Frequency Shift Keying plays a great role in wide range of applications in the field of communication and was considered efficient one in data transmission of wireless modems.This above circuit uses a simple IC NE555 in order to produce FSK signal with respect to the given input signal.In this circuit it was wired as a simple Astable multivibrator and in addition a transistor was connected through which input signal was given into the base of the transistor.The resistors Ra,Rb and C determines the frequency of the FSK modulated signal in the astable mode of operation. Also check out the matching FSK Demodulator using IC 565 circuit. Lets move into the working of this circuit.


The output frequency of the signal was based on the input digital signal given to the base of the transistor.When the given input was high that is of logic 1 the PNP transistor was Q is off and IC 555 timer works in the normal Astable mode of operation giving out the series of square wave pulses thus there will be no change in the frequency of the output signal.Here the resistors Ra, Rb and Capacitor C was selected in such a way to obtain output frequency of 1070Hz.The output frequency when the input was high was given by the equation
                                                  f= 1.45/(Ra + 2Rb)C

When the input binary data if logic 0, the PNP transistor is on and its connects the resistance Rc across resistance Ra.The resistors Rc is selected in such a way that the value of 1270Hz.Here the value of Rc added in addition to the Ra, Rb and C to contribute the working of the  NE555.This makes the charging and discharging quicker resulting in high frequency waves as output.The Ra, Rb, Rc and C values was selected in such a way to obtain output frequency of 1270 Hz.This was given by the equation
                                                 f=1.45/((Ra || Rc)+ 2Rb)C

Thus the resultant output FSK will give frequency of 1070Hz when input is high and frequency of 1270 when input is low.Thus by this way the FSK signal was obtained using NE555.For more NE 555 circuits visit our IC 555 circuits page.

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  1. an what king of Q PNP can I use, is a lot of tipes on PNP

  2. You can use 2N2907 for the above circuit

  3. This circuit have 200Hz interval. I want 1KHz interval, Should i modify the capacitor C or C1 to obtain better result?

    1. 200 Hz interval? Cant understand your point kindly brief a bit. The capacitor C1 was used to reduce to noise in the above circuit and there will be no impact in frequency due to this. If you are intend to change frequency you can alter the values of Ra, Rb and C above.

  4. hello,
    what should be the mark and space frequencies if i want to apply digital voice input(at least at 8KHz), and how can i get them?
    thank you.

    1. As i said above you can alter the mark and space frequency based on your requirement by altering the component Ra,Rb,Rc and C values in the above circuit diagram. Only switching of the state between the mark and space frequencies will be rapid if you increase the frequency of the digital input.

  5. so how can i change the circuit into an active high circuit with the f1 frequencies being higher then f2


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