Simple and sensitive metal detector

Here is an extremely simple, but worthy of existence, metal detector, for the production of which only one cheap integrated circuit op-amp TL084 is required.

What is needed to assemble the circuit

• Quad operational amplifier TL084
• Variable capacitor 100 pF
• Search coils
• Headphones

The project was featured in Everyday Practical Electronics Magazine in 2006 and has since become quite popular among novice radio amateurs, as the simplicity of the circuit resulted in excellent repeatability and really good results.

The device is characterized by high stability and sensitivity, for example, a small coin can be detected at a distance of 15 cm, and a larger metal object at more than 50 centimeters.

The concept is based on BFO (Beat Frequency Oscillator) and IB (Induction Balance) principle, so the circuit can be called BB (Beat Balance) metal detector. Its main characteristics are as follows:

• Depending on the design, a BB metal detector has the potential to have the same sensitivity as an IB.
• It does not require a receiver, amplifier or level detector, which greatly simplifies the design and reduces the assembly cost.
• Both search generators are identical, so BB has high resistance to voltage and temperature fluctuations.
• Each search coil has an opposite reaction to metal, so the BB circuit has a high degree of immunity to ground mineralization.

Manufacturing of metal detector coils

The two coils must be identical, the method of their manufacture is as follows:

_{Each coil is 12 cm in diameter and consists of 70 turns of 0.3 mm enameled copper wire. The coils are then folded into a D shape (that's why they are often called DD), taped, and shielded with aluminum foil tied with tinned copper wire. Make sure to leave a small gap so that the shield does not act as a short-circuited turn. Then we need to connect the coils to the MD circuit using decent microphone cables, trying to correctly identify the start and end wires as shown in the picture.}

Setting up a metal detector circuit

Setting up the metal detector is very simple:

1. First, you need to arrange the two windings so that the gap between them is the smallest, and the variable capacitor is in the middle position. A very high frequency tone will be heard.
2. Then gradually increase the overlap, decreasing the frequency of the sound in the speaker. We continue this way until the sound disappears.
3. Then we move the variable capacitor until the sound with the lowest frequency is heard.
   Now the detector has reached maximum sensitivity.

The detector works surprisingly stable, but from time to time it needs to be fine-tuned with a variable capacitor. For clearer sound, you can connect the headphone output to an audio amplifier, otherwise the volume of the headphones is quite sufficient for working outdoors.

And keep in mind that on the ground the situation with metal detection depends on its composition and humidity. Good luck in assembling the MD!

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