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DIY Metal detector

acko1989

Jan 19, 2020
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Hi,

I have questions about this 555 metal detector circuit in attachment down below. I understand that this works in astable mode and the induction of a coil changes when there is metal near by, so frequency changes. My question is: Why is one side of inductor and resistor connected to output (Pin 3) of 555 timer?
 

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Audioguru

Sep 24, 2016
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Pin3 provides positive feedback so the circuit can oscillate. When pin2 goes low it causes pin3 to go high which slowly charges the capacitor at pin2 and pin6. When the voltage on pin6 gets high enough then it causes pin3 to go low which causes the capacitor at pin2 to discharge until its voltage is low enough for the circuit to begin again.

The 555 IC will fail in that circuit because its maximum supply is 16V and you show 18V.

The LC is in the wrong place. With it where it is shown then its current is very low and metal near it will not change the frequency and change its current.

A metal detector is supposed to use a circuit to detect a frequency charge. This circuit does not.
 

acko1989

Jan 19, 2020
5
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Jan 19, 2020
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Pin3 provides positive feedback so the circuit can oscillate. When pin2 goes low it causes pin3 to go high which slowly charges the capacitor at pin2 and pin6. When the voltage on pin6 gets high enough then it causes pin3 to go low which causes the capacitor at pin2 to discharge until its voltage is low enough for the circuit to begin again.

The 555 IC will fail in that circuit because its maximum supply is 16V and you show 18V.

The LC is in the wrong place. With it where it is shown then its current is very low and metal near it will not change the frequency and change its current.

A metal detector is supposed to use a circuit to detect a frequency charge. This circuit does not.
Thank you for answer.
Is capacitor discharged only through 47k ohm resistor? How LC circuit effect frequency in this case? I understand that LC circuit make resonant frequency, but i can't see whole picture.
 

Audioguru

Sep 24, 2016
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I found a few of the same metal detector circuit in Google that are demo'd in You tube. (They might be from the same guy from "over there" who can barely speeky zee Engrish). They also use a 555 circuit that has an LC series circuit in it. Some of them wrongly say the 555 is a monostable but actually it is an ordinary astable oscillator with the inductor changing the pitch when metal is near it. They barely work and the pitch changes only slightly when a huge iron is touching the coil.
In one demo, the pitch changed a lot when a huge iron touched the coil because it wiggled the coil wire making a bad connection on the breadboard. Also the pitch changed a little even when there was no iron near it.
Most authors wrongly say the sound maker is a buzzer but actually it is a piezo transducer or speaker.

A real metal detector uses a BFO (beat frequency oscillator) circuit that has two oscillators that have their frequencies compared and one of them has its frequency changed by metal. They are very sensitive.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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Really good site thanks for the link, just what I'm after
Colin Mitchell, the Australian owner of the Talking Electronics website, used to post here quite often a few years ago. His website is a valuable resource for beginners trying to make sense of all things electronical (yeah, I know electronical isn't a "real" word, but IMO it should be) by combining a little theory with a LOT of experimentation. I believe, and I think Colin believes this too, that you learn by DOING not by "simulating" or simply performing "monkey see, monkey do" activities. It is worthwhile to spend a few hundred hours exploring http://www.talkingelectronics.com.
 

plutes

Mar 4, 2021
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Colin Mitchell, the Australian owner of the Talking Electronics website, used to post here quite often a few years ago. His website is a valuable resource for beginners trying to make sense of all things electronical (yeah, I know electronical isn't a "real" word, but IMO it should be) by combining a little theory with a LOT of experimentation. I believe, and I think Colin believes this too, that you learn by DOING not by "simulating" or simply performing "monkey see, monkey do" activities. It is worthwhile to spend a few hundred hours exploring http://www.talkingelectronics.com.
It is very good, a few weeks of tinkering on just one site. I've spoke to him and will be ordering the nail finder metal detecting kit soon.
 
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