AM transmitter

Discussion in 'The Projects Forum' started by peter_morley, May 16, 2011.

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  1. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    Here is a link to the project I was considering...http://www.sciencebuddies.org/science-fair-projects/project_ideas/Elec_p024.shtml
    What I wan't to do with it is keep everything intact except the oscillator. Instead of an oscillator I want to use a 555 timer with a pot so I can adjust the frequency being transmitted. If you look at the schematic attached this is what I have done. I know there are limitations to the 555 timer such as max output current can only be 200mA. So would this modification work?...this is my first time doing radio signals so I'm a bit confused as to what is happening.
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    The 555 is not suitable for RF transmitter applications.
     
  3. Wendy

    Moderator

    Mar 24, 2008
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    You are trying to do an analog function. Anything digital is not going to work well.
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Yes, that will sort of work. Except your resistor values are too low. Using a 100 ohm resistor for RA might cause some smoke, or at least put too much demand on the chip's current sinking ability. A frequency modulated version of the circuit (modulation applied to pin 5, and the power fully fixed) that somebody sent me uses a .001 uf capacitor, a 390 ohm resistor to V+ and a 1k resistor between pins 6 and 7, and ran it from +9 to +12 volts.

    If your modulating signal is voice, you might even be able to understand what is being said. Do not connect this to a very long antenna because this is certain to radiate signals out of the AM broadcast band, and that might be bad.

    If you are in the U.S., you can transmit on the AM band if your power input to the final RF stage is is 1/10 watt or less and your antenna ls 3 meters long or shorter, and the harmonics are suppressed (can't remember 30 or 40 db) -your harmonics will be beyond spec, hence the warning not to connect to much of an antenna.

    It looks like a fun first RF project, but don't expect a lot from it, and move on to a "cleaner" circuit soon!
     
    Last edited: May 17, 2011
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Hrmmmm... Interesting concept. It is barely possible a 555 will work that high in frequency. Spec'd rise & fall times give a max frequency of 5MHz. It may not work at the top end of the AM band but might actually make a signal at the low end. It is worth a try anyway as you can grab all the parts from Radio Shack. (Do they still have those 8 ohm transformers? I used then in a few things!)

    Do look into making those resistors each 100 times larger while making the cap 100 times smaller, the resistors are just too low a value. (10K, 1.6K, 100 pf). Don't but an antenna, just use a piece of wire. If you have a hand held receiver next to the circuit chances are it will pick it up without any antenna at all.

    To all: the basic concept here is to modulate the supply voltage to a fixed oscillator running at some frequency in the AM band. When you modulate the supply voltage you necessarily modulate the output voltage, and hence you have amplitude modulation.
     
  6. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Hah, I just noticed RS sells two varieties of the 555, and the TLC555/TLC555CP Catalog #: 276-1718 is actually spec'd to run between 1.2 minimum to 2.1 typical MHz.

    See if you can get that one. The other one they sell is not spec'd for max frequency but may work too, so you might want to get both and see if one works better.

    I assume you do not have an oscilloscope; that would make checking this little circuit out simple. Sorry I can't think of anything else simple that would work.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Goodbye RF spectrum. This sucker will splatter all over. A filter between the 555 and the oscillator would help.

    Personally I don't think this has as snowballs chance. If you do get on the air with any power the FCC might take an interest, but I don't put much chance on that either.
     
  8. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Of course it will, it's modulating a square wave and not a sine wave.

    Another reason not to use an antenna, but seriously the FCC ain't gonna track you down for briefly trying this.

    I'm going to take that as a personal challenge and get me one of these to work!

    Now where did I stick those transformers...
     
  9. CDRIVE

    Senior Member

    Jul 1, 2008
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    Feeding a square wave to an antenna is a recipe for making friends in your neighborhood! Don't be surprised if they show up at your door step with a 12 gauge though!

    To the rest of the membership: Pa...leeeeeze don't encourage him!!!!
     
  10. simo_x

    Member

    Dec 23, 2010
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    I would look for crystal oscillators..
     
  11. peter_morley

    Thread Starter Member

    Mar 12, 2011
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    this is great i didn't think anyone would respond to this, well ErnieM if you can get this to work could you show me a schematic of it so I can see what you did? I love the enthusiasm!...just hope the FCC doesn't come chasing after me haha
     
  12. beenthere

    Retired Moderator

    Apr 20, 2004
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    Step on any commercial broadcast and there will be a knock at your door. This does not need to go further.
     
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