PWM Circuit Based on the 555 Timer

Introduction In applications LED Brightness Control we may want to vary

voltage given to it. Most often we use a variable resistor for that. The main problem with this control is the power dissipation across the variable resistor.

This problem can be avoided by using Pulse Width Modulation (PWM). In this method the average voltage delivered to the load is controlled by varying the duty cycle of the rectangular wave.

What is PWM? Pulse width modulation is the process of switching the power to a

device on and off at a given frequency, with varying on and off times. These on and off times are referred to as "duty cycle".

These signals are sent to the LED at a high enough frequency that the pulsing has no effect on the LED. The end result of the PWM process is that the overall power sent to the LED can be adjusted from off (0% duty cycle) to full on (100% duty cycle) with good efficiency and stable control.

Advantages Using PWM The signal remains digital all the way from the processor to the

controlled system, no digital-to-analog conversion is necessary. By keeping the signal digital, noise effects are minimized. Noise

can only affect a digital signal if it is strong enough to change a logical 1 to a logical 0

555 Timer In this project the NE555 should operate in the astable

multivibrator. The resistors R12, R2, potentiometer R4 and capacitor C2 will play the important role.

Pins 2 and 6 have upper comparator and lower comparator respectively. The upper comparator has the value 2/3 of the vcc and lower comparator has 1/3 of the vcc.

If the astable multivibrator is high at starting and capacitor C2 is started to charge through R12, R2 and pot R4. When the capacitor reaches the voltage of 2/3 its vcc then upper comparator makes the astable multivibrator to goes low.

Then the voltage in the capacitor starts to discharge when it reaches the voltage of 1/3 its vcc then lower comparator flips and make the astable multivibrator to go high.

Here diode D1 plays the key role diode will allow the voltage when the capacitor is discharging and it will not allow reverse current when the multivibrator is at high or capacitor is charging.

The output of the NE555 is taken form pin 3 and connected to the led panel through the transistor Q1 and resistor R1.

The resistor R1 is used to limit the base current of the transistor  and transistor is used as a amplifier to limit or enhance the current which is given to the LED panel.

The overall frequency of the PWM signal in this circuit is determined by the values of R12, R2, potentiometer R4 and capacitor C2.