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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?

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    Yes, with a relay. Ask google, it knows how. – Majenko Dec 3 '15 at 13:19
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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.

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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:

  1. Safety – keeping the mains voltage separate from the Arduino and the other low voltage components so that nobody gets hurt.

  2. 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.

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You can use a relay shield like these: Relay Boards - eBay

Be sure to check the amp draw of the relays. Sometimes you need a transistor, but I don't think you should for these shields. Just be careful working with mains voltage.

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If you want to switch it on/off, there is a simple solution using a relay and an optocupler (safer option) or transistor. Here is a schematic:

Schematic

but if you want to control intensity of light, then you have to use triac control, which you have explained here: AC Phase Control.

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