metal detector

This circuit usually is called as by the name of metal detector. in principle the circuit based on usual superheterodyning is applied in acceptor superhet. This circuit applies two oscillators RF. Frequency from the two oscillator determined by 55 MHZ. oscillator RF which firstly covers transistor T1 ( BF 494) and a filter 55MHz. Second oscillator is an oscillator Colpitt is coming from transistor T3 ( BF494) and inductor L1 shunted by capacitor VC1. Two this oscillator frequencies ( tells Fx and Fy) mingled in mixer transistor T2 ( other BF 494) and difference or beat frequency ( Fx-fy) output from collector from transistor T2 is jointed to which detector phase comprised diodes D1 and D2. The output is a beating DC passed a low-pass filter realized with help of from a resistor 10k R12 and two capacitors 15nF C6 and C10. Then is give to AF amplifier IC1 ( 2822M) via volume. controls VR1 and the output fed to one speakers 8-ohm/1W. Inductor L1 can be built with 15 rotations from 25SWG wires in a 10cm ( 4-inch) diameter air core. To operate for correct of circuit which is the stall frequency the two oscillators is same in order to obtaining zero blows in absence about all metals in near by around, almost circuit. Straightness from oscillator 2 can be done with help of from capacitor VC1. When frequency both having the character of same, the beat frequency is zero, that is. beats frquency=Fx-Fy=0, and like that there is no sound from loudspeaker. When search coil L1 through for metal, thereby changes second oscillator frequency. Hence now Fx-Fy is not empty and sound loudspeaker. Thereby one is can detect presence from metal.