Metal detector... Maybe

519

06:35:35

Usually metal detectors work with the difference of a reference oscillator and the tank circuit with the air coil. I was wondering if this maybe works aswell. So first I have a collpits tuned at around 500kHz with the 10uH being the air coil. This is then fed in a pulse counting frequency demodulator and then through a LPF to creat a average voltage according to the frequency of the LC tank. This is then fed into a VCO which is tuned to audible frequencies. Now the VCO's cap is a 1000 times smaller than it would be just to compensate for sampling rate. I guess the low pass will have to be adjusted accordingly. And I think the sim is having some issues with the PCFDM. Pls let me know what you think of that. 

published 6 years ago

hurz

6 years ago

problem is your colpitts just change for a few Hz when little pieces of metal come close and this will be dificult to hear cuz the filter and VCO do constantly run away, only a little but maybe similar strong as the colpitts does change if metal come close. I dont think it will work very sensitive only with big changes in colpitts frequency

ViolationMad

6 years ago

And if I try to keep the inductance of the coil as small as possible? Wouldn't that make the effect of the metal bigger?

hurz

6 years ago

10uH is alread very small for an air coil this plate size maybe less then ten turns

ViolationMad

6 years ago

And if I multiply the averaged voltage maybe? So the range of the VCO's input is used more.

hurz

6 years ago

this will multiply also the runaway error. I think you must give it a try, without VCO and just measure the LPF voltage in depended of an object coming close. See how many millivolt delta you get

hurz

6 years ago

here a 5kHz change for a LPF cause only 1.1% amplitude change. 5kHz is a lot i expect just 50Hz or less http://everycircuit.com/circuit/6619923275841536

hurz

6 years ago

guess you are searching for money at playa to finance your holiday. So a little coin 20cm or more below the ground?

hurz

6 years ago

another application is a car detection at a traffic light. You see that is totaly different usecase

ViolationMad

6 years ago

A friend of mine who is also learning electronics asked me to make a metal detector. So I just put this together in my head while I was doing my assembly line job to fincace my holiday haha. Honestly I dont expect much from it but I just wondered because I didn't look anything up for reference. I think it could still do good to get familiar with the single blocks in it.

ViolationMad

6 years ago

And I will also build it just out of curiosity.

ViolationMad

6 years ago

You expect the delta in maginute of millivolts? Well that doesn't sound to promising...

hurz

6 years ago

Setup quickly an oscillator and check it out. Thats my guess

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