Simple Metal Detector Circuit

You can find a metal detector at airports, theaters and various other public places. They are used for the safety of people to detect anyone carrying a metal (Arms etc). In this project we are going to design a simple metal detector circuit. There are so many metal detector designs but most of them are complex in design so here we are going to design a simple metal detector circuit using 555 Timer IC.

Before going further we need to understand concept of inductor and RLC circuits. Let us first talk about inductors. Inductors are nothing but coils of enameled copper wire comes in different shapes and sizes. Based on various parameters the inductance of an inductor is calculated. In all of those parameters we are basically interested in core on the inductor as depending on the core, inductance value changes drastically.

In the figure below you can see the air cored inductors, In these inductors there will be no solid core. They are basically coils left in air. The medium of flow of magnetic field generated by inductor is nothing or air. These inductors have inductances of very less value.

These inductors are used when the need for values of few micro Henry. For values greater than few milli these are not a suitable one. In below figure you can see a inductors of ferrite core ,

When inductor coil is wound on a core may be ferrite or an iron core, the inductance of the coil increases enormously. This value is much more than the air cored one of the same size and shape.

Now for a RLC circuit as shown in figure, the reactance or impedance between terminals “a” and “c” depends on the values of L and C if the applied signal frequency is constant.

So if the inductance value changes the value of reactance or impedance changes. How these two concepts are used together for a metal detector circuit, is explained in working section of this project.

Metal Detector Circuit Components

• +9 supply voltage
• 555 IC
• 47KΩ resistor
• 2.2µF capacitor (2 pieces)
• Speaker (8Ω)
• 170Turns of 10cm diameter coil (any gauge would work)

Metal Detector Circuit Diagram and Working

The figure shows the circuit diagram of metal detector. The 555 IC timer here acts as a square wave generator and it generate pulses with frequencies audible to human. The capacitor between pin2 and pin1 should not be changed as it is need to generate audible frequencies. 

In the circuit there is an RLC circuit formed by 47K resistor, 2.2µF capacitor, and 150turn inductor. This RLC circuit is the metal detection part. Now as mentioned earlier in previous section, a metal core inductor has a high inductance value over a air cored one.

Remember the coil wound here is a air cored one, so when a metal piece is brought near the coil, the metal piece acts as a core for the air cored inductor. By this metal acting as a core, the inductance of the coil changes or increases considerably. With this sudden increase in inductance of coil the overall reactance or impedance of the RLC circuit changes by a considerable amount when compared without the metal piece.                  

At first when there is no metal piece the signal fed to speaker causes some audible sound. Now with the reactance change around the RLC circuit the signal sent to speaker will no longer be the same as before, because of this the sound produced by the speaker will be of different to the first one.                                

So whenever a metal is brought near the coil, the impedance of RLC changes making the signal to change resulting in variation to sound generated in speaker. You can also check this Arduino based metal detector.

Common Tips:

• Enamel should be removed at the tips of coil for soldering connections.
• With different gauge we will have different RLC impedance, so one should experiment with resistance in RLC circuit for sensitive metal detection.
• The speaker can be of any type. But with resistance less than 8Ω, the timer might get heated.
• Use supply voltage higher than 5V.