Making a simple fm transmitter from Morse code from a mcu

Discussion in 'The Projects Forum' started by solexious, May 24, 2012.

  1. solexious

    Thread Starter Active Member

    Nov 20, 2007
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    Hello all,

    I designed a small AM transmitter that I could broadcast Morse code from an attiny25. Using a 2n7000 attached to the clock out for the frequency generation, and a io pin with a cap to modify the amplitude of the signal. I worked it out from reading bits around the net.

    But, I now want to make an fm version and I'm finding it quite hard going.

    I want to make it with as few parts as possible, and I don't mind if it has a short range. I've found lots of scmatics around the net, but they all use mics, and I'm not so sure how to swap it for a io pin, and most of them use quite a few extra components as they don't use a mcu...

    Any pointers as what I should look at?

    Sol
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Do you want the Morse Code to activate a tone that modulates the FM transmitter?
    Then attenuate the tone down to the same level of a microphone. The attenuator is simply 2 resistors. Feed the attenuated tone to the mic input of the FM transmitter.
     
  3. solexious

    Thread Starter Active Member

    Nov 20, 2007
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    I'm looking to make it so by using the io out of the attiny it can product the tones to be sent.

    For my AM version it turns on and off the output pin for 1ms at a time to make a nice tone sound.

    While trying to find circuits online to do this I came across this post, think this could be used to make a very simple transmitter (by swapping out the chip for parts)? http://www.8051projects.net/tags/5v-fm-transmitter
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    A very simple FM transmitter is made with one transistor.
     
  5. solexious

    Thread Starter Active Member

    Nov 20, 2007
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    Now your just teasing me! ;)

    I have heard of such a mythical circuit, but my google fu fails at finding it :/
     
  6. Audioguru

    New Member

    Dec 20, 2007
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  7. solexious

    Thread Starter Active Member

    Nov 20, 2007
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    Ah, I took "single transistor" to mean the only component needed! (hence its supposed magical properties)
     
  8. DickCappels

    Moderator

    Aug 21, 2008
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    Unabashed plug for one of my old projects. You can also just stare at the schematic until it becomes clear how to convert a transmitter with a microphone to one that takes an audio signal as an input.

    By the way, notice the 470k resistors on the input? They are that large because the audio signal to the base has to be really low amplitude, otherwise the signal splatters all over the FM band (and probably other places as well) and sounds distorted. Keep that in mind when adjusting the audio level.

    http://cappels.org/dproj/FMdist/fmdis.htm
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    It will sound awful (with music or speech) because the input cuts high audio frequencies instead of boosting them with pre-emphasis like all FM radio stations. Then the de-emphasis in all FM radios will cut the high audio frequencies even more.
    But for a single tone of Morse Code, who cares what it sounds like.
     
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