Metal Detector Project

[Midas Souza]

Metaldetector is a very common device that is used for checking persons, luggage or bags in shopping malls, hotels, cinema halls, etc. to ensure that person is not carrying any metals or illegal things like guns, bombs etc. Metal Detectors detect the presence of metals.

There are different types of metal detectors like hand held metal detectors, walk through metal detectors and ground search metal detectors. Metal detectors can be created easily and the circuit for a basic metal detector is not that complex.

TDA0161 Proximity Detector IC: TDA0161 is a Proximity Detector IC manufactured by STMicroelectronics. It can be used detect metal objects by detecting the slight changes in the high frequency Eddy current losses.

The TDA0161 IC acts as an oscillator with the help of externally tuned circuit. The changes in supply current will determine the output signal i.e. current is high when a metal object is near and it is low when there is no metal object.

There are three main parts in the metal detector circuit: the LC Circuit, the Proximity Sensor , output LED and the Buzzer. The coil and the capacitor C1, which are connected in parallel, will form the LC circuit.

LC Circuit: LC circuit has inductor and capacitor connected in parallel.This circuit sarts resonating when there is same frequency material near to it. The LC circuit charges capacitor and inductor alternatively.When the capacitor is charged fully ,charge is applied to inductor.

Inductor starts charging and when charge across the capacitor is nil, it draws charge from the inducutor in reverse polarity. Then inductor charge is reduced and again the process repeats.Note inductor is a magnetic field storage device and capacitor is electric field storage device.

Proximity Sensor: The proximity sensor can detect the objects with out any physical interference. The proximity sensor will work same as infrared sensor, proximity also release a signal, it will not give output unless and until there is no change in the reflected back signal.

If there is a change in signal it will detect and give the output accordingly. There are different proximity sensors for example to detect plastic material we can use capacitive type proximity and for metals we should use inductive type.

The LC Circuit, which consists of L1 (coil) and C1, is the main metal detector part of the circuit. With the help of this LC Circuit, which is also called as Tank Circuit or Tuned Circuit, the TDA0161 IC acts as an oscillator and oscillates at a particular frequency.

When the LC circuit detects any resonating frequency from any metal which is near to it, electric field will be created which will lead to induces current in the coil and changes in the signal flow through the coil.

Variable resistor is used to change the proximity sensor value equal to the LC circuit, it is better to check the value when the coil is not near any metal object. When the metal is detected, the LC circuit will have changed signal.

The changed signal is given to the proximity detector (TDA 0161), which will detect the change in the signal and react accordingly. The output of the proximity sensor will less than 1mA when there is no metal detected and it will be around 10mA (usually greater than 8mA) when coil is near to the metal.

I built it and it works pretty good. Not sensative enough to use for digging up treasures at the beach, but I think it will be useful for finding that lost bolt or screw in the grass. Next step will be building a soldered up version and mounting it on a handle. No problem finding the IC on ebay.

I have a need for similar circuit but the only difference is, the circuit I need should function the same way as this one does IN MAGNETIC FIELD. Do you know if this should function the same way under the influence of magnetic field?

A DIY BFO Metal Detector project is a project where individuals create their own metal detector using a Beat Frequency Oscillator (BFO) circuit. This type of metal detector is commonly used for finding metallic objects in the ground.

A BFO metal detector works by using a coil of wire to create an electromagnetic field. When this field comes into contact with a metallic object, it causes a change in the frequency of the oscillator, which is detected by the circuit and produces an audible signal.

The materials needed for a DIY BFO Metal Detector project include a BFO circuit board, a 9-volt battery, a speaker, a 555 timer IC, capacitors, resistors, a potentiometer, a coil of wire, and a metal detector probe. These materials can often be found at electronics stores or online.

Yes, anyone with basic knowledge of electronics and access to the necessary materials can build a DIY BFO Metal Detector. However, it is important to follow proper safety precautions and instructions to avoid any potential hazards.

The benefits of a DIY BFO Metal Detector project include the satisfaction of building something with your own hands, the ability to customize and modify the detector to fit your specific needs, and the potential cost savings compared to purchasing a pre-made metal detector.

As mentioned in the last post, I have some ideas about improvements to the circuit. I haven't tried any of these out (and won't be able to in the immediate future since school's starting again), I wanted to write them down anyway.

I'm not going to go to deep here, basically BFO detectors aren't a super good way of making a metal detector, and if I was going to build a metal detector 'for real' I'd order parts so that I could build a different architecture. @mircemk has a number of metal detector projects posted that use non-BFO architectures, and seem to achieve better results than what I was able to get. If you need a better metal detector start looking there.

I didn't try to optimize the coil design[1] at all while I was building. I think if one knew more about coil design one could probably come up with a better design, but I don't so there's not much for me to comment on here.

The sense oscillator and BFO designs actually both worked reasonably well in isolation. The only issue is that when the 555 timer shares power rails with the colpitts oscillator the two oscillators tend to couple when the sense oscillator drifts to a frequency near the beat frequency. This is a problem since it means that minimum difference frequency generated by the mixer is limited to how close you can bring the two oscillators without them coupling. In practice this means that the operator hears a pretty high pitched whistle, small variations in which are hard to discern. I think if you could get the oscillator frequencies closer (thereby making the whistle pitch lower) it would be easier to discern small variation, essentially making the detector 'more sensitive'.

I'm not entirely sure how one would go about decoupling the two circuits however. Decoupling caps on the 555 certainly help, but the circuits still couple a bit. I think that moving the circuits farther apart, and possibly trying to separate ground connections/planes might improve coupling a little as well.

The other obvious area for improvement is the FET mixer. I didn't really design this circuit, so much as just stick stuff into a breadboard until things worked enough. Eventually I did end up spending an afternoon messing around in LTspice, and came up with the following as an improvement for the mixer:

(Note: I guessed at the biasing values for the old FET mixer (the voltage divider is a trimpot in the real thing, and I didn't measure it to see what the final position was for the simulation), so its possible that the old circuit might actually perform better than what's shown here, but I don't think that its doing that much better)

Finally I think that a better amplification stage is in order. Specifically the signal from the sense oscillator was attenuated before feeding into the mixer in order to stop the amp from making a horrific screeching when hooked up to the earphones. If I'd been less lazy I think I could have redone the amp circuit to have a gain lower than the default (20V/V), and then done away with the attenuation, which would probably also help with overall sensitivity.

Its also possible that the better mixer stage might not need an amplifier at all, but that would require some real world testing. If that were the case though you could see a pretty spectacular reduction in the BoM magnitude, which is always welcome.

We know for sure that there is a metal stake driven into the ground on one corner of the property, supposedly put there by a surveyor, since we've found that stake previously. As a result we assumed that there would be a surveyor's pin on all of the corners of the property. However, after an unsuccessful search for it we did a bit more research, and it seems like pins are only placed if the property owners request a survey for and no pins were placed when the government initially surveyed the land as we had assumed. Therefore, it seems like there is a pin in the opposite corner of the property since the far neighbour must have done a survey at some point and driven a stake then. Since the property line that's under dispute is between us and the near neighbour it seems like there's probably never been an official survey, and there is likely no pin to look for.

Additionally, the ground near were the corner is supposed to be according to the government maps of the area is pretty rocky, and as a result its probably not possible drive a steel peg into the ground there anyway. In talking to some other people afterwards I've also heard that surveyors can sometimes use naturally occurring landmarks (such as large identifiable rocks) instead of a pin, so its possible that even if a survey had been carried out there wouldn't be a pin left.

So while my failure to find a metal object that doesn't exists isn't entirely my fault, I think that there is still a lesson to be learned here which is: make sure the solution you've come up with is actually a solution to the problem at hand...

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