VIT UNIVERSITY

School of Electrical Engineering

Metal Detector

Atit S Gaonkar – 14BCE1144

Subject: EEE101

FALL 2014 – 2015

ABSTRACT

It is a low cost metal detector using a transistor circuit.

Same frequencies beat off to produce no sound.

The buzzer, and the LED is used in such a way that the

current provided to it is not sufficient to it.

Inductance of its coil changes, and so do the frequency of

oscillations when placed near metal. Now when there is a

change in the frequency, the buzzer will produce a sound

with the glowing LED.

BASIC CUIRCUIT DIAGRAM

This is the basic circuit diagram of a metal detector. It is called

as the Colpitts Oscillators. The resistors and capacitors are

should be specified and of the same type so that the current

would be able to trigger the sound in buzzer and glow the LED.

The series capacitors in parallel with the inductor is called as an

oscillator. First the capacitors get charged and as soon as it gets

fully charged, it starts to discharge the energy to the inductor.

The inductor gets energy and then again gives it to capacitor.

This process repeats itself and makes oscillations and has a

certain frequency.

This effect links back into the receiver coil(s) in the search head,

where it induces an electrical voltage which is detected and

converted, for example, into an audio signal.

BLOCK DIAGRAM

COIL ---> DETECTOR ---> RF ---> LED & BUZZER

AMPLIFIER

First the search head or the coil is made to interact with the

metal and then the oscillator fails to maintain the frequency,

hence the metal is detected. Then the signal is amplified and

given to LED and Buzzer to indicate the presence of a metal.

Only those metal can be detected which can change the

frequency to RF range.

CIRCUIT DIAGRAM

COMPONENTS

Resistors :

56K, 33K, 22K, 27K, 22K, 680, 15K, 5K

Capacitors :

1PF, 220PF,270PF,12KPF,1KPF,100KPF

Transistors :

BC 547

Diode :

IN 4148

LED

Coil

Buzzer

Battery

PCB

It will nearly cost you around Rs.500

WORKING

The operation of metal detectors is based on the principles of

electromagnetic induction. It generates a rapidly changing

magnetic field by running the alternating current though coil,

which will generate eddy currents inside the metal objects.

Thereby, the eddy currents will create a new magnetic field that

affects the original one, and then the metal detectors will utter a

high-pitched tone. The accuracy and reliability of the metal

detectors depend on the stability of frequency of electromagnetic

launchers. Generally, the frequency is between 80 to 800 KHz.

This metal detector can be used to detect slightly bigger size

metallic object. It uses a sensing coil. This coil should be kept

near metallic objects for detection.

Input is a weak Colpitts Oscillator RF. When coil is brought

near a metallic object, magnetic energy is absorbed and

oscillator fails to maintain the frequency. Then the final

transistor conducts, hence the Buzzer and the LED is activated.

Use a 9V battery only to make it work correctly.

APPLICATION

The metal detectors are able to detect all kinds of metal impurity

of different metal materials. The metal detectors can be used

widely in food detecting. They are specially used in detecting

the metal impurity in meat, beverage, fruits, vegetables, dairy

products, food additives, and health care products and so on.

Besides, they can be used to detect the metal impurity in

chemical raw materials, rubber, plastics, textile, leather,

chemical fiber and toys, etc.

Metal detectors are mainly used in security systems.

REFERENCE

• www.circuiteasy.com

• www.instructables.com

• www.electronicshub.org

• www.circuitstoday.com

• www.slideshare.com