555 timer fsk generator problem

Discussion in 'The Projects Forum' started by nwo4life, Feb 24, 2007.

  1. nwo4life

    Thread Starter New Member

    Feb 23, 2007
    3
    0
    Hi,

    I need to make an FSK (frequency shift keying) generator using the 555 timer. My marks are as follows,
    '0' = 1000 Hz '1' = 1200 Hz

    I have calualeted all the resistor and capacitor values for both frequencies, but I don't know how to hook everything up with one 555 timer. One idea is to use an active logic switch (NPN BJT) but I don't know how?

    If anyone has any ideas or suggestions please post a reply.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    I would use a VCO for the FSK generator. That way, there is only a voltage level to change, instead of capacitors and resistors. You also get a sine instead of square wave signal.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    655
    If you don't mind square waves, and want to use a 555, here is a schematic with ideal component values. You might want to make R2 and R3 adjustable +/- 20% to allow for component tolerances. Alternately, if you can match R2 and R3 (two ~12k resistors, matched to each other), make R1 adjustable (e.g., 20k with a 10k pot wired as a rheostat).
     
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