Frequency shift keying


To send logical zero and logical one, at a long distance, this method is adopted. “Zero” is detected by sensing low frequency and  “one” by high frequency. Frequency is transmitted on a long line without any change in its value and at the other end it is detected unambiguously.

Circuit diagram


Short Description

Frequency shift keying is abbreviated as FSK. It uses frequency modulation technique.

  • A PLL (Phase Locked Loop) IC, CD4046 is used to implement the circuit.
  • Only the VCO (Voltage Controlled Oscillator) section of this IC is used to implement the circuit.
  • The component values shown in the above diagram, produce two frequencies.
    • 250Hz are generated when the key is pressed and pin no. 9 is connected to the circuit ground.
    • 1.6KHz when pin no. 9 is connected to 9V, by not pressing the key.
    • When the key is pressed the speaker produces bass sound.
    • When key is not pressed, the speaker produces treble sound (i.e. sharp note).
    • Depending upon the capacitor value tolerance, the frequency will change a little.

Material List

Sr. No Component Specifications Quantity
1 CD4046 standard IC 1
2 IC socket 16 pin 1
3 Resistors 10k,0.25W 2
4 Resistors 100k,0.25W 1
5 Resistor 3k3,0.25W 1
6 Capacitor 100kpf,disc (104) 1
7 PCB general purpose, 4″X4″ 1
8 Battery 9V, flat, with snapper 1
9 Transistor BC547 1
10 Speaker 8 Ohm,3 ” dia 1
11 Key (Button) small, push to ON 1
12 Misc solder metal, hook up wire

This project is available in the kit form. Contact Omkar to buy it. This site has no commercial connection with his activities.