Metal detect with audio amp?

Thread Starter

triops124

Joined Mar 21, 2016
6
Hi All,

I am considering ordering some parts for a theoretical circuit, and I am wondering if you think it would work.

Basically, it's an induction balance very low frequency metal detector circuit that would use a PAM8403 audio amp IC, a speaker, a battery, and two inductor coils.

My theory is that if I position the transmitter coil just outside of the feedback loop range of the receiver coil, it will not make a siren sound on the speaker, but if a ferrous object passes by the coils it will unbalance the circuit and cause a feedback loop, which would cause the speaker to emit a siren.

The siren is generated in a manner similar to putting your microphone too close to a speaker - causing a loud tone from the feedback, except the microphone is a search coil instead inputting into both left and right audio channels. Left output driving a speaker, and right output driving the transmitter coil.

Circuit diagram attached.
Circuit description: battery ground to module ground, battery 5vcc to module power in vcc, speaker to left output, transmitter coil to speaker right output, search coil from line in ground to both left and right channel line in.1647799624098708665585086944951.jpg


Does this circuit make sense or am I missing something?
 

Audioguru again

Joined Oct 21, 2019
4,680
The datasheet for the PAM8403 amplifier shows important input coupling capacitors that you do not have. They prevent the inputs from being DC shorted to 0V.
Your circuit has nothing to set the audio frequency which will probably be higher than you can hear.
Gold and silver are a good find but they are not ferrous and will not make the circuit work.
Your coil on the amplifier output will probably short it.
 

MrSalts

Joined Apr 2, 2020
1,762
Hi All,

I am considering ordering some parts for a theoretical circuit, and I am wondering if you think it would work.

Basically, it's an induction balance very low frequency metal detector circuit that would use a PAM8403 audio amp IC, a speaker, a battery, and two inductor coils.

My theory is that if I position the transmitter coil just outside of the feedback loop range of the receiver coil, it will not make a siren sound on the speaker, but if a ferrous object passes by the coils it will unbalance the circuit and cause a feedback loop, which would cause the speaker to emit a siren.

The siren is generated in a manner similar to putting your microphone too close to a speaker - causing a loud tone from the feedback, except the microphone is a search coil instead inputting into both left and right audio channels. Left output driving a speaker, and right output driving the transmitter coil.

Circuit diagram attached.
Circuit description: battery ground to module ground, battery 5vcc to module power in vcc, speaker to left output, transmitter coil to speaker right output, search coil from line in ground to both left and right channel line in.View attachment 263294


Does this circuit make sense or am I missing something?
Conceptually, you are on the right path for a magnetizable metal finder - this type of locator is used to find property line markers ("corner irons") used in the US. As others have pointed out, your circuit needs some help.
 

MisterBill2

Joined Jan 23, 2018
11,879
If this concept for metal detection would work then detectors using it would be on the market already. And, NO, it wil not function as described because oscillators are more complex than that. Check out "Inductive proximity switches"

In addition, it does not seem like that is actually your goal.
AND, AG is correct about that amplifier needing isolation capacitors.

What could work is to have an oscillator feed one amplifier to drive one coil, and any detected ferrous meta will alter the coupling to a pickup coil, changing the volume of the tone in the speaker. Some expensive metal detectors use that scheme, except that they use balancing circuits to null the signal so that the tone appears when metal is detected. Simiar principle, far more complex.
 
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