user7754: I want to use LDR to control a light bulb (12W) which runs on AC supply using Arduino.
Is it possible to do this?
If yes, how?

user10907: The quick and dirty answer is that you need something that will safely handle the mains voltage that you can control with the Arduino (which is running at 5 (or 3.3) volts). There are two issues you need to consider:

Safety – keeping the mains voltage separate from the Arduino and the other low voltage components so that nobody gets hurt.
Capability of components – avoiding damage to the Arduino and related components due to the mains voltage.

A common way of achieving the second goal is with either a relay or a solid-state component like a triac.
The first goal is often accomplished by using an optocoupler – a device that isolates the low-voltage circuitry from the higher mains voltage by means of an LED and a photo transistor in a common housing.
The circuitry around the relay and optocoupler aren't all that complicated but it isn't trivial either. You might look on e-bay – a search for "arduino relay" will find you lots of modules that will do the job for you.

user16061: Try the PowerSwitch Tail II.
This is suitable if you are living in the USA (120VAC power).
You will find 240VAC versions as a kit on that web page as well.

7754: <p>I want to use LDR to control a light bulb (12W) which runs on AC supply using Arduino.</p>

<p>Is it possible to do this?</p>

<p>If yes, how?</p>
 10907: <p>The quick and dirty answer is that you need something that will safely handle the mains voltage that you can control with the Arduino (which is running at 5 (or 3.3) volts). There are two issues you need to consider:</p>

<ol>
<li><p>Safety – keeping the mains voltage separate from the Arduino and the other low voltage components so that nobody gets hurt.</p></li>
<li><p>Capability of components – avoiding damage to the Arduino and related components due to the mains voltage.</p></li>
</ol>

<p>A common way of achieving the second goal is with either a relay or a solid-state component like a triac.</p>

<p>The first goal is often accomplished by using an optocoupler – a device that isolates the low-voltage circuitry from the higher mains voltage by means of an LED and a photo transistor in a common housing.</p>

<p>The circuitry around the relay and optocoupler aren't all that complicated but it isn't trivial either. You might look on e-bay – a search for "arduino relay" will find you lots of modules that will do the job for you.</p>
 16061: <p>Try the <a href="http://www.powerswitchtail.com/Pages/default.aspx" rel="nofollow">PowerSwitch Tail II</a>.</p>

<p>This is suitable if you are living in the USA (120VAC power).
You will find 240VAC versions as a kit on that web page as well.</p>
