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Relay concept in 8051 microcontroller and its interfacing

A relay is an electrically operated switch or an electromechanical switch. Relays are used where it is necessary to control an electronic circuit by a low-power signal or where several circuits must be controlled by one signal.
Principle:
Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts.

Construction and working:

Relays are made up of electromagnet and a set of contacts generally based on Single Pole Double Throw (SPDT) or Double Pole Double Throw (DPDT) switching method. It has 3 pins to perform function –

• COM = Common, always connect to NC; it is the moving part of the switch.
• NC = Normally Closed, COM is connected to this when the relay coil is off.
• NO = Normally Open, COM is connected to this when the relay coil is on.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical.
An electric current through a conductor will produce a magnetic field at right angles to the direction of electron flow. If that conductor is wrapped into a coil shape, the magnetic field produced will be oriented along the length of the coil. The greater the current, the greater the strength of the magnetic field, all other factors being equal:

It can be connected to any port of the microcontroller according to your circuit diagram. It is generally connected to P2 or P3 port of the 8051 microcontroller and thus, used to control the high power devices like motors, transformers, heaters and bulbs etc.
Interfacing:
Interfacing with Bulbs

Interfacing with Motor

Code:

#include "REG52.h"

sbit RELAY1=P3^2;
long int i;
void main()
{
RELAY1=0;
for(j=0;j<15000;j++);

while(1)
{
for(i=0;i<6;i++) // run loop for 6 times ON/OFF
{
RELAY1=0;
for(j=0;j<15000;j++);
RELAY1=1;
for(j=0;j<15000;j++);
}
}
}

Explanation of Code:

- Relay has 3 pin in which 2 pins acts as a Open and Closed to make circuit. Here we have used 12V relay and connected to the motor. Now, just write the program to make it ON/OFF with delay. You can also add for loop so that relay makes a transition continuously many a times.

By connecting a bulb or LED, you can use relay as a switch which turns OFF/ON bulb according to the code burnt in microcontroller. Now, you can control the LED or bulb through relay switch.
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