Hi guys,

I'm working on a metal detector project and I need some external input/ideas on how to increase its range.
Here's how it works:

Its a basic Colpitts oscillator featuring a self wound Inducting coil. This coil is on a diameter 125mm former with 0.25mm enameled copper wire consisting of 26 turns. The circuit works fine and produces an oscillating frequency of approximately 160 kHz.

When a metallic object is brought close to the coil, the varying magnetic field induces Eddy currents in it which lower the inductance of the coil hence increasing the frequency of the oscillator. The closer the object the greater this frequency deviation. At the moment the frequency deviation begins to get slightly noticeable when the object is approximately 10cm away. Problem is, I need this coil to be able to detect objects at approximately 20cm away or greater if possible.
My thoughts are:
Either try to increase the current through the coil
or increase the number of turns on the coil.

I can perhaps increase the coil diameter up to 200mm, and I don't mind having oscillating frequency reduced as much as to 120kHz (which may come from increasing turns).

Any input will be much appreciated!

 

Increasing diameter would be simple if you have a coil you can substitute to try.

 

A stronger magnetic field in your coil would help increase range. Can you increase current consumption in your oscillator?

 

Why not add another 9 volt battery in series?

 

10cm is about the best you can get with these circuits.
You can try the 2 coil arrangement with 50 and 70 turns as the additional number of turns adds to the sensitivity.
The only way to get deeper detection is to go for pulse circuits that inject an enormous amount of energy into the surroundings and then wait for a return pulse.

 

The magnetic field most definitely needs to be increased, which requires an increase in current. At the moment the current draw from the coil is approximately 4-5mA and to increase this, the voltage input to the transistor must be increased. However this requires too great a increase in voltage in order to compensate and I'm afraid as Colin55 has indicated, this circuit may not be able to generate the energy required.

Colin55 I have tried to implement your circuit but I must be honest, I don't fully understand how it works. Are the two coils supposed to operate like a conventional BFO detector, or do both these coils in combination with the two BC547's form a single oscillator? Upon setting up this circuit the speaker produces a constant high pitched tone and since I'm unaware of how this circuit works I'm not too sure where to start debugging.

Furthermore, the inductors are 70 and 50 turn, but on what size/shape coil? Are these single or multilayer coils? I ask because when winding my own coil I calculated the turns required in accordance to the frequency required and the core and wire diameter used.

Thanks for your responses guys

 

See Metal Detector MkII on Talking Electronics website. The size of the coils are important as well as how much they overlap.
More turns on the coil(s) produces more sensitivity and although the circuit is sensitive, I would prefer to detect the change in frequency and not the change in output voltage from the detecting coil.
In addition, the coil should be lightly loaded and not heavily loaded as in the circuit above.
However none of the requirements have been implemented in any circuits and none are much more sensitive than 10cm to 15cm. Even the $1,000 Metal detectors have about the same penetration.

 

The BIG trade off here is battery life.
This is supposed to be field portable equipment.
Preferably light enough for all day use.

 

Yes, but at 5mA, the battery will last 1,000 hours. I have 6,000mAHr 18650 cells at 3.7v
You can take 200mA and they will last all day and 4 will give 14.8v

 

Hi Colin55.

I managed to find your MKII description on your site. It is very helpful, thank you sir. Following your setup instructions I've gotten the circuit to work. However at best it detects a large metal object about 12cm away. Do you have any suggestions on how to increase this range?

 

You can see the Metal Detector circuits article on my website and the links to hundreds of circuits.
They are all the same depth as this is all you can get with 15mA
These circuits are working down to 1 cycle in 300,000

 

Good Day Collin55.

I have thoroughly revised your metal detector article. All of these circuits should provide more/less the same depth. I see that you have listed the oscillating detector "circuit 8" as your most sensitive detector. This circuit is not very different to the original design I have proposed and I'm quite sure it will achieve the same depth as well.

Just for experimental purposes I've hooked up the MKII to circuit to a PSU of mine and began increasing the voltage. At around 24V it seemed to be capable of detecting a large metal object around 18cm away. Better than any of the circuits I've tested so far!

However the problem with this circuit is that it continually needs to be tuned via the potentiometer and you always have to hit the "sweet spot" to get maximum range which isn't always the easiest thing to do. Furthermore this circuit works of varying voltage levels and I'd prefer to track varying frequencies.

I feel as if I'm left to conclude with the fact that it's not possible to create a detector to reach depths of 20cm or greater without going into very complex designs such as the induction balance detectors??

 

If this will do the job:

Have a look here: https://my.vanderbilt.edu/mriinterlock/files/2013/01/Induction-Balance-Metal-Detector.pdf