I don't think that would work, a 555 works by charging a capacitor up, it's not 'really' an oscillator at all.

Here's the oscillator circuit from the EPE LCF meter, with the values shown it runs about 500KHz, it's nice and simple!. The preset is adjusted to ensure reliable oscillation.

As you are at school, presumably you have access to scopes and frequency counters - you could use either (or both) to check the amount of frequency shift you get, and find the optimum frequency and size and shape of coil. Your teacher should love you for it :lol:

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Ok, maybe a very stupid question. But how would you attach that thing? I mean, if you charge a capicitor it won't oscillate forever... don't you need some amplifier or something? I'm sorry , but i'm just a first grade student I found this sheme http://larc.ee.nthu.edu.tw/~jcbor/Ci...b_Proj1_23.PDF, can't I use that? thx in advance

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\"The fear of the Lord is the beginning of wisdom\" - Proverbs 1:7

 

Sorry, do you mean 'where are the supply rails?', it's normal not to bother showing them on logic circuits - you do (of course) need power for it. The CMOS NAND gate is the amplifier, it's a standard circuit.

I can't comment on the web page you gave, as I can't get it to work.

 

Yes that :P I feel really stupid you know :? but erhm... bottom is plus ? It dc right? And ehm.. where would be the output then, after the nand gate?? pff, i should really try this with a oscilloscope, much easier than guessing Where would I be without you

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\"The fear of the Lord is the beginning of wisdom\" - Proverbs 1:7

 

Perhaps I should have labelled the diagram :lol:

Here's a slightly better diagram, you will need to sort out the pin connections for the chip yourself.

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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

 

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:

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\"The fear of the Lord is the beginning of wisdom\" - Proverbs 1:7

 

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

__________________
\"The fear of the Lord is the beginning of wisdom\" - Proverbs 1:7

 

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.

 

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?

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\"The fear of the Lord is the beginning of wisdom\" - Proverbs 1:7