There are lot of free apps in iTunes store to do this purpose. Here is the one simple application for the same purpose. Available for the iPhone 4 LED Light is a pretty simple, single-use app. It turns on your light as soon as you launch the app, and it turns off just as easily.

There’s nothing more to it beyond that, but it’s definitely worth downloading if you want to put that light to good use when you’re running around in the dark. LED Light is a free download from the iTunesApp Store. Here is the  download link.

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