VTech Alphabert s Magic Readport User Manual download pdf (Page 67)

BM Dept., Govt. Engg. College, Gandhinagar Page 67

When port pin P1.1 is low, LED of opto-isolator will glow which makes transistor of

opto-isolator ON. Current flows through base of transistor TIP120 and motor gets DC supply.

Transistor of opto-isolator and TIP120 forms Darlington pair. When port pin P1.1 is high (i.e.

set), LED will not glow, both transistors remains off and motor will not get DC supply. By

controlling ON and OFF time of the LED, we can control speed of the DC motor. Program will

be very much similar to the previous program, except we will interchange SETB P1.1 and CLR

P1.1.

H-Bridge configuration for bi-directional rotation:

Direction of DC motor can be changed by changing polarity of DC voltage. H-Bridge

configuration is very popular for bi-directional speed control. Interfacing of H-bridge with

microcontroller is shown in the following circuit diagram. We need four microcontroller pins to

control direction and speed of the DC motor. When port pins P1.0 and P1.4 are high, motor

rotates in clockwise direction and when pins P1.1 and P1.2 are high; motor rotates in

anticlockwise direction because of reversal in current. Optical isolation is not shown in the

circuit but it is better to use optical isolation for safety of microcontroller.

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VTech Alphabert s Magic Readport User Manual Page 67