This document describes a student project to control the speed of a DC motor using pulse width modulation. It includes a list of components used, an overview of the theory behind pulse width modulation and DC motors, diagrams of the PWM waveform and 555 timer IC, and schematics of the circuit both simulated and on breadboard/PCB. The circuit uses a 555 timer and potentiometer to vary the duty cycle and thereby control motor speed. Construction, working, conclusions and resources are also summarized.

1. Paper presentation of project work
Submitted By::
• 14-16-016 - Dhanjit Sutradhar
• 14-16-038 - Upendra Chokka
• 14-16-044 - Bishal Roy

2. Project Name :
Speed control of DC motor using pulse width modulation
(PWM).
Components Used:
• Potentiometer(10k ohm)
• 3 Diodes( 1N4148)
• 3 Resistors(1k,220,220 ohm)
• IC 555 timer(8 DIP)
• DC Motor(6V)
• 2 Capacitors(100uF,1uF)
• Battery(9V)
• PCB
• Transistor( BD 139)

3. Theory of this project ::
 Pulse Width Modulation
 There are many different ways to control the speed of motors but one very simple and easy way is
to use Pulse Width Modulation. But before we start looking at the in’s and out’s of pulse width
modulation we need to understand a little more about how a DC motor works.
 Next to stepper motors, the Permanent Magnet DC Motor (PMDC) is the most commonly used
type of small direct current motor available producing a continuous rotational speed that can be
easily controlled. Small DC motors ideal for use in applications were speed control is required such
as in small toys, models, robots and other such Electronics Circuits.
 A DC motor consist basically of two parts, the stationary body of the motor called the “Stator” and
the inner part which rotates producing the movement called the “Rotor”. For D.C. machines the
rotor is commonly termed the “Armature”.
 Generally in small light duty DC motors the stator consists of a pair of fixed permanent magnets
producing a uniform and stationary magnetic flux inside the motor giving these types of motors
their name of “permanent-magnet direct-current” (PMDC) motors.
 The motors armature consists of individual electrical coils connected together in a circular
configuration around its metallic body producing a North-Pole then a South-Pole then a North-
Pole etc, type of field system configuration.

4. Pulse Width Modulated Waveform ::
 The use of pulse width modulation to control a small motor has the advantage in that the power
loss in the switching transistor is small because the transistor is either fully “ON” or fully “OFF”. As a
result the switching transistor has a much reduced power dissipation giving it a linear type of
control which results in better speed stability.
 Also the amplitude of the motor voltage remains constant so the motor is always at full strength.
The result is that the motor can be rotated much more slowly without it stalling. So how can we
produce a pulse width modulation signal to control the motor. Easy, use an Astable 555
Oscillator circuit as shown below.
Figure-1: Pulse width modulation waveform

5. IC 555 Timer ::
Figure-2: IC 555 timer 8 DIP Block Diagram

6. :-
1. Ground: This pin is used to provide a zero voltage rail to the Integrated circuit to divide the supply
potential between the three resistors shown in the diagram.
2. Trigger: As we can see that the voltage at the non-inverting end of the comparator is Vin/3, so if
the trigger input is used to set the output of the F/F to ‘high’ state by applying a voltage equal to or less
than Vin/3 or any negative pulse, as the voltage at the non-inverting end of the comparator is Vin/3.
3.Output: It is the output pin of the IC, connected to the Q’ (Q-bar) of the F/F with an inverter in
between as show in the figure.
4.Reset: This pin is used to reset the output of the F/F regardless of the initial condition of the F/F
and also it is an active low Pin so it connected to ‘high’ state to avoid any noise interference, unless a
reset operation is required. So most of the time it is connected to the Supply voltage as shown in the
figure.
5. Control Voltage: As we can see that the pin 5 is connected to the inverting input having a voltage
level of (2/3) Vin. It is used to override the inverting voltage to change the width of the output signal
irrespective of the RC timing network.
6. Threshold: The pin is connected to the non-inverting input of the first comparator. The output of the
comparator will be high when the threshold voltage will be more than (2/3) Vin thus resetting the output
(Q) of the F/F from ‘high’ to ‘low’.
7. Discharge: This pin is used to discharge the timing capacitors (capacitors involved in the external
circuit to make the IC behave as a square wave generator) to ground when the output of Pin 3 is
switched to ‘low’.
8. Supply: This pin is used to provide the IC with the supply voltage for the functioning and
carrying of the different operations to be fulfilled with the 555 timer.

7. Construction & Circuit Diagram ::
 Firstly, we had simulated our circuit design in proteus software.
 Then after circuit designing was completed we had implemented it in
breadboard to check the working.
 Actually it worked in breadboard and then circuit was implemented on
printed circuit board (PCB).
Figure-3 : Speed Control of DC Motor using PWM circuit simulated in Proteus Software
1N4148

8. • This simple circuit based around the familiar NE555 or 7555 timer chip is used to produced the required
pulse width modulation signal at a fixed frequency output. The timing capacitor C is charged and
discharged by current flowing through the timing networks RA and RB as we looked at in the 555 Timer
tutorial.
• The output signal at pin 3 of the 555 is equal to the supply voltage switching the transistors fully “ON”.
The time taken for C to charge or discharge depends upon the values of RA, RB.
• The capacitor charges up through the network RA but is diverted around the resistive network RB and
through diode D1. As soon as the capacitor is charged, it is immediately discharged through diode D2 and
network RB into pin 7. During the discharging process the output at pin 3 is at 0 V and the transistor is
switched “OFF”.
• Then the time taken for capacitor, C to go through one complete charge-discharge cycle depends on the
values of RA, RB and C with the time T for one complete cycle being given as:
• The time, TH, for which the output is “ON” is: TH = 0.693(RA).C
• The time, TL, for which the output is “OFF” is: TL = 0.693(RB).C
• Total “ON”-“OFF” cycle time given as: T = TH + TL with the output frequency being ƒ = 1/T
• Resistor R1 plus the “top” part of the potentiometer, VR1 represent the resistive network of RA. While the
“bottom” part of the potentiometer plus R2 represent the resistive network of RB above.
• These values can be changed to suite different applications and DC motors but providing that the 555
Astable circuit runs fast enough at a few hundred Hertz minimum, there should be no jerkiness in the
rotation of the motor.
• Diode D3 is our old favorite the flywheel diode used to protect the electronic circuit from the inductive
loading of the motor. Also if the motor load is high put a heatsink on the switching transistor or MOSFET.

9. Schematic Diagram of Circuit:
On PCBOn Breadboard
Working of this circuit is explained also in a video and YouTube link was : https://youtu.be/3To3Zgr1Jao

10. Precautions :
• Care must be taken while soldering the circuit on printed circuit board (PCB)
because we had experienced so many problems while soldering.
• We should aware of choosing a battery of a certain low voltage of a range of 6-12
volts to get rid of any overloading conditions.
Conclusion :
• Speed control of DC motor can be achieved using Digital or Analog Pulse Width
Modulation technique.
• When digital PWM is used, control is obtained at two levels, high and low.
Whereas using analog PWM, control can be obtained over a wide range of
values.
• In the proposed method, duty cycle is varied from 0 percent to 90 percent and
motor is controlled at different speeds. Intervals are taken at every 10 percent. At
50 percent of duty cycle, speed of DC motor is observed to be half of that at full
voltage.

11. Resources :
 Wikipedia
 Electronics Tutorials Website : http://www.electronics-
tutorials.ws/blog/pulse-width-modulation.html
 Certain You Tube videos