Metal Detector

 Metal detectors serve a wide range of utilitarian functions such as:

  • Item recovery - help someone search for a lost item, such as a piece of jewelry
  • Archaeological exploration - find metallic items of historical significance
  • Geological research - detect the metallic composition of soil or rock formations

Operating a metal detector is simple. Once you turn the unit on, you move slowly over the area you wish to search. In most cases, you sweep the coil (search head) back and forth over the ground in front of you. When you pass it over a target object, an audible signal occurs.

Metal detectors use one of three technologies:

·         Very low frequency (VLF)

·         Pulse induction (PI)

·         Beat-frequency oscillation (BFO).

We are here using very low frequency concept. It consist of detector circuit and coil loop.

DETECTOR CIRCUIT:

Detector circuit has been made using 555 timer IC which is working in astable mode. (For details on astable mode you can refer to the tutorial on 555 timer on this same site under tutorial tab). 

The RC circuit is responsible for generating the frequency in the coil L1. L1 is a coil loop of copper of 10mh is placed at position. This coil loop can be made at home by using a copper wire of 10mh and rotating it over a cylindrical hollow plate. Remember that wire do not break. The frequency generated by the 555 timer is sent along this wire, first in one direction and then in the other, thousands of times each second. The number of times that the current's direction switches each second establishes the frequency of the unit. The frequency of 2.2 KHz is generated by the RC values taken here. You can generated any frequency by taking desired values of RC.

Each time the current changes direction, the polarity of the magnetic field changes. This means that if the coil of wire is parallel to the ground, the magnetic field is constantly pushing down into the ground and then pulling back out of it. As the magnetic field pulses back and forth into the ground, it interacts with any conductive objects it encounters, causing them to generate weak magnetic fields of their own. The polarity of the object's magnetic field is directly opposite the transmitter coil's magnetic field.If the transmitter coil's field is pulsing downward, the object's field is pulsing upward.

Initially when the circuit is switched on, the initial frequency of 2.2 kHz is present across the coil loop and the output is the continuous sound from the speaker due to this 2.2 kHz frequency. The coil loop generates electric field around it and when any metal object comes across in between these electric field, the frequency changes and the change freq. leads to change in the sound from the speaker. This change of sound is observable and it signals that a metal object is detected or is near the detector. 

On the other side if any non- metallic object is brought or comes across the electric field then sound does not change and hence the aim of the project is accomplished.

It is a tested project with following observations: (initial freq. 2.2 KHz)

 

object 

frequency

metallic knife

2.1 khz

plier

1.9 khz

iron 

2.05 Khz

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