Metal Detector

Best Metal Detector-Homemade

Nowadays, metal detection has become a hobby of many people. Besides as a funny and interesting hobby for them, they also wished indeed a treasure that is embedded in the soil when excavated. For this one hobby, you have to have a tool known as a metal detector.

To undergo this hobby is quite expensive to buy. But for those of you who want to try to make yourself a metal detector, the following will be presented a simple schematic that relates to metal detection.

The operation of metal detector is based on superheterodying principle, which is generally used in a heterodyne receiver. This circuit uses two RF oscillators. Both oscillator frequency is fixed at 5.5 MHz. The first RF oscillator comprises transistor T1 (BF 494) and 5.5 MHz ceramic filter commonly used in TV sound-IF section.

The second oscillator is an oscillator Colpitt realization with the help of the transistor T3 (BF494) and inductor L1 (follow the details of construction) was driven by trimmer capacitor VC1.

These two oscillator frequencies (say Fx and Fy) are mixed in a mixer transistor T2 (another BF 494) and the difference or the frequency of beats (Fx-Fy) output from the collector of transistor T2 is connected to phase detector consists of diodes D1 and D2 (both OA79).

The Output is a pulsating DC, which is passed through a low pass filter realized with the help of 10K resistor R12 and two 15nF capacitors C6 and C10. It is then forwarded to the AF amplifier IC1 (2822M) via volume control VR1 and the output fed to the speaker 8 ohm/1W.

Inductor L1 can be constructed using 15 turns of wire 25SWG at 10 cm (4-inch) diameter air-core former and then cementing with insulating varnish. For proper operation of the circuit, it is important that the frequency of the oscillator is equal to obtain zero beats in the absence of any metal in close to the circuit.

Adjustment of oscillator 2 (to match the frequency of oscillator 1) can be done with the help of the trimmer capacitor VC1. When two frequencies are the same, the beat frequency is zero, i.e. beat frequency = Fx-Fy = 0, and thus there is no sound from the loudspeaker.

When the search coil L1 through the metal, the metal changes its inductance, thus changing the frequency of the second oscillator. So now Fx-Fy is zero and the loudspeaker voice. So one is able to detect the presence of metal.

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