PWM (pulse width modulation) with LM555

pwm

Today we will learn how to generate PWM (pulse width modulation) out of a very famous chip 555 & little bit of circuit. This is really simple and it is very handy if you want to control your led, light bulb, servo motor or dc motor (brushless also works). The following discussion is especially made easy, keeping the newbies in mind. These circuits can be easily built by them for testing and analysis purposes.

Before working on this first we have to know about what is actually PWM.

What Is PWM?
Pulse Width Modulation (PWM) is a technique for delivering partial power to a load via digital means. PWM is a method for changing how long a square wave stays “on”. The on-off behaviour changes the average power of the signal.
If signal toggles between on and off quicker than the load, then the load is not affected by the toggling. The duty cycle (the width of the signal) is modulated. It is a percentage measurement of how long the signal stays on.

Application:
PWM signals are used for a variety of control applications. Their main use is for controlling DC motors but it can also be used to control LED Lights, Power supplies, audio amplifiers, valves, pumps, hydraulics, and other mechanical parts. The frequency that the PWM signal needs to be set at will be dependent on the application and the response time of the system that is being powered.

Three types of PWM are possible (FIG 1)
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  1. The pulse centre may be fixed in the centre of the time window and both edges of the pulse moved to Compress or expand the width.
  2. The lead edge can be held at the lead edge of the window and the tail edge modulated.
  3. The tail edge can be fixed and the lead edge modulated.

Three types of PWM signals (blue): leading edge modulation (top), trailing edge modulation (middle) and cantered pulses (both edges are modulated, bottom). The green lines are the saw tooth signals used to generate the PWM waveforms using the intersective method.

Let’s learn a simple PWM generation circuit Using IC 555.
The IC 555 is well known for its features which become readily suitable for an unlimited range of electronic circuit applications. Though the chip is fundamentally designed for generating time delays and oscillations periods, it can be modified or manipulated to do a number of other functions.

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Part list:
1) 555 timer IC – 1
2) 100K variable resistor – 1
3) 1N4148 Diode – 2
4) 0.01uF capacitor – 1
5) 1K ¼ resistor – 2

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CIRCUIT EXPLANATION:

  1. After making this circuit, we connect 9 volt battery for giving power to this circuit.
  2. We see that at the output pin no. 3. (FIG.3) LED is glowing & when we turn the variable Resistor the intensity of LED changing.
  3. The 0.01 uf capacitor at Pin 2 start charge and discharge through path variable resistor to D3 and D4 and output in PWM form we get on pin 3.
  4. Changes to the Potentiometer value as well as the .01 cap on pin 2 of the 555 can be made to change the operating frequency. A lower value capacitor will result in a higher frequency.
  5. With help of the D.S.O we can clearly see the PWM waveform at pin no. 3. & by turning a variable resistor of 100k duty cycle pwm waveform is varying and the intensity of LED light also vary.

References:
LM 555 Datasheet – http://www.national.com/ds/LM/LM555.pdf