can someone please help me design a simple induction circuit.
In my first circuit i used a LDR as a sensor to track a white/black line. but for my second circuit i want to replace the LDR with an inductor and be able to notice the difference when i put an inductor (coil with steal rod in the middle) close to it. i believe an inductor run on ac supply. please help me and with explanations too.

thanks

 

Not as easy as it sounds, you need a circuit that detects the slight changes in inductance and reluctance as a metallic object moves in front of the coil.

 

If you just want to detect the presence of a steel object - not any metal - then a Hall Effect sensor might be easier. As the iron or steel object gets closer, the magnetic field gets concentrated and the analog output changes.

See attached drawing. You will have to experiment.

A possible sensor:-
http://uk.farnell.com/allegro-microsystems/a1301eua-t/hall-effect-sensor-sip-3-1301/dp/1198085

You will need a slightly more complicated circuit with a comparator to detect possibly slight changes in voltage.