This is a metal detection circuit that detects coins or metal sheets under the carpet across the carpet. This small device is very suitable for home-made.
First, the component preparation circuit The NPN type transistor type is 9014, and the magnification of the triode VT1 is not too large, which can improve the sensitivity of the circuit. VD1-VD2 is 1N4148. The resistance is 1/8W.
The probe of the metal detector is a key component, which is an inductor with a magnetic core. The magnetic core can be selected from the Φ10 radio antenna magnetic bar, and the interceptor is 15mm. Then use the insulating plate or thick cardboard to make two baffles with a diameter of 20mm. Dig a hole of Φ10mm in the middle and then cover the two ends of the core, as shown in Figure 1. . Finally, the Φ0.31 enameled wire wraps around 300 turns on the core. The probes that do this work best. If you can't make it yourself, you can buy a 6.8mH finished inductor, but it must be a vertical inductor wound around the "work" shaped core, and the smaller the resistance of the inductor, the better.


Second, the circuit fabrication and debugging Figure 2 is the metal detector electrical schematic diagram, Figure 3 is its circuit board installation diagram, Figure 4 is its circuit board component installation diagram. The lead wires of the components used are cleaned and tinned before assembly. According to the three figures, resistors, diodes, capacitors, transistors, LEDs, and trimmer resistors are soldered to the circuit board, and the inductor probe, switch, and battery clip are connected to the circuit board. The circuit is installed and the power can be debugged without checking. Turn on the power and slowly adjust the resistance of the trimming resistor RP from large to small until the LED is lit. Then use a metal object to approach the end face of the inductive probe, and the LED will go out. Adjusting the trimming resistor RP can change the sensitivity of the metal detector. The resistance of the trimming resistor RP is too large or too small to work. If the adjustment is good, the detection distance of the circuit can reach 20mm. However, be careful that the inductive probe of the metal detector is not too close to the component, and do not use a metal casing when packing. If necessary, the inductive probe of the metal detector can also be taken out and fixed with a non-metallic material.





Third, the circuit works the main part of the metal detector circuit is a critical state of the oscillator, when a metal object close to the inductance L (ie the probe of the detector), the electromagnetic field generated in the coil will be induced in the metal object Eddy current, this energy loss is derived from the oscillating circuit itself, which is equivalent to the addition of loss resistors in the circuit. If the metal object is closer to the coil L, the loss in the circuit is increased, and the coil value is lowered, so that the oscillator that is originally in the oscillation critical state is stopped. Thereby controlling the backlight of the rear LED.
In this circuit, the transistor VT1 and the peripheral inductor and capacitor form a capacitor three-point oscillator. Its AC equivalent circuit (not considering the role of RP and R2 is shown in Figure 5. When the base of the transistor in Figure 5 has a positive signal, its collector signal is negative due to the reverse action of the transistor. Two capacitors The signal polarity at both ends is shown in Figure 5. Through the feedback of the capacitor, the signal on the base of the triode is in phase with the original. Since this is positive feedback, the circuit can generate oscillation, the existence of RP and R1, weakening the positive in the circuit. The feedback signal keeps the circuit in the state of just starting up.


The oscillation frequency of the metal detector is about 40KHz, which is mainly determined by the inductance L and the capacitors C1 and C2. Adjusting the potentiometer RP reduces the feedback signal so that the circuit is in the state of just starting to oscillate. Resistor R2 is the base bias resistor of transistor VT1. The weak oscillating signal is amplified by a capacitor C4 and a resistor to a voltage amplifier composed of a transistor VT2, resistors R4, R5, and a capacitor C5. It is then rectified by diodes VD1 and VD2, and capacitor C6 is filtered. The rectified and filtered DC voltage turns on the transistor VT3, its collector is at a low level, and the light-emitting diode VD3 is bright.
When the inductive probe L of the metal detector approaches the metal object, the oscillation circuit stops, no signal passes through the capacitor C4, and the base of the transistor VT3 does not get a positive voltage, so the transistor VT3 is turned off, and the LED is extinguished.