DIY Metal Detector

Compared with other types of metal detectors, the working principle of this circuit is as follows: When the inductance of the detection inductor changes, the oscillation frequency of the L oscillator also changes. When any metal body approaches this detection inductor, its inductance changes.

How the frequency changes depends on the characteristics of the metal and the operating frequency used by the circuit. If the operating frequency is high, the metal object can be regarded as a short-circuit loop, which will reduce the inductance of the detection inductor, thereby increasing the operating frequency of the oscillator; if the operating frequency of the oscillator is low enough to ignore the eddy current loss, the detector may be able to distinguish between ferrous metals and non-ferrous metals.

It is very difficult to make an oscillating coil with a frequency not higher than 200Hz, so the oscillation operating frequency of this oscillator circuit is selected to be about 300KHz. In this way, the inductor is very easy to make. It only needs to be made by winding a turn of a coaxial cable according to the dimensions in the figure.

The circuit includes oscillator T1, frequency-voltage converter IC1 and MOS dual op amp IC2. The diameter of the detection head coil is 440mm. The values ​​of C1 and C2 can ensure that the oscillator frequency is about 300KHz. If a smaller diameter detection coil is used, the coil needs to be wound with more turns.

The oscillator signal level must be at least 500mVpp to drive the 4046 IC well. At this level, the phase comparator ensures that the phase-locked loop inside the IC is always locked in sync. The source follower output on pin 10 is then sent to IC2 CA3130 for greater amplification.

The center frequency of the phase-locked loop, that is, the zero point of the microammeter at the center, is adjusted by potentiometer P1. If the sensitivity of the op amp is very high, it is necessary to carefully and repeatedly use P2 for fine adjustment. The sensitivity of the unit is adjusted by P3, which is connected to the negative feedback loop and the inverting input of IC2; at the same time, there is a positive feedback added to the non-inverting input of IC2 through the microammeter and R10. Of course, different impedance meters can also be used, but the values ​​of R9, R10 and R11 must be changed. Note: When detecting metal, there is a certain relationship between the size of the detected object and the detection coil. It is futile to use a detection coil with a diameter of 440mm (17.5 inches) to detect metal the size of a coin.