FSK Using 555-Timer
The astable multivibrator with selective frequency input gives FSK Frequency Shift Keying. We may manually change the frequency through a potentiometer, but if it has to be changed by a digital signal, this circuit can be used. By inserting a resistance in parallel to the existing timing resistors, the time period can be varied. This is done with the help of a transistor.
Waveform of FSK