Metal Detector inside trashbin?

[Staigen]

One thing come to my mind, why not use a 4046 for detecting. The oscillator feeding the 4046 and the output from the comparator feeding the 4046 oscillator thru a rc-filter and also feeding a gate of some sort. I can post a little schematic to clarify if you want.

//Staigen

 

[Wbdsgnr]

Yes could you give me some? I was thinking, how would you program in ASM to detect a change? Proberbly with a bunch of bitscf commands, but if it could be done easier I would really like to know Thanks anyway for your great support :roll:

 

[Wbdsgnr]

I was thinking.. isn't it possible to convert the frequency to a analog voltage? This way with a comparator it would be much easier, since I don't have much knowledge about programming a PIC in ASM and stuff

 

[Nigel Goodwin]

I was thinking.. isn't it possible to convert the frequency to a analog voltage? This way with a comparator it would be much easier, since I don't have much knowledge about programming a PIC in ASM and stuff

Yes you could, a 555 can be used to make a simple frequency meter.

You could also use an NE567 tone detector PLL chip, have this set to the same frequency as the oscillator, and when the oscillator goes far enough off frequency the lock pin of the 567 will change state.

This obviously all depends on how far the frequency of the oscillator changes!. The advantage of a processor solution is that you don't need to rely on an absolute value (as you would with a 567 or an analogue comparator) - you could write the software so it detects short term changes, regardless of the long term frequency.

But the first thing you really need to do is build a simple oscillator, the one I gave above would be a simple one to build, wind the coil, and find how far the frequency moves when the item in question enters the coil.

 

[Wbdsgnr]

I made the circuit, and attached it to a small coil from a transformer. It was like 3 square cm (around 1 inch roughly) and had 1200 windings. The change was massive when I put my key trough it but the coil wasn't large. The frequency was around 1 khz I think, didn't calculate it But does that mean I should make the large coil with the same amount of windings? Or should I rather use a toroid coil? or oter capicitors?