Babar, some time ago, I had to make a coin detector for a cofee-machine. I used two coils, one infront the other. One of the coils was connected to an AC power supply and gave me a magnetic field. I connected the other to a diode and a resistive divider. When the coin passes between the two coils, The magnetic field is altered, giving me a spike at the other coil because I was altering the air core. Then detection is easy with a schmitt trigger. Hope it can give you a clue.