This document discusses interfacing a relay with an 8051 microcontroller. It defines a relay as a switch that can be controlled by a low-power signal to switch high-power circuits. It provides a diagram showing how an 8051 port pin can provide control signals to drive a transistor and energize a relay coil to open and close the main relay switch. It also gives examples of assembly and C code to output high and low signals from a port pin to toggle a relay on and off.

1. RELAY INTERFACING WITH
8051
PRATIK PHADTE
COURSE : ELECTRICAL AND ELECTRONICS ENGINEERING
@pratikphadte19@gmail.com

2. TOPICS
• What`s a relay
• Interfacing relay with 8051
• Assembly code for interfacing
• C code for interfacing

3. WHAT`S A RELAY
• A relay is nothing but a switch, it can be controlled(ON/OFF) by a low power signal(ex:
current in microamps).
• Its mainly used as a switch in high power circuits(230v,50hz).
• When control signal is given
(ex:HIGH) ,then the switch is
CLOSED
• Similarly when control signal is
LOW ,then switch in OPEN

4. WHAT`S A RELAY
• The figure shows practical internal
operation of a relay.
• Here ,when the small switch is closed ,the coil gets
energized becoming an ELECTROMAGNET
• Thus ,attracting the metallic plate and hence
CLOSING the main switch.
• When the small switch is open, the coil is
de-energized ,hence the SPRING pulls the
metallic plate back therefore OPENING the
main switch.

5. INTERFACING RELAY WITH 8051
For interfacing with the controller we need a setup like below:
• Here the 8051 uc can be programmed to
to set its any port pin as output(HIGH/LOW
(one /zero)
• A HIGH signal from port pin P0.1 will turn o
transistor and energize the coil, hence MAIN
switch will CLOSE thus the bulb GLOWS.
• When LOW signal is given by 8051 uc ,
the transistor is off and the coil gets
de-energized via the free wheeling diode
hence the metallic plate is lifted and
main switch is OPEN.

6. ASSEMBLY CODE FOR INTERFACING
For assembly code we need to just send a high signal to the used Port PIN
High signal=LOGIC ONE
CODE:
org 00h
setb P0.2 [output of port zero pin two is set to HIGH(one)]
acall delay
clr P0.2 [output of port zero pin two is set to LOW(zero)]
END
NOTE: delay code is not mentioned here

7. C CODE FOR INTERFACING
#include<reg52.h>
sbit relay_pin = P0^2; [naming port zero pin two]
Void delay(int); [Define delay function]
Void main( )
{
do
{
relay_pin=0; [relay switched ON]
delay( );
relay_pin=1; [relay switched ON]
delay( );
} while(1);
}
NOTE: delay code is not mentioned here