1. There should be a cap in parallel with that zener
2. There's no sense in putting that 220k in series with that 1k
3. That dual-comparator thing is pointless
4. The LM324 is garbage and won't work at those frequencies
5. That circuit is awful, don't use it, try one of these instead - Sensors / Detectors: Metal Detectors electronic circuits
1. There should be a cap in parallel with that zener
2. There's no sense in putting that 220k in series with that 1k
3. That dual-comparator thing is pointless
4. The LM324 is garbage and won't work at those frequencies
5. That circuit is awful, don't use it, try one of these instead - Sensors / Detectors: Metal Detectors electronic circuits
There's much that can be said for that simple circuit, however for us to explain it as an easily understandable lesson requires you to know some basic electronics as well. So you are already at a disadvantage. Try a far simpler circuit instead.. like a battery, switch and l.e.d. Observe correct polarity.... there's a good start!
I have designed many metal detectors for the electronics press, including the world's simplest self-contained metal detector: see Urban Ministry Live And Unplugged: BB Metal Detector (my blog, which has an "Electronics" section with other simple designs).