Background Info
Another person sent me an article from Radio TV Experimenter, I Think 1967 Issue.

It was stated as an: "Underwater Metal Detector", by James E Pugh Jr., using a Balance Bridge Design.
He claimed that just an ordinary "100 foot Lamp Cord" could be used between the detector
and the coil, with no problems. It was a Transistor design.

Unfortunately at least one part to build this (a 5.4 Henry Adjustable Choke) is no longer
available, Or would definately be Very Difficult to obtain in todays market place.

In the Origional Article he claimed it would detect a "Camera" at about 1 Foot.
(But Remember a Camera back than Was Bigger and Different Construction than Todays Cameras.)
He also claimed it would detect an "Outboard motor" at about 4 feet.

Lastly, He said Maximum Detection Distance was about 5 Feet.



My Design

First I came up with a Simple IC, Sine Wave circuit design, powered by a single supply.

This is followed by a "Maxwell Inductance Bridge".

In Place of that Bulky Inductor which was just used as a Simple 1 Khz BP Filter,
I designed an Active BP Filter, set to 1 Kc with a "Q" of 25.
Somewhat of an OverKill, but it definately Rejects any Unwanted Interference.

With Further Testing I feel a Q of 10 is better.
** I may update the schematic with these resistor changes,
If there is sufficient interest in this.

Finally an LM386 Power Amplifier, set with a gain of 200.
Definately more gain than needed, but the Volume control allows for a good adjustment.

I just used a Pizo Transducer for my proto.
However a Speaker could be used, or by adding a 100 Ohm current limiting
resistor, it would make it quite suitable for Headphones at some future time.

I used an LM358 for the Oscillator and a seperate LM358 for the Filter.
Partly for ease of a PC Layout and partly to keep these Seperated so less
chance of "Cross talk" of the 1 Khz Signal.

This Circuit is only at 1 Khz, so the use of Better, High Performance IC's,
really won't make any difference in its performance.

I have now built the Circuit on a PCB and it Works.

For My Test Coil, I made a 12 inch diameter coil of about 300 turns # 30 AWG wire.
Actual Measured Inductance was 65 Mhy

In the Origional artical the author made Two coils, A larger Oval Shaped coil for underwater
and a Smaller 6 inch Round coil for other searching of small objects, on the ground or in his house.
I calculated the Inductance of those coils to also be about 65 Mhy.

Reguardless of which coil you make,
I Doubt this detector is practical for
searching for coins or jewerly.

I believe you would need a Much Higher Frequency for that to be practical.
And that would also require many substantial changes to rest of the circuit.

But if you want to Experiment with it, Go For It!

I have not tested this underwater, But it should work fine.
Definately Water Proof the Coil to prevent corrosion.

However, I DID test it with a 50 foot long, Cheap Radio Shack Speaker wire.
No problem.

Latest Info, Feb 19 2006:
I have revised the schematic to match my circuit board that is now shown below.
It is working and has a fairly good sensitivity on larger objects.







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My My Schematic
The small transformer is a "Mouser" Part Number "TL012"


My Prototype PCB.



Here is a Picture Overlay.

All Imformation in this Article is "Copyright protected".

 Chemelec 
 *Copyright © 2006*