Colpitts oscillator metal detector

Discussion in 'The Projects Forum' started by shmoseph, Sep 18, 2010.

  1. shmoseph

    Thread Starter New Member

    Sep 18, 2010
    2
    0
    Hey
    I've built a metal detector using a colpitts oscillator. I used two 1uF capacitors in series. Ive been measuring the voltage output from the center tap between the two capacitors. When I bring metal close to the coil, the amplitude of the oscillations goes down to zero.
    Can anyone please explain why this happens? Someone told me that it has the same effect as placing a resistor in parallel to the inductor, and in simulation this works very well..but i still don't understand the reason for this effect!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    Post a schematic so that we can see for ourselves what you are describing.

    You can attach it here using the forum's attachment tools.

    hgmjr
     
  3. shmoseph

    Thread Starter New Member

    Sep 18, 2010
    2
    0
    ok so ive attached three pictures. The first shows the circuit and waveform before any metal is brought close to the coil. The second two show the effect on the waveform as metal is brought slowly closer to the coil.Please explain how bringing metal close to the coil gives the same effect as adding a resistor in parallel.
     
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