Frequency Modulation using 555 timer

Frequency modulation (FM) is a popular technique for transmitting information over long distances. It involves varying the frequency of a carrier signal in accordance with the message signal amplitude to be transmitted. This technique finds widespread use in radio and television broadcasting, mobile communication, and other similar applications. One of the most commonly used components for generating FM signals is the 555 timer. In this blog post, we will discuss how to use a 555 timer for frequency modulation.

The 555 timer is a versatile integrated circuit that can be used for a wide range of applications, including timing, oscillation, and pulse generation. It is a monolithic timing circuit that consists of two comparators, a flip-flop, and a discharge transistor. The circuit is capable of generating square waves, pulse-width modulated signals, and other waveforms. In frequency modulation, we use the 555 timer to generate a carrier signal whose frequency varies in accordance with the message signal. This is same as using 555 timer as a voltage controlled oscillator.

The following shows the circuit diagram of 555 Timer used for frequency modulation.


555 timer frequency modulation circuit

The first step in using a 555 timer for frequency modulation is to set up the basic astable oscillator circuit. The astable oscillator generates a square wave whose frequency is determined by the values of the resistors and capacitors in the circuit. The formula for calculating the frequency of the oscillator is given by:


where R1 and R2 are the resistors and C1 is the capacitor in the circuit.

If we want to have 100KHz square wave then we can use the following equations to derive the values of R1, R2 and C1.


\( T_{L}=0.693R_{2}C_1\)

 The calculated values for 100KHz are:

R1 = 32 KΩ using standard value R1 = 33 KΩ,

R2 = 56 KΩ and C1 = 100pF

The duty cycle can be calculated using the following formula.


with the component values the duty cycle is 61.11 %.

These component values for astable mode can be directly calculated using the online 555 Timer calculator.

 Next, we need to modulate the frequency of the oscillator in accordance with the message signal. This can be done by applying audio signal or message signal to control voltage pin of the 555 timer. The control pin is connected to the junction of a 10KOhm resistor and a 10uF coupling capacitor. The coupling capacitor is used to block the DC component of the signal and allow only the AC component to pass through.

The message signal is applied to the control voltage pin. The amplitude of the message signal modulates the voltage at the control pin of the 555 timer, which in turn modulates the frequency of the oscillator. The result is a frequency modulated signal that contains the message signal.

The following shows the input message signal of 10KHz applied to the control voltage pin 5 and the output FM signal at the output pin 3.

FM modulation signal

The following shows FM modulation using the NE555 Timer simulation in Proteus electronics design software.


It is important to note that the values of the resistors and capacitors in the circuit should be chosen carefully to ensure that the frequency of the oscillator is within the desired range. Also, the coupling capacitor should be chosen to provide sufficient coupling between the message signal and the control voltage while blocking the DC component.

In conclusion, the 555 timer can be used to generate frequency modulated signals for a variety of applications such as FM transmitter. The basic astable oscillator circuit can be modified to generate FM signals by modulating the voltage at the control pin using a variable resistor and coupling capacitor. With careful selection of components, it is possible to generate high-quality FM signals using the 555 timer.

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