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resonant circuit ->need some help

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Elvis P.

Junior Member level 3
Aug 28, 2001
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the following problem:

i need to detect if an object is present on a table.
i want to do that with an resonant circuit which will be out of tune if something metallic infulences the resonant point of the solenoid.
if someone have any expereience with such circuit (troubles generally and especially with hf [mobile phone]) i would really appreciate any help. (not to make to many pcb revisions .... :)

Elvois P.

what object you want to detect?if it metallic,and it is big enough,it is easyto do so.tell us what you want to detect,so we can help you.


the objcect is are a cube and a ball. t
he material will be plastic, but it is possible to put metall in it to detect the presence.
the diameter will be something like 15cm.

if you need any more information just pm.

thx a lot for the help


ok,if can be metall,it will be good.especially something like iron,or magnetics metall.the you can use a LC resonant circuit.when the metall is neer the indutor coil,the value of inductor will be higher,so the frequence will be lower.if the ball is taken away ,the frequence will be ,use a demodulator can detect the change of frequence,so you will know the ball is near you or be taken away.this is one method.
method two:if the ball is too far away from the detector,method one maybe can not can use a IR ,either use the reflect signal or the signal be cutted off by the ball,the you can get the change signal of IR,you can also know whether the ball is take away.
hope can be help.

REsonant circuit

Dear shanren!

thx for your advice! IR will not work there for different reasons. My plan is to use an L/C Combination. my question is if you have already done such a project and or have some experience... the theoretics are maybe trivial but the realization is not, and i want to avoid print revisions if possible.

Elvis P.

Hi Elvis P

what is the size of the table you have to observe???
If it is really big maybe you could build a resonant circuit
where the capacity is tuned. A big metalized plate under or
on the surface of the table and when a also metalized
piece comes close the capacity will change and togehter as
a swinging circuit with an inductor your resonant frequency will change.
(this principle is realized in some contactless switches like
in light controller or older TV)

Else you could use a LC resonant circuit and by adding a metalized
(permability about 1) device close to the inductor the quality of
the resonance by having loss currents induced to the metalized
object the will decrease and thus the amplitude of the oszillation.

All those principles sound simple but are tricky in realisation.
(sorry for that)

I build such a capacity detector years ago but it took a long time
to adjust and it was not really error tolerant.

For this you need a band pass filter with high order minimizing the peaks.

Sorry, have no more help to give.

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