Simple Metal Detector Circuit

A special technique is used in this article for designing a metal detector.

It only uses a circuit design that is different from the others circuit and needs only one transistor.

The circuit picks up signals reflected off metal using a regular radio.

What is a Metal Detector:

A metal detector is an electrical circuit that is designed to identify whether or not there is a metal detector nearby.

It works on the basis of electromagnetic induction which is a process wherein metal changes the resonance frequency of a tuned circuit.

Applications for metal detectors are many and include industrial operations, treasure seeking, security screening and archeological research.

Circuit Working:

Parts List:

Below is a detailed explanation of the working of the metal detector circuit:

Wind 30 turns of wire around a plastic base or bobbin with a diameter of 3 to 4 inches.

This forms the oscillator coil L1.

Make L1 into a search head or sensing coil by connecting it to the circuit using a three wire connection.

For ground sweeping metal detection place the search head at the bottom end of a long wooden or plastic pole.

For increased versatility the complete circuit is housed in a portable box for detecting metal pipes or wires hidden behind walls.

The circuit depends upon the presence of a metal detector interfering with the electromagnetic field generated by coil L1.

Both the inductance value of L1 and the frequency of the electromagnetic field change when a metal detector attacks.

When a battery powered portable broadcast band radio is brought close it acts as a detector and amplifier.

The radio detects variations in frequency caused by the metal detector and emits a loud signal.

One should set the radio initially to a local station.

Check the radios speaker for a low frequency beat or flutter.

Adjust capacitor C1 to observe a low frequency beat or chirp indicating successful calibration.

When placed near the hidden metal detector the locating circuit significantly changes the rhythm tone.

The circuit measures changes in the electromagnetic field produced by the oscillator coil as a result of metal detector noise.

These differences are amplified and signaled by the radio receiver which makes it a useful metal detector finder with use in a variety of settings.

The circuits components which involves capacitor C1 is adjusted to provide accurate calibration for maximum output.

Formula:

Below mentioned formula helps in figuring out the metal detector circuits initial working frequency.

The desired oscillation frequency is adjusted by choosing certain values for a capacitor C and the coils inductance L:

f = 1 / 2π√L * C

here,

• In hertz Hz f is the circuits resonance frequency the resonant frequency is the frequency at which the circuit oscillates most naturally and with the greatest amplitude.
• 2π usually occurs in calculations involving oscillations and other periodic processes it is the product of two times the mathematical constant pi or about 3.14159.
• For the LC circuit L is the inductors measured inductance in henries H inductors resist changes in current by generating a voltage.
• The capacitance of the capacitor in the LC circuit in an electrostatic field is C which is measured in farads F capacitors are used to store electrical energy.
• This symbol stands for the square root √.

The formulas operation:

By calculating the square root of the product of capacitance C and inductance L the formula finds the LC circuits resonant frequency in reverse.

The circuits resonance frequency will decrease with increasing capacitance or inductance.

However a greater resonant frequency will result from a lower capacitance or inductance.

How to Build:

The following are the connections steps involved in building the metal detector circuit.

Get the Oscillator Coil L1 ready:

• Wrap 30 insulated wire turns around a 3 to 4 inch diameter plastic bobbin.
• Make that the turns are securely and firmly coiled.

Attach the circuit to the oscillator coil:

• Connect the oscillator coil L1 to the circuit using a three wire cable.
• L1 can serve as a sensing coil or search head because to this cable.

Put the Circuit Together on a Breadboard:

• The transistor should be placed on the breadboard.
• Decide which pins are the emitter, base and collector.
• Usually the emitter and collector terminals are used to connect the oscillator coil L1 to the proper locations on the transistor.
• Add resistors if necessary to bias the transistor.
• The variable capacitor C1 should be integrated.
• To adjust the circuit connect the variable capacitor C1 to it.
• To fine tune the circuit for the best results change the capacitance of C1.

Attach the radio receiver:

• The radio receiver should be placed near the circuit.
• Use the proper wires to connect it to the circuit.
• For better receiving make sure the radios antenna is positioned correctly.

Source of Power:

• Connect the battery so that the circuit is powered.
• Be sure the transistor used can handle the voltage.

Set the Circuit to Calibration:

• Switch the radio to a local station and turn on the circuit.
• Check the radios speaker for a low frequency beat or flutter.
• To ensure proper calibration adjust variable capacitor C1 until a low frequency beat or chirp is heard.

Examine the metal detection system:

• See how the radios output changes when one brings the circuit close to a metal detector.
• When a metal detector is present the frequency should visibly change causing the radio to respond.

Improving and fine tuning:

• Improve performance for certain metal detecting factors by experimenting with circuit designs and capacitor values.

Conclusion:

One can create a working metal detector circuit by following these instructions.

Based on ones specific needs for metal detecting application, can change the circuit settings such as the values of the capacitors and the design of the coil.

References:

Design of a Beat Frequency Oscillator Metal Detector