by ABSF » Tue Nov 29, 2011 7:14 am
Since the circuit is simple, you might just construct it and see the effect to judge whether it is suitable for your need. I guess the circuit is outputing a continuous tone and when the inductor is brought near a metal, the inductance would change and thus changes the pitch of the tone. So you need some kind of op-amp circuit with rectified output to convert it to a logic H or L when the wanted tone is detector. You might also need a band-pass filter between the 555 and the op-amp to select the frequency that you're interested.
It is a simple circuit so the sensitivity may not be satisfactory but anyway it is a good start.
Allen
The next war will determine NOT who is right BUT what is left.