555 timer fsk generator problem

Thread Starter


Joined Feb 23, 2007

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.


Joined Apr 20, 2004
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.

Ron H

Joined Apr 14, 2005
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).