Pwm n613g

Objective: To study Pulse Width Modulation and Demodulation using 555 Timer IC

Required Components: 1. 555 timer IC -1

2. 10K potentiometer -1

3. 100ohm resistor -1

4. 0.1uF capacitor -1

5. 1k resistor -1 (optional)

6. Bread board -1

7. 9v Battery -1

8. LED -1

9. Multimeter or CRO -1

10. Jumper wire –

11. Battery connector -1

Theory: Pulse Duration Modulation (PDM) or Pulse Width Modulation (PWM) In pulse time modulation (PTM), the modulating signal is sampled, just as it is in PAM. However, in PTM, the amplitude of the sample is indicated by a timing variation of the modulated pulse, rather than an amplitude variation. The variable timing characteristics may be the duration, position, or frequency of the pulses. Therefore, there are three basic types of PTM: pulse duration modulation, pulse position modulation, and pulse frequency modulation.

This type of PTM is also called pulse width or pulse length modulation, however, pulse duration modulation (PDM) is the preferred term. There are three different classifications of PDM: symmetrical PDM, leading edge PDM, and trailing edge PDM.

In a symmetrical PDM waveform, the modulation signal as sampled and both the leading and trailing edges of the pulse are varied in accordance with the sample amplitude. When the sample is a high negative difference duration, the spacing between the center of the pulses remains constant as shown.

In leading edge type of PDM, the sample amplitude varies the leading edge of the pulse. The trailing edge of each pulse is fixed and, therefore, the spacing or timing between each pulse’s trailing edge is constant.

In trailing edge PDM, the sample amplitude varies the trailing edge of the pulse, with the leading edge remaining fixed.

It is a type of analog modulation. In pulse width modulation or pulse duration modulation, the width of the pulse carrier is varied in accordance with the sample values of message signal or modulating signal or modulating voltage. In pulse width modulation, the amplitude is made constant and width of pulse and position of pulse is made proportional to the amplitude of the signal. We can vary the pulse width in three ways: 1. By keeping the leading edge constant and vary the pulse width with respect to leading edge 2. By keeping the tailing constant. 3. By keeping the center of the pulse constant. We can generate pulse width using different circuitry. In practical, we use 555 Timer which is the best way for generating the pulse width modulation signals. By configuring the 555 timer as monostable or astable multivibrator, we can generate the PWM signals. We can use PIC, 8051, AVR, ARM, etc. microcontrollers to generate the PWM signals. PWM signal generation has n number of ways. In demodulation, we need PWM detector and its related circuitry for demodulating the PWM signal. There are two important parameters for a PWM as discussed below:

Duty cycle of the PWM: The percentage of time in which the PWM signal remains HIGH (on time) is called as duty cycle. If the signal is always ON it is in 100% duty cycle and if it is always off it is 0% duty cycle.

Duty Cycle =Turn ON time/ (Turn ON time + Turn OFF time)

Frequency of a PWM: The frequency of a PWM signal determines how fast a PWM completes one period. One Period is complete ON and OFF of a PWM signal as shown in the above figure. In our tutorial we will set a frequency of 5 KHz. We can notice if LED being OFF for half second and LED being ON for other half second. But if Frequency of ON and OFF times increased from ‘1 per second’ to ’50 per second’. The human eye cannot capture this frequency. For a normal eye the LED will be seen, as glowing with half of the brightness. So with further reduction of ON time the LED appears much lighter.

Generation of PWM: 

PWM signal can be generated by using a comparator, where modulating signal and sawtooth signal form the input of the comparator. It is the simplest method for PWM generation.

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The PWM generation

PWM generation by a comparator 

As shown in the figure, one input of the comparator is fed by the input message or modulating signal and the other input by a sawtooth signal which operates at carrier frequency.

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Considering both ±ve sides, the maximum of the input signal should be less than that of sawtooth signal.

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The comparator will compare the two signals together to generate the PWM signal at its output as shown in the third waveform of Fig6.

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The rising edges of the PWM signal coincides with the falling edge of the sawtooth signal.

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When the sawtooth signal is at the minimum value which is less than the minimum of the input signal, then the positive input of the comparator is at higher potential which gives the comparator output as positive.

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When the sawtooth signal rises and is at the maximum value, the negative input of the comparator is at higher potential, which will produce the comparator output to be negative.

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Thus the input signal magnitude determines the comparator output and its potential, which then decides the width of the pulse generated at the output.

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In other words we can say that the width of the pulse generated signal is directly proportional to the amplitude of the modulating signal.

PWM and PPM signal generation

DEMODULATION OF PWM PWM pulse can be detected using Ramp generator and some circuit combinations. We have discussed PWM generator circuit using 741 op amp in previous articles. The coded message in the form of PWM can be easily decoded with the help of a synchronous pulse. The most simple demodulation principle of PWM signals are describes in this article The block diagram itself explaining all the decoding principles. Waveforms at different sections of Pulse width demodulation are also given here. PWM signals have wide verities of applications like PWM Inverter circuit, PWM LED brightness control or Dimmer circuit etc.

PWM Demodulator Block Diagram

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PWM coding can be done using 741 op amp that we discussed before. Here the modulated (PWM) wave is applied to the decoder system for getting the message signal.

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The

basic

theory

behind

Pulse

width

demodulation

is

that converting the PWM signal to PAM (Pulse Amplitude Modulation) signal. PAM can be easily detected by suitable low filter.

PWM



PWM to PAM Converter

PAM

Low Filter

Decoded

Input PWM wave is applied to Ramp generator and Synchronous Pulse generator

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Synchronous pulse generator will generate a pulse waveform such that the pulse will end at the beginning of each PWM pulse (See the fig below)

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Ramp generator will produce a ramp signal whose amplitude is proportional to width of the PWM signal (See the fig below)

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Now apply these Ramp and Synchronous pulse to an Adder circuit which adds these signals together.

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The next block is a positive Clipper with a specific voltage; Clipper clips the waveform at a particular level as shown in fig.

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The output of clipper will be PAM signal, now the PWM signal gets converted to PAM signal.

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The PAM can be demodulated by Low filtering method. Thus our next block is Low Filter.

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Ramp Generator + Synchronous Pulse + Adder + Clipper = PWM to PAM Converter

Advantages of Pulse Width Modulation (PWM): 

As like pulse position modulation, noise interference is less due to amplitude has been made constant.

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Signal can be separated very easily at demodulation and noise can also be separated easily.

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Synchronization between transmitter and receiver is not required unlike pulse position modulation.

Disadvantages of Pulse Width Modulation (PWM): 

Power will be variable because of varying in width of pulse. Transmitter can handle the power even for maximum width of the pulse.

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Bandwidth should be large to use in communication, should be huge even when compared to the pulse amplitude modulation.

Applications of Pulse Width Modulation (PWM):

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PWM is used in telecommunication systems.

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PWM can be used to control the amount of power delivered to a load without incurring the losses. So, this can be used in power delivering systems.

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Audio effects and amplifications purposes also used.

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PWM signals are used to control the speed of the robot by controlling the motors.

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PWM is also used in robotics.

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Embedded applications.

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Analog and digital applications etc.

555 Timer PWM Generator Circuit Diagram and Explanation:

In this PWM generator circuit, as we mentioned above we have used 555 Timer IC for generating PWM signal. Here we have controlled the output frequency of the

PWM signal by selecting resistor RV1 and capacitor C1. We have used a variable resistor in place of fixed resistor for changing duty cycle of the output signal. Capacitor Charging through D1 diode and Discharge through D2 diode will generates PWM signal at 555 timer's output pin. Below formula is used for deriving the frequency of the PWM signal: F = 0.693*RV1*C1

Simulating PWM generation using 555 Timer IC: Below are some Snapshots: