RF TX Audio Input Source

Discussion in 'Wireless & RF Design' started by GolfGate772, Jul 10, 2013.

  1. GolfGate772

    Thread Starter New Member

    Jul 10, 2013
    5
    0
    Good day all,

    I was wondering if I could ask you all for some advice/assistance on how to make a modification to a RF transmitter circuit I am working on.

    The link to the schematic of the transmitter I am working on can be found here: http://www.bugplan.com/bbg.htm

    I am building this transmitter (for a purpose other than a bug), and figured out that I do not need a microphone (as pictured on the schematic), but an 1/8th inch audio cable input instead.
    However, I am not sure how to go by swapping the microphone with that kind of audio input, or if it is even possible. If you guys have any info on how to do this, or, better yet, know how, please let me know. That would be a great help!

    Oh, and in terms of legality, I own a business and would be using this circuit to transmit on a frequency I am licensed to do so by installing a different crystal into the transmitter.

    Thank you for your time and expertise! :)
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    The signal from a microphone is much weaker than that from an mp3 player.
    You will have to attenuate the signal with a voltage divider using two resistors.

    What carrier frequency are you planning on transmitting?
     
  3. GolfGate772

    Thread Starter New Member

    Jul 10, 2013
    5
    0
    464.500Mhz (Brown Dot)
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    I doubt very much that that circuit will work at such a high frequency.
     
  5. GolfGate772

    Thread Starter New Member

    Jul 10, 2013
    5
    0
    Well, let's just theoretically say that it will work (even though it might not). I will work out the frequency issue later. However, I am still interested in how to swap out the microphone for the 1/8th inch input.
     
  6. GolfGate772

    Thread Starter New Member

    Jul 10, 2013
    5
    0
    Our business is also licensed under 151.955Mhz (Purple Dot). I would rather not use that frequency for this transmitter, but I can if need be.
     
  7. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Disconnect R7 from the Collector of Q2 and connect the core of the 1/8 there, shield to ground.

    Ramesh
     
  8. GolfGate772

    Thread Starter New Member

    Jul 10, 2013
    5
    0
    Thanks! However, I have never really worked with 1/8th inch mounts before. So, of the three pins on the mount, do you know which ones go where? Also, do I just remove (or not add) the microphone all together, seeing that it is now unnecessary? And when you say disconnect R7, do you mean remove it completely and bridge it with the audio input mount, or solder he end of R7 to the audio mount?
     
  9. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Your 1/8 inch is actually 3.5 mm. It's an European standard and is known as a 3.5 mm Plug / Socket.

    I've attached the schematic. By default, the original ckt is retained, plugging in will bypass the mic amp.

    Ramesh
     
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