Download Pulse Width Modulation

Pulse Width Modulation (PWM)
Put simply, PWM is the process of switching power ON and OFF to a device in pulses at a
specific frequency. The same approach used in commercial light dimmers, DC motor speed
controller, CPU fan speed controllers and etc. It is a very energy-efficient means of controlling
electrical and electronic devices.
The fundamentals: Imagine your everyday incandescent light bulb. When you turn the power
ON, it doesn't light up to it's full brightness instantly, instead it will take (approximately) a couple
of seconds to do so. Similarly when you turn the power OFF, it will take a couple of seconds for
the bulb to die off.
Now, imagine you're able to flick the power switch ON/OFF fast enough. If you can time your
ON/OFF flicking just right (of course it's difficult), the light bulb will actually dim instead of flicker.
The ON/OFF cycle is known as the duty cycle. Below are the visual representation of 50%, 80%
and 20% duty cycle.
The 555 Timer IC
The 555 timer is arguably one of the most popular IC’s ever made. There are thousands of
resources online if you're interested to delve deeper into the subject. I'm just going to give the
simple description directly relevant to the build.
PIN 1 - Ground
DC Ground
PIN 2 - Trigger
When LOW, it causes the Output pin to go HIGH. Activated
when voltage fall
below 1/3 of +V.
PIN 3 - Output
Output is HIGH when Trigger pin is LOW. Output is LOW when Threshold pin is
HIGH. Output is LOW when reset pin is LOW.Output pin is able to source or sink
current.
PIN 4 - Reset
Short to +V when not in use.
PIN 5 - Control Voltage
Grounded through a 0.01uF capacitor when not in use.
PIN 6 - Threshold
When voltage reaches 2/3 of +V, this pin will cause Output to be driven LOW.
PIN 7 - Discharge
Grounded when Output pin goes HIGH.
PIN 8 - +V
DC Power
How the circuit works
When the circuit is powered up, the C1 capacitor will initially be
in a discharged state. Thus, the Trigger (pin 2) will be LOW,
driving the Output (pin 3) to go HIGH. Discharge (pin 7) goes
HIGH and shorts to ground. The cycle begins.
The HIGH Output will cause C1 capacitor to be charged
through the R1 and D1 path. Upon C1 voltage reaching 2/3 of
+V, the Threshold (pin 6) will be activated and drive the Output
(pin 3) LOW. Discharge (pin 7) goes LOW. The time it takes for
C1 to charge depends on the position of R1.
Since Output (pin 3) is now LOW, capacitor C1 will start to
discharge through the D2 and R1 path. When the voltage of C1
drops below 1/3 of +V, Trigger (pin 2) will be LOW, driving
Output (pin 3) to go HIGH, and Discharge (pin 7) to go HIGH
and shorts to ground. The cycle repeats itself.
You've probably noticed by now that the circuit is using
Discharge (pin 7) to drive the motor, simply by shorting to
ground in each cycle. You can add some amount of protection
if you're concerned about back EMF from the motor.
Pin 4 and 5 are not used, and pin 1 is simply tied to ground.
The circuit can take between +3v to +18v. The Frequency is
around 144Hz. Do note that, doubling the value of C1 will
reduce frequency to half, tripling will will reduce frequency to 1/3, and so on.
NOTE: The circuit, as is, can safely pass current of up to 200mA (max) .
Parts list
1) 555 timer IC - 1
2) 100K variable resistor - 1
3) 1N4148 Diode - 2
4) 100nF capacitor - 2