VTech Alphabert s Magic Readport User Manual download pdf (Page 65)

BM Dept., Govt. Engg. College, Gandhinagar Page 65

EXPERIMENT NO. 15

AIM: Interface DC motor with AT89C51 microcontroller. Write a program to rotate motor with

different speed using PWM.

Circuit Diagram:

Basic concept:

DC motor speed control is useful in controlling motion in industrial control systems as

well as arm of robots. To control speed of DC motor, we can use variable DC voltage source.

When supply is given to DC motor, it takes some time to reach at full speed. If we switch OFF

DC power supply before it gets maximum speed, it starts to slow down. If we switch ON and

OFF DC power supply continuously, speed of DC motor will be in between zero and full rated

speed. If duty cycle is more (i.e. ON time is more than OFF time) speed is more and if duty cycle

is less (ON time is less than OFF time), speed of the motor is also less. Thus, if we apply PWM

(Pulse Width Modulated) waveform to the motor, we can change speed of the motor. When

width of pulse is highest, speed of motor is also highest and when width of pulse is lowest, speed

of motor is also lowest.

Interfacing details:

Simple DC motor control circuit is shown in the circuit diagram. Common anode seven

segment display is connected to port P0 to show the speed of the motor in numeric form. DC

motor is connected to the collector of transistor (We can use transistor TIP122 or 2N 3055).

Transistor is controlled by signal from microcontroller port pin P1.1. Two push button switches

SW1 and SW2 are connected at external interrupt pins to control speed of the DC motor. We can

write program such that width of pulses increases when we press switch SW1 and width of

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VTech Alphabert s Magic Readport User Manual Page 65