro, but it depends on the signal you need to sense.