Metal Detector Circuit

Discussion in 'The Projects Forum' started by ro169, Sep 18, 2016.

  1. ro169

    Thread Starter Member

    Oct 10, 2014
    60
    0
    Hi all.

    Been working on a metal detector project. I've come across quite a few schematics online.
    I've tried to replicate the MKII by Talking Electronics: http://www.talkingelectronics.com/projects/MetalDetectorMkII/MetalDetectorMkII.html
    Here is the Schematic:
    MetalDetectorMkIICct.jpg
    However I have changed a few things due to availability and the circuit doesn't perform as expected.
    I have used 1.25mm gauge fluxed wire forming 100mm diameter coils (According to the site the size of wire or coil isn't important)
    I have replaced one of the BC547 transistors with a BC237
    In place of "390R" surface mount I have used a 330R
    When switched on I have observed the speaker voltage to be a constant 2.6V despite the position of the coils relative to each other or the position of the dial on the potentiometer - i.e no tone is produced on the speaker.

    Any input will be appreciated :)
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    Did you read the "setting up" part?
    Bertus
     
    ro169 likes this.
  3. ro169

    Thread Starter Member

    Oct 10, 2014
    60
    0
    Yes I have.
     
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,908
    378
    Use the two BC547 transistors that you have as the two leftmost transistors in the schematic as they are the most critical. Use the BC237 as the third transistor in the schematic.
     
  5. ro169

    Thread Starter Member

    Oct 10, 2014
    60
    0
    Yes this was my thinking as well! I did do this :). Do you think that using a BC237 could be the problem? I will try using a BC547 as soon as I get my hands on another.

    One of my concerns is the 390R surface mount.
    390R.jpg
    I'm not sure exactly how the resistor "adjusts for the type of wire and coil-size".
    However I've assumed that this is a standard resistor and not something unique and used the 330R in place of it as the site states that its purpose is to help you customize and build your own projects using the blocks in this 1.
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,908
    378
    It is pretty good equivalent to the '547 so I don't think it would be a problem in the rest of the circuit.

    Check the diodes are the right way round and that the LED lights.
     
  7. ro169

    Thread Starter Member

    Oct 10, 2014
    60
    0
    Yes. Diodes are the right way around.

    LED lights up indicating there is power on the board.
     
  8. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    This circuit looks very simple, making it attractive to a beginner, but it's actually very tricky to get it working.
    More complex circuits are actually easier to troubleshoot.
     
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