How to Use a Radio as a Metal Detector

A typical metal detector is composed of basic circuits such as dual oscillators, mixers, and difference frequency amplifiers. When the detection coil is close to a metal object, the inductance changes, causing the oscillation frequency to change. Although the metal detector circuit is not too complicated, it is not easy for beginners with limited experience to get the desired effect. The metal detector introduced below only needs a medium wave oscillator, and other functions are replaced by a radio, so it is very easy to make and debug.

The circuit is shown in the figure, which uses the commonly used common base capacitor three-point oscillator circuit. The detailed principle will not be repeated. First, use the radio to receive a certain medium wave radio station, use it as a comparison signal, and then adjust C2 to make the oscillation frequency close to the radio frequency. At this time, the speaker will emit a beat frequency whistle. Select a suitable tone. When the inductor coil L is close to a metal object, the tone changes, and it can be judged whether there is a metal object.

The inductor coil L is wound 16 times on a mold of about +300mm with 0.52mm diameter enameled wire, and tied with insulating tape after being removed from the mold. The diameter of the detection coil can be changed appropriately according to needs. Generally speaking, the larger the diameter, the deeper the detection depth, but the detection sensitivity to smaller metal objects is reduced. In addition, when the detection coil L is close to the ground, it is easily affected by the distributed capacitance and causes the frequency to drift. The coil can be shielded with aluminum foil or soft metal mesh. Note that the shielding layer cannot form a short-circuit loop for the coil, that is, the shielding layer in the schematic diagram can only be grounded at one end, otherwise the oscillation circuit will not be able to oscillate. There are no special requirements for other components, which can be determined according to the conditions at hand.

As long as the circuit is welded correctly and the power is turned on, the oscillation can start. The frequency adjustment range of this circuit is also wide, and it is easy to succeed. The inductor and circuit board are fixed at the lower end of the insulating rod, and the radio is placed at the upper end.

Under normal circumstances, the tone generated by the beat is only determined by the difference between the radio frequency and the oscillation frequency of this circuit, and has nothing to do with the tuning of the radio. The accuracy of the radio tuning only affects the volume, but during use, it will be found that the beat frequency sound can be received at multiple frequency points of the radio, and the tone will also change with the tuning of the radio. This phenomenon is particularly obvious at the high end of the frequency. This is an intermodulation signal generated by the radio, the detection oscillation circuit and the radio local oscillator. It is a good thing for using metal detection circuits, because adjusting the tuning knob of the radio is more convenient than adjusting the variable capacitor of the oscillation circuit. Secondly, if the howling sound is a tuning product of the higher harmonics of the oscillation signal, it can also increase the sensitivity of detection.