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FM Transmitter circuit help

Discussion in 'General Electronics Discussion' started by asder, Apr 17, 2011.

  1. asder

    asder

    3
    0
    Apr 17, 2011
    Hello! First of all, I'm new here, nice to meet you all. I have been trying to follow and build this circuit: http://www.cie-wc.edu/fmtransmitter.asp
    Then I noticed that if I wanted to use a 3.5" jack instead of a microphone, I would need to change something else in the circuit. What would be changed in order to use an audio jack instead of a electret microphone to transmit?

    Also, I couldn't find a 5-30pf variable capacitor, but I have these two:

    One like this: http://www.kpsec.freeuk.com/photos/rapid/capvar.jpg taken from an old radio. It has 7 pins, four small knobs on one side, and a bigger one on the other side. It does not have any model number.
    And another one, a small two-pin trimmer that goes up to 70 pf.
    Could one of those be used for the circuit, or should I try to get the correct one?

    One more thing, I hope it doesn't bother you: Where in the circuit could I add a white LED so I know the battery isn't dead? I think it's 3V, but maybe it's more, since it doesn't look very bright on a cell battery.

    Thanks in advance.
     
  2. serious coinage

    serious coinage

    13
    0
    Mar 26, 2011
    If you're happy using a trimmer, here's a 6.2-30pF variable capacitor, which should be good enough.
     
  3. asder

    asder

    3
    0
    Apr 17, 2011
    Anybody else?:confused:
     
  4. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    You disconnect the collector of Q1 from C2, and connect the jack to C2.
    Connect a 47pF capacitor in series with the 70pF trimmer you have - and use that.
    The tuner capacitor you have contains multiple sections - and it's virtually impossible to remote-identify if there's a suitable section and where it's connected.
    Adding a LED will only drain the battery faster, but if you want to you connect it from + to - in series with a 3.3V zener and a 1000 ohm resistor.
     

    Attached Files:

  5. asder

    asder

    3
    0
    Apr 17, 2011
    Can I then leave out the microphone and connect C1/R1 directly to the negative? Would you explain the jack's connecting further, please? I don't get it.

    Sorry for all these questions, I don't really know a lot about electronics.
     
    Last edited: Apr 28, 2011
  6. Resqueline

    Resqueline

    2,848
    1
    Jul 31, 2009
    If you never need the mike then you can leave out half the circuit. Hope this pic helps explain it.
     

    Attached Files:

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