Simple Metal Detector Circuit

This post shows you a special way to build a metal detector.

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

The circuit works with a regular radio to pick up signals that bounce off metal.

What is a Metal Detector:

A metal detector is an electronic circuit designed to detect the presence of metal detector in its vicinity.

It operates on the principle of electromagnetic induction or the change in the resonant frequency of a tuned circuit in the presence of metal.

Metal detectors are widely used for various applications including security screening, treasure hunting, archaeological exploration, and industrial processes.

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.

Securely connect L1 to the circuit using a three wire cable transforming it into a search head or sensing coil.

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

If detecting metal pipes or wires concealed by walls house the entire circuit in a portable box for enhanced adaptability.

The circuit relies on 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 intrudes.

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.

Set the radio initially to a local station.

Listen for a low frequency rhythm or flutter from the radios speaker.

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

The locating circuit, when positioned close to the concealed metal detector causes a drastic change in the beat note.

The oscillator coil generates an electromagnetic field, and the circuit detects variations in this field caused by the interference of metal detector.

The radio receiver amplifies and signals these variations making it an effective metal detector finder with applications in diverse scenarios.

Adjustments to the circuits components, such as capacitor C1 enable precise calibration for optimal performance.

Formula:

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

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

f = 1 / 2π√L * C

here,

• f: This is the circuits resonance frequency, expressed in hertz Hz, the frequency at which the circuit oscillates most naturally and with the largest amplitude is known as the resonant frequency.
• 2π: is the product of two times the mathematical constant pi or roughly 3.14159, it typically appears in computations involving oscillations and other periodic processes.
• L: This is the inductors measured inductance in henries H for the LC circuit, by producing a voltage inductors resist variations in current.
• C: This is the capacitance, expressed in farads F of the capacitor in the LC circuit, in an electrostatic field, capacitors store electrical energy.
• √: The square root is represented by this symbol.

How the formula functions:

In essence, the formula says that the square root of the product of capacitance C and inductance L determines the resonant frequency of the LC circuit in an inverse manner.

The circuits resonance frequency will decrease with a greater inductance or capacitance.

On the other hand, a greater resonant frequency will result from a lower capacitance or inductance.

How to Build:

Building the metal detector circuit involves in the following process.

Prepare the Oscillator Coil L1:

• Wind 30 turns of insulated wire around a plastic bobbin with a diameter of 3 to 4 inches.
• Ensure the turns are wound tightly and securely.

Connect the Oscillator Coil to the Circuit:

• Use a three wire cable to connect the oscillator coil L1 to the circuit.
• This cable will transform L1 into a search head or sensing coil.

Assemble the Circuit on a Breadboard:

• Place the transistor on the breadboard.
• Identify the collector, base and emitter pins.
• Connect the oscillator coil L1 to the appropriate points on the transistor typically using the emitter and collector terminals.
• If needed add resistors for biasing the transistor.

Integrate the Variable Capacitor C1:

• Connect the variable capacitor C1 to the circuit for tuning purposes.
• Adjust the capacitance of C1 to fine tune the circuit for optimal performance.

Connect the Radio Receiver:

• Place the radio receiver close to the circuit.
• Connect it to the circuit using appropriate wires.
• Ensure the radios antenna is positioned for better reception.

Power Supply:

• Connect the battery to power the circuit.
• Ensure the voltage is suitable for the transistor being used.

Calibrate the Circuit:

• Turn on the circuit and set the radio to a local station.
• Listen for a low frequency rhythm or flutter from the radios speaker.
• Adjust the variable capacitor C1 until a low frequency beat or chirp is heard indicating successful calibration.

Test the Metal Detection:

• Bring the circuit close to a metal detector and observe the changes in the radios output.
• The presence of a metal detector should cause a noticeable variation in the frequency triggering a response from the radio.

Fine Tuning and Optimization:

• Experiment with different capacitor values and circuit configurations to optimize performance for specific metal detection scenarios.

Conclusion:

By following these steps, you can construct a functional metal detector circuit.

Adjustments to the circuit parameters such as capacitor values and coil construction can be made based on the specific requirements of your metal detection application.

References

Design of a Beat Frequency Oscillator Metal Detector