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  1. #1
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    Help with Metal detector circuit

    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 33mHClick image for larger version. 

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    Thank you.

    •   Alt25th January 2015, 16:09

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  2. #2
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    Re: Help with Metal detector circuit

    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.
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    •   Alt25th January 2015, 23:17

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  3. #3
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    Re: Help with Metal detector circuit

    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



    •   Alt29th January 2015, 14:12

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  4. #4
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    Re: Help with Metal detector circuit

    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.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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