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How Siri knows when you're near

The iPhone 4S's infrared sensor tells Siri when you're near, so you don't even have to press the button to activate it. Neat.

We were all aware that the iPhone's personal assistant Siri was a clever little helper -- but weren't aware of exactly how clever.

iFixit has found a new function for the infrared LED proximity sensor on the iPhone 4S. Previously this dimmed the screen when you held the handset to your face to make a call, so that you didn't accidentally press the screen with your ear -- it still does that, but adds a cool feature to Siri too.

The sensor gives out a constant infrared beam whenever the screen is activated, whereas on earlier models of iPhone it only did so when you made a call. This is what allows you to hold the handset to your face to activate Siri. The assistant's always ready to take orders, no pressing of the screen required. Very clever indeed.

It could also provide one of the clues to the ongoing battery problem mystery. Some iPhone 4S owners have claimed longer battery life when disabling Siri's 'Raise to Speak' feature, by going to Settings>General>Siri>Raise to Speak>Off. No constant infrared signal being pumped out, less drain on the battery it would seem.

Apple has admitted there are battery issues with the 4S, announcing a fix was on the way soon. It should be in the iOS 5.0.1 update that'll be along shortly. This LED infrared sensor could also be one of the main reasons we're not likely to see Siri on older models of iPhone, as their lack of sensor would mean you wouldn't get the full experience. (As Siri is one of the main selling points of the 4S, it would also likely harm sales.)

Siri still can't find local info and services in the UK due to Apple failing to secure partners, but this should be fixed next year. The service went down in the US recently. But still, at least it won't rack you up a massive phone bill.