# understanding circuit operation

Discussion in 'General Electronics Discussion' started by ami85t, Sep 8, 2014.

1. ### ami85t

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0
Feb 19, 2014
Hi,
I have build up a metal detector circuit based on this implementation:
http://www.instructables.com/id/How-to-make-a-cheap-and-simple-metal-detector/
I am trying to figure out it's working operation.
This implementation appears in a lot of sites in the web but there's no real explanation of what's going on there
and why are the specific components chosen.

In theory the LC's frequency is f=1/(2*pi*sqrt(LC). I used a 22uF cap and 33mH inductor which produced a 470hz frequency.
by theory it's supposed to be 186hz - that's more then times two. Now of course that the 555 is influencing this result via the (I think) astable frequency and I am trying to understand how.
Shortening up the two legs of the inductor results in a square wave with 2.6hz frequency and shortening up also the two legs of the capacitor results in a 4.54khz frequency. all frequencies I measured in the 555's output (pin 3).

Can anyone help me or direct me how to figure out the working operation?
Thanks,
Amitai

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2. ### Colin Mitchell

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312
Aug 31, 2014
That is my circuit from my website.
Most of my circuits are copied by others.
The circuit is only designed to pick up a pipe in a wall at a few cm.

3. ### Gryd3

4,098
875
Jun 25, 2014
I found the same circuit here : http://www.555-timer-circuits.com/metal-detector.html
But it does not explain the details of operation.
Colin, if this is your circuit perhaps you can help ami85t understand how it works. I can't go into much detail with it other than a very generic idea.

4. ### Colin Mitchell

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312
Aug 31, 2014
Basically the circuit is a square wave oscillator using the 47k and lower 2u2.
These get the circuit operating.
The inductor and 2u2 (in series) have a certain "resistance" at this frequency and now the circuit operates at a completely different frequency because the two components are just a resistor and this resistor is in parallel with the 47k.
Now we have a completely new higher frequency of operation.
The voltage on the top of the inductor is rising and falling - from 12v to 0v to 12v to 0v and is charging and discharging the 2u2 and the current through the 2u2 is charging and discharging the lower 2u2.
The complicated part of this discussion is the charging and discharging effect of the inductor.
When the voltage is 12v on the top of the inductor, current flows through the inductor and this produces magnetic flux and this flux cuts all the turns of the coil to produce a back voltage that acts against the voltage we are applying. The result is only a small forward voltage is available to flow through the inductor and through the 2u2.
A lot of the flux produced by the inductor escapes into the air and is lost.
But when a magnetic object is placed near the inductor, some of the flux enters the magnetic object and it absorbs it and and becomes a magnet and it then sends the magnetism back to the coil where it interferes with the magnetism produced by the coil. This changes the current taken by the inductor and also the current fed to the 2u2. This effectively changes the "resistance" of the combination and thus the frequency of the circuit changes.
It's that simple.

ami85t and Gryd3 like this.
5. ### ami85t

71
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Feb 19, 2014
Hi Colin, thank you very much for your the circuit and the answer!
I'm trying to find the best values of the components so to receive the maximum distance detecting.
I have done some experiments with the circuit and got good results for a 33mH inductor and a 22uF caps instead of the 2.2uF.
Instead of a speaker I'm using a frequency capture using a uC.
What brought you to use these specific values for the components? Is it based on
any equation of the astable option of the 555 as seen in the picture below? if yes why didn't you use the 7th leg as shown below?
Have you done some experiments which brought you to these features which can help me in my process?
Thanks,
Amitai

6. ### Colin Mitchell

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312
Aug 31, 2014
Once you get over 10cm for 2cm coin, let me know.
Pin 7 is the same as pin 3

7. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Pins 3 and 7 of a 555 are not the same. Pin 3 is designated as the "output" and pin 7 is designated as the "discharge" pin. They are both driven from the same internal signal. Pin 7 is an open collector output and pin 3 is a totem pole output. You can avoid the "R1" resistor in the standard oscillator configuration if you use pin 3 to drive the oscillator capacitor via the "R2" resistor.