Proximity Detectors

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.

Outline

  • Circuit Diagram
  • Components Required
  • Component Description
  • Metal Detector Circuit Explanation
  • Block Diagram of Metal Detector
  • Working
  • Advantages
  • Disadvantages
  • Applications

Circuit Diagram

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

Metal Detector

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.

TDA0161 Proximity Detector IC is no longer available, according to STMicroelectronics. If it’s on the market, go ahead and make this enjoyable project. If it isn’t available, look for a compatible IC. If a similar IC becomes available, we will endeavour to update. Please let us know if you find any Proximity Detector ICs in the comments area.

Coil (Inductor): For this project, we used 30 AWG copper wire. The coil is then coiled using a 5.8cm diameter reference. There are 140–150 spins in the coil.

Metal Detector Circuit Explanation

  • When any metal that is close to the LC circuit, L1 and C1, receives a resonant frequency, an electric field is formed, which induces current in the coil and affects the signal flow through the coil.
  • To make the proximity sensor value equivalent to the LC circuit, a variable resistor is needed. It is best to check the value while the coil is not close to the metal. The LC circuit will have changed signal when the metal is identified. The modified signal is sent to the proximity detector (TDA 0161), which detects the change and responds appropriately. When no metal is detected, the proximity sensor’s output is 1mA, and when metal is found, it is about
  • 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

block diagram

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.

The inductor and capacitor are connected in parallel in the LC circuit. When material of the same frequency comes close to this circuit, it begins to resonate. The LC circuit alternates between charging the capacitor and inductor. Charge is applied to the inductor when the capacitor is fully charged.

The inductor begins charging, and when the charge across the capacitor is zero, it reverses polarity and drains charge from the inductor. The inductor charge is then lowered, and the process begins all over again. Note that an inductor stores magnetic fields while a capacitor stores electric fields.

Proximity Sensor: Without any physical interference, the proximity sensor can detect objects.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 major metal detector element of the circuit is the LC Circuit, which consists of L1 (coil) and C1. The TDA0161 IC functions as an oscillator with the help of this LC Circuit, also known as a Tank Circuit or Tuned Circuit, and oscillates at a specific frequency.

When the LC circuit detects a resonating frequency from a metal that is close by, an electric field is formed, causing current to flow through the coil and changes in the signal flow.

The proximity sensor value is changed using a variable resistor in the LC circuit; it is best to examine the value while the coil is not near any metal item. The LC circuit will be activated when the metal is identified.

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.

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