Friday, November 14, 2014

METAL DETECTOR CIRCUIT

The device we then build will function as a metal detector that can scout out metal objects, such as coins, nails, keys such as car keys you may not be able to find, and even gold if youre looking for in a beach (though this one may not have industrial strength). This metal detector can detect certain kinds of metal- especially iron-containing metals, which are called ferrous metals, even if under a half-inch of drywall or sand.

How this metal detector works is that it uses an IC that generates an AC signal that goes through a coil. Metal objects are objects which conduct electricity, so a current can be induced in these metal objects. When the coil in the metal detector comes near a metal object, the electromagnetic field in the coil induces currents in the metal object. The electromagnetic field generated by the metal changes the current in the coil. When the signal changes, the IC turns on an LED i.e. alerting the user to the presence of a metal.

Circuit Diagram

Metal Detector Circuit
Parts Needed
  • TDA0161 Proximity Detector (IC1)
  • 2 1KΩ Resistors (R1, R4)
  • 10KΩ Potentiometer (R2)
  • 330Ω Resistor (R3)
  • 120Ω Resistor (R5)
  • 2N3904 Transistor (Q1)
  • 2 4.7nF ceramic capacitors (C1, C2)
  • 680 picohenry bobbin-type Inductor
  • Battery Holder for 4 AA batteries (6V)
  • LED
  • SPST Switch

Explanation of Circuit

Inductor L1 : The inductor L1 forms a parallel circuit with the capacitor C1 to form an LC parallel circuit. When a signal that oscillates at several KHz passes through this circuit, the signal creates an electric field around the coil. When you bring the coil near a metallic object, that electric field induces an oscillating signal in the object. So when the oscillating signal has been induced in the metallic object, the signal in the object creates an electric field that induces current in the coil. This current changes the oscillating signal running through the LC parallel circuit.

TDA0161 Proximity Detector IC : This IC is a proximity detector. This IC suplies the oscillating signal that is sent through the LC parallel circuit. The IC also responds to any changes in the signal. The IC has an output of 1 milliamperes (mA) or less if the coil is far from a metallic object and an output of 0mA or higher if the coil is near a metallic object. 
Thus, this IC is at the heart of this circuit. When the object is far from a metallic object, the current which the IC produces is insufficient to drive the LED. Thus, the LED does not turn on. When the coil is near a metallic object, the IC produces sufficient power to drive the LED and it turns on. 

Resistor R1 and Potentiometer R2 : These resistors are used to calibrate the TDA0161 IC to the LC circuit. You calibrate it by adjusting the potentiometer to change the current output it creates in accordance with the proximity of metal to the coil. You can adjust so that it can detect metals at the distances which you want it to. By increasing the potentiometer resistance, the IC will create less current output. Therefore, a metal must be placed closer to the coil in order for the LED to light. By decreasing the potentiometer resistance, the IC produces less current output, so the metal doesnt have to be placed as close to the coil. Its up to you to set the adjustment.

2N3904 Transistor (Q1) : The 2N3904 transistor provides amplification, so that there is sufficient current to power the LED. Without this transistor, there would not be enough power to turn on the LED. 

LED : The LED in the circuit serves as an indicator to when there is a presence of a metal. When a metal is in close proximity to our electronic circuitry, the LED turns on. This shows we have found metal. When the LED is off (not lit), then our metal detector has not detected metal and indicates no metal is in close proximity.

6 volts : The 6 volts is the supply power to the entire circuit. This 6 volts is supplied through 4 AA batteries in series. Being that each battery supplies 1.5 volts, 4 AA batteries (1.5V * 4) provides 6V. This 6 volts gives power to every component in the circuit.

SPST Switch : The SPST switch allows us to shut off power to the circuit, if we want the metal detector power shut off, just like any electronic device would have. This just serves asn an on/off switch.

This circuit detects metal through the coil, L1. Once metal is placed near the inductor L1, it will trigger current production from the proximity detector IC, which in turn lights the LED. So to test this circuit, just place a metallic object near this inductor. When done, the LED should turn on. When the metallic object is moved away, the LED should shut off. 
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