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Help with Metal detector circuit

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2gk4

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I need help understanding this metal detector circuit.
The values for different components are
R1,R4-56K
R2-3K3
R3-22K
R5-2K7
R6-2K2
R7,R9-680E
R8-15K
P1-5K pot
C1,C6-.1pF(104)
C2-1kpF(102)
C3-220pF
C4-270pF
C5-12kpF
c7-100uF
The diodes are IN4148 signal diodes, but they are connected in reverse bias(I do not understand why)
and the transistors are BC547. The coil is 33mH IMG-20141209-WA0008.jpgWP_000370.jpg
Thank you.
 

This isn't a very clever circuit - don't expect good results!

I think the way it is supposed to work is Q1 is a weak oscillator, whether it oscillates or not depends on the setting of P1 and the coils proximity to something that might change it's inductance. If it starts oscillating, the signal it produces is rectified by the voltage doubler of D1, D2 C6 and the hand drawn capacitor. The resulting DC makes Q3 conduct which in turn stops Q4 conducting and turns the LED/buzzer off.

I would guess this device is for tracing wiring in walls where the cables are thick and the distance very short.

Brian.
 

And it continously keeps beeping instead of the buzzer going off only when some metal is placed around the coil and I'm unable to figure out what the reason could be
 

You should be able to adjust P1 so it stops and starts the beeping. The optimum setting will be on the borderline between the two.
Can you confirm turning P1 starts and stops the beep please?

Brian.
 

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