Thursday, October 3, 2013


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Figure 1.1 Representation of Relay

What is a Relay?

A relay is an electromechanical switch, which perform ON and OFF operations without any human interaction. General representation of double contact relay is shown in fig 1.1

Generally, the relay consists a inductor coil, a spring (not shown in the figure), Swing terminal, and two high power contacts named as normally closed (NC) and normally opened (NO). Relay uses an Electromagnet to move swing terminal between two contacts (NO and NC). When there is no power applied to the inductor coil (Relay is OFF), the spring holds the swing terminal is attached to NC contact.
Whenever required power is applied to the inductor coil, the current flowing through the coil generates a magnetic field which is helpful to move the swing terminal and attached it to the normally open (NO) contact. Again when power is OFF, the spring restores the swing terminal position to NC.

Advantage of relay:

A relay takes small power to turn ON, but it can control high power devices to switch ON and OFF. Consider an example; a relay is used t control the ceiling FAN at our home. The ceiling FAN may runs at 230V AC and draws a current maximum of 4A. Therefore the power required is 4X230 = 920 watts. Off course we can control AC, lights, etc., depend up on the relay ratings. Relays can be used to control DC motors in ROBOTICs.

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Types of Relays:

a.       SPST (Single Pole Single Throw)
b.       SPDT (Single Pole Double Throw)
c.       DPST (Double Pole Single Throw)
d.       DPDT (Double Pole Double Throw)

Interfacing relay to 8051 microcontroller

There are many ways to interface a relay to 8051 microcontroller. But simple and easy way for beginners is by using ULN2003/ULN2803. In this tutorial, a 5volts operated relay is taken for the demonstration. But the circuits shown can be useful to 12Volts operated relays also.

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