Tasel

Based on super hetero dyning principle. The circuit using 2 RF oscillators. The frequencies of all oscillators are at 5.5 MHz. The first RF oscillator consist of transistor T1 (BF 494) and a 5.5MHz ceramic filter. The 2nd oscillator is a Colpitt’s oscillator accomplished with the help of T3 (BF494) and inductor L1 shunted by trimmer capacitor VC1. These two oscillators’ frequencies (say Fx and Fy) are blended in the mixer transistor T2 (another BF 494) and the difference or the beat frequency (Fx-Fy) output from collector of transistor T2 is connected to detector stage comprising diodes D1 and D2.

The output is a pulsating DC which is passed through a low-pass filter realized with the help of a 10k resistor R12 and two 15nF capacitors C6 and C10. It is then passed to AF amplifier IC1 (2822M) via volume control VR1 and the output is fed to an 8-ohm/1W speaker. The inductor L1 can be constructed using 15 turns of 25SWG wire on a 10cm (4-inch) diameter air-core former and then cementing it with insulating varnish. For proper operation of the circuit it is critical that frequencies of both the oscillators are the same so as to obtain zero beat in the absence of any metal in the near vicinity of the circuit. The alignment of oscillator 2 (to match oscillator 1 frequency) can be done with the help of trimmer capacitor VC1. When the two frequencies are equal, the beat frequency is zero, i.e. beat frequency=Fx-Fy=0, and thus there is no sound from the loudspeaker. When search coil L1 passes over metal, the metal changes its inductance, thereby changing the second oscillator’s frequency. So now Fx-Fy is not zero and the loudspeaker sounds. Thus one is able to detect presence of metal.

Component list:

Resistor
R1,R5,R6,R9,R10__________330K
R2_______________________330
R3,R4,R7,R8,R11__________1K
R12,R13__________________10K
R14,R15__________________4.7

Capacitor
C1,C14____________________100uF/25V
C2,C3,C4,C7,C11,C13,C16___10nF
C5________________________100pF
C6________________________470pF
C8,C12____________________0.1uF
C9,C10____________________15nF
C15_______________________1u/16V
C17,C18___________________100nF

Variable Capacitor
VC1_______________________0-22pF

Diode
D1,D2_____________________OA79

Transistor
T1,T2,T3__________________BF494

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