This document describes the design and working of a low-cost metal detector circuit. It uses a Colpitts oscillator circuit with resistors, capacitors, transistors, diodes, an LED, coil, buzzer and battery. When the coil is brought near a metal object, it absorbs magnetic energy and causes the oscillator frequency to change. This triggers the final transistor to conduct, activating the buzzer and LED to indicate metal detection. The metal detector can be used to detect metallic objects for applications like food safety inspection and security systems.

1. VIT UNIVERSITY
School of Electrical Engineering
Metal Detector
Atit S Gaonkar – 14BCE1144
Subject: EEE101
FALL 2014 – 2015

2. 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.

3. 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.

4. 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.

5. CIRCUIT DIAGRAM

6. 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

7. 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.

8. 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.

9. 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.

10. REFERENCE
• www.circuiteasy.com
• www.instructables.com
• www.electronicshub.org
• www.circuitstoday.com
• www.slideshare.com