Looking for a metal detector expert :)

Discussion in 'General Electronics Chat' started by brucester, Jan 20, 2013.

  1. brucester

    Thread Starter New Member

    Dec 24, 2012

    I have found a simple metal detector circuit which i have simulated here:


    Here is a pic of the full schematic:


    I was hoping someone could tell me roughly how it works.

    I understand the 555 timer that generates a pulse width and frequency of the 9 volt battery short over the coil.

    I understand the damping resister in paralell with the coil.

    I understand the clamper diodes and the preamp stage (opamp 1).

    What i dont understand is the next opamp stage.

    It is set up as a comparator and compares the amplified flyback decay signal from the coil with what?

    If someone could explain in simple terms the final stages of this circuit it would be greatly appreciated.

  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    What are the two op amps Op1 Op2 called, the oscillations are clamped at 600mV by the diodes and compared using op1 which acts as a squelch control, the op2 is an audio amp.
  3. brucester

    Thread Starter New Member

    Dec 24, 2012
    TL072 or similar

    What signals are compared? Is one signal compared to a previous signal?

    I thought the first op amp, amplified as it is rigged up a non inverting amp.

    Op2 is set up as a comparator.

    You sure about this?
  4. ifixit

    AAC Fanatic!

    Nov 20, 2008
    OP2 is comparing the input signal with an integrated version of the same signal. Since the integration times are very different, there will only be an audio output, or output change in tone or level, when the signal changes in phase, frequency, or level. The audio level will; drop off to nothing, or low, if there is no change in the input. This is what you want for a metal detector.

    Have fun with it,