Interfacing Relays with 8051

    Relays allow us to control the high power circuits or instruments using low power circuits and voltage levels such as output of microcontrollers. Interfacing relays to 8051 or any other microcontroller is just simple as controlling a LED.

    A relay consists of a coil which is driven by low power circuits such as microcontrollers in order to control high power ac circuits through output switches. Normally there are two types of switches in a relay, they are Normally Open (NO) and Normally Closed (NC) contacts. 'NO' contacts are open when relay is not activated where as the 'NC' contacts are closed, when relay is activated by supplying power to the input coil, the 'NO' contacts are now closed and the 'NC' contacts are open.

The circuit diagram for interfacing relay to a 8051 microcontroller is shown below.

The assembly program for interfacing relay is given below.

;************************************************* ; ;Program: Interfacing relay ;Author: Mahesh ;Description: Relay turns ON when the switch ;connected to P1.0 and turns OFF when pressed ;again ; ;************************************************* relay equ P0.0 sw equ P1.0 ;************************************************* org 0000h Main:clr relay        ;Configure inp and outp             setb sw up:  jnb sw,on        ;wait for switch to be pressed      clr relay      acall delay      sjmp up on:  setb relay       ;Turn ON relay      acall delay      acall delay here:jb sw,here       ;wait for switch to be released      clr relay        Turn OFF relay      acall delay      acall delay      sjmp up          ;Loop ;************************************************* delay:mov r7,#0ffh    ;delay subroutine again:mov r6,#0ffh      djnz r6,$      djnz r7,again      ret ;************************************************* end