A Simple PWM Circuit Based on the 555 Timer

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One of the most fundamental problems in robotics is DC motor speed control. The most common method of speed control is PWM or pulse width modulation. 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”. The diagram below shows the waveforms of 10%, 50%, and 90% duty cycle signals.

View Circuit

When the circuit powers up, the trigger pin is LOW as capacitor C1 is discharged. This begins the oscillator cycle, causing the output to go HIGH.

When the output goes HIGH, capacitor C1 begins to charge through the right side of R1 and diode D2. When the voltage on C1 reaches 2/3 of +V, the threshold (pin 6) is activated, which in turn causes the output (pin 3), and discharge (pin 7) to go LOW.

When the output (pin 3) goes LOW, capacitor C1 starts to discharge through the left side of R1 and D1. When the voltage on C1 falls below 1/3 of +V, the output (pin 3) and discharge (pin 7) pins go HIGH, and the cycle repeats.

Pin 5 is not used for an external voltage input, so it is bypassed to ground with an 0.01uF capacitor.

Note the configuration of R1, D1, and D2. Capacitor C1 charges through one side of R1 and discharges through the other side. The sum of the charge and discharge resistance is always the same, therefore the wavelength of the output signal is constant. Only the duty cycle varies with R1.

The overall frequency of the PWM signal in this circuit is determined by the values of R1 and C1. In the schematic above, this has been set to 144 Hz.

Read More Source:
http://www.dprg.org/tutorials/2005-11a/index.html
Thank you.

This entry was posted on Thursday, November 15th, 2007 at 9:14 am and is filed under 555, motor speed controller. You can follow any responses to this entry through the RSS 2.0 feed.