Hi;
I would like to know what is the uses of these ports on relay. NO (Normally Open) COM (Common) NC (Normally Closed). Can any one explain me further about this relay.
Thank you;
zakirox
Those three are the switch connections from the relay. You wire them to the circuit you want to control.
That relay is an electronically driven single pole change over switch. Sometimes called a SPST.
Why have you got a relay if you do not know what it does?
A relay is simply an electrically controlled switch. You can look-up how an SPDT switch works, and you should be able to understand what the relay is doing.
The common and NO contacts are normally disconnected, or "off". When you apply power to the relay coil, these two contacts are connected together ("closed" or "shorted").
The common and NC work the other way. These two contacts are connected together ("on", "closed", "shorted") when the coil is not powered. When the coil is powered, these contacts open-up.
The NO & NC contacts are never connected together.
The terminology & logic is reversed from a water faucet... For a switch or relay, closed means the contacts are connected an current flows. Open means the contacts are not connected (the circuit is broken) and current doesn't flow.
Depending on your application, you may not need to use all of the contacts.
Many relays have a clear plastic cover that you can see through. I was just wiring-up [u]this relay[/u] about an hour ago. You can see the internal wiring and you can see the contacts opening & closing as the relay operates. (It's not for an Arduino project. This particular relay has a 12V coil, and you can't operate it directly from an Arduino.)
In my application, I'm using the common & NO contacts. The NC contacts are not connected to anything.
I found the attached pic, which may help. What would we do without Google?
If you wire C and NC, your external circuit in usually on and goes off when you energise the relay. The dotted line in that pic indicates that the coil pulls the lever down when the coil is energised. The lever is spring-loaded into the NC position.
If you wire C and NO, your external circuit in usually off and goes on when you energise the relay.
[hint]
Google Is Your Friend 8)
[/hint]
wow thanks every one i love u all 
heheehhe i understood 
simple version ;
it is a way to connect high voltage, high power to a micro-controller.
the Comm/NO/NC side is totally isolated from the signal side.