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FM transmitter circuit connection

Discussion in 'General Electronics Discussion' started by Yap, Mar 16, 2017.

  1. Yap

    Yap

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    Feb 13, 2017
    Hi everyone. I am a newbie in electronic field. I need some helps. Recently i trying to construct a FM transmitter circuit for learning purpose. However when i try to connect a headset into the audio input, i found that the antenna isn't transmitting anything. May i know the real method for checking the circuit connection and method to connect a headset mic input into a stereo audio jack?
     

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  2. Audioguru

    Audioguru

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    Sep 24, 2016
    Your schematic is so small that we cannot see its details when enlarged.
    Your parts are WAY too far apart. They are supposed to be close together at the Very High Frequency of the FM band. your transistors in an old metal can look like they are 40 years old.

    My FM transmitter has many more parts than yours but measures as small as a tea bag:
     

    Attached Files:

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  3. davenn

    davenn Moderator

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    Sep 5, 2009
    hi ya
    welcome to EP :)

    firstly, this is a really bad way of building an RF circuit, the long track lines between components and the mutual inductance and capacitance
    between parallel tracks virtually ensures that the circuit will never work as planned

    I'm trying to find an example photo of a good basic method

    secondly the schematic you posted is so tiny that it is unreadable. As a result we don't really know what the circuit is like
    do you have a larger version you can post ?


    Dave
     
  4. Tha fios agaibh

    Tha fios agaibh

    1,892
    599
    Aug 11, 2014
    The long leads of inductor will change its inductance value.

    Make sure your using the right type of microphone. Usually an electret type. It will have two wires, hook positive side to the capacitor next to base of input transistor. hook negative side to negative ground rail.

    Some capacitors might need to be ceramic type instead of electrolytic type?
     
  5. davenn

    davenn Moderator

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    1,746
    Sep 5, 2009
    OK couldn't find one

    here instead is a very fast and rough drawing of one method of a quick and nasty board layout for experimenting
    the individual pads of remaining PCB pads could be around 5mm to 10mm square and make sure there's a good gap
    of around 2-3 mm of blank board area between the pads

    PCB.GIF
     
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  6. Audioguru

    Audioguru

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    Sep 24, 2016
    We have all seen that very old circuit using obsolete transistors in metal cases. I wanted to see the project at Circuits Today but my browser said "Beware, they have a security issue".
    Of course a modern headset uses an electret microphone that will not work in that simple circuit.
    A stereo headset with mic uses 4 or 5 wires and only 2 of them are for the microphone. A stereo headphones jack will not work.

    I have never built "dead bug" or "manhattan" styles, instead I use stripboard with the strips cut short.
     

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  7. Yap

    Yap

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    0
    Feb 13, 2017
    Oh i see. Thank you everyone. I will try to change my connection to a smaller scale and post once again.^~^
     
  8. Audioguru

    Audioguru

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    576
    Sep 24, 2016
    Broadcast band FM radio uses equalization called "pre-emphasis" to boost high audio frequencies for transmitting and "de-emphasis" to cut the high frequencies back down to normal for receiving so that hiss picked up is reduced. All FM radios have de-emphasis so your simple transmitter that is missing pre-emphasis will sound muffled (no high audio frequencies) when heard on a normal FM radio. My FM transmitter has pre-emphasis and a capacitor that can be selected for the amount that is different at different parts of the world and it sounds like high fidelity.
     
  9. GhostLoveScore

    GhostLoveScore

    71
    5
    Nov 27, 2016
    This seems less than ideal for RF circuits since it doesn't have large ground plane?
     
  10. davenn

    davenn Moderator

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    Sep 5, 2009
    using double sided board, gives a good GP
    it isn't for permanent use rather it is for basic testing and will do a very much better job than a breadboard
     
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