Metal Detector Circuit

Metal detector 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.

In this project, we have designed a simple DIY type Metal Detector Circuit using very simple components that can be used in our homes and gardens.

Circuit Diagram

The following image shows the circuit diagram for the metal detector circuit.

Components Required

• 1 x TDA0161 Proximity Detector IC
• 2 x 47nF Capacitors (Ceramic Capacitor code 473)
• 1 x 1 KΩ Resistor (1/4 Watt)
• 1 x 330 Ω Resistor (1/4 Watt)
• 1 x 100 Ω Resistor (1/4 Watt)
• 1 x 5 KΩ Potentiometer
• 1 x 2N2222A (NPN Transistor)
• 1 x 5V Buzzer
• Coil (copper wire of 26 – 30 AWG is taken and it is wound in to a coil of diamater 5 – 6 cm and 140 – 150 turns)
• Additional Components (for LED)
  • 1 x 220 Ω Resistor (1/4 Watt)
  • 1 x 5mm LED

Component Description

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.

TDA0161 has 8 pins and it comes in Dual in – line Package (DIP). The following image shows the pin diagram of TDA0161 IC.

NOTE: According to STMicroelectronics, TDA0161 Proximity Detector IC is obsolete. If it is available in the market, go ahead and make this fun project. If it isn’t available, try to find a replacement IC. We will try to update if any similar IC is available. If you find any Proximity Detector ICs, please mention it in the comments section.

Coil (Inductor): We have taken a 30 AWG Copper wire for this project. It is then wound in to a coil using a 5.8cm diamater reference. The coil consists of 140 – 150 turns.

Metal Detector Circuit Explanation

• When the LC circuit that is L1 and C1 has got 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 there is coil not near to the metal. 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 be of 1mA when there is no metal detected and it will be around 10mA when coil is near to the metal
• When the output pin is high the resistor R3 will provide positive voltage to transistor Q1. Q1 will be turned on and led will glow and buzzer will give the buzz. Resistor r2 is used to limit the current flow.

Block Diagram of Metal Detector

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.

Proximity sensor(TDA0161), is triggered by this LC cirucit if any metal is detected.The Proximity sensor will then turn on the led and produces alarm using buzzer.

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.

Working

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.

When the output pin is high, the resistor R3 will provide positive voltage to transistor Q1. Q1 will be turned on and LED will glow (not shown in the circuit) and buzzer will be activated.

Advantages

• The Proximity Detector IC TDA0161 based Metal Detector Circuit is a very simple and easy to construct metal detector that can be used to detect small metals in our homes, offices and gardens.
• There is need for any microcontroller as the Proximity Sensor will be sufficient to implement the project.

Disadvantages

• The main disadvantage of this Metal Detector Circuit is the range of detection. The metal object has to be at a distance of 10mm for the detector to detect it.

Applications

• This simple Metal Detector can be used to identify metals like iron, gold, silver etc.
• Since it is a simple project, we can use this in our home to scan for nails, metal scraps etc. which are not easily spotable by naked eye.