Relay -why it acts the other way - help needed!

I'm trying to use relay in my project . Relay shield is designed for arduino 5v

I have a doubt in NC COM AND NO
NC normally closed
NO normally open
COM common connection

When I connect hot wire to COM and wire from the load light to NO . To act like a switch.
When I code it acts the other way .
digitalWrite( relay , HIGH) It turns off the relay I don't under stand it . Usually I used HIGH command to turn on a led or anything
digitalWrite(relay , LOW) it activates the relay and load gets the power. Someone plz tell me,why it is acting the other way?

If it is Normally open then it should turn on only when I give HIGH to it . But why it trigges,r when I give LOW.

Am I doing anything wrong .

And about NORMALLY CLOSED . Why it is needed ? It is always connected! When I turn off arduino , it powers up the load !
Could someone explain me where and when NC is used . I don't find any application where NC suits?

It seems like the NO and NC markings on the relay are reversed. Connect your load to the contact marked NC instead of the one marked NO.

One application for NC is in a power fail circuit.
If your relay is powered by AC mains the NC contact will be open.
You connect a battery to the contact then to a bell.
When your AC mains fails the relay drops and the NC contact closes.
Therefore the battery is connected to the bell and the power fail is annunciated.

If it's a relay board, they are often set up with optoisolators - you can sink current (low signal) to turn it on, or it can be wired so that you source current (high signal) to turn it on.
Most that I've seen recently need a low to turn them on.

NC vs NO and COMMON - relays can be used to connect 1 of 2 signals to input for example, or to switch an output to 1 of two inputs - or just as a power switch as you are doing. So three different uses right there - need to think broadly about these things.

Can we get a photo ?

I think you've got one of the relays described on this page:
http://arduino-info.wikispaces.com/ArduinoPower

Scroll down to "Important Note" for an explanation.

Kindly refer the link for relay image

white wire is hot line-COM
yellow wire is line to load - NO

Can we get a photo ?

White wire is hot line
Yellow is line to load

Your wiring is correct. What is wrong is your coding:

digitalWrite( relay , HIGH)   It turns off the relay I don't under stand it  . Usually I used HIGH command to turn on a led or anything 
digitalWrite(relay , LOW) it activates the relay and load gets the power. Someone plz tell me,why it is acting the other way?

For that and most Asian relay modules digitalWrite( relay , HIGH) will turn the relay off and
digitalWrite( relay , LOW) will turn the relay on. You just need to get over the fact that you have in your mind that a HIGH output always means ON and a LOW means always OFF, it’s not the case with that specific relay module.

Lefty

LarryD:
One application for NC is in a power fail circuit.
If your relay is powered by AC mains the NC contact will be open.
You connect a battery to the contact then to a bell.
When your AC mains fails the relay drops and the NC contact closes.
Therefore the battery is connected to the bell and the power fail is annunciated.

WOW! perfect application. I never thought this way. but how can i power relay with ac 220v when it takes 5 v as input .

retrolefty:
Your wiring is correct. What is wrong is your coding:

digitalWrite( relay , HIGH)   It turns off the relay I don't under stand it  . Usually I used HIGH command to turn on a led or anything 

digitalWrite(relay , LOW) it activates the relay and load gets the power. Someone plz tell me,why it is acting the other way?




For that and most Asian relay modules digitalWrite( relay , HIGH) will turn the relay off and
digitalWrite( relay , LOW) will turn the relay on. You just need to get over the fact that you have in your mind that a HIGH output always means ON and a LOW means always OFF, it's not the case with that specific relay module.

Lefty

CrossRoads:
If it's a relay board, they are often set up with optoisolators - you can sink current (low signal) to turn it on, or it can be wired so that you source current (high signal) to turn it on.
Most that I've seen recently need a low to turn them on.

NC vs NO and COMMON - relays can be used to connect 1 of 2 signals to input for example, or to switch an output to 1 of two inputs - or just as a power switch as you are doing. So three different uses right there - need to think broadly about these things.

i thought, i was wrong in wiring r coding.if relay works this way then It is cool . Thanks lot guys !
Once again thanks to all for helping.