Siri?s Secret: An Always-On Infrared Sensor

There may be an actual hardware reason why Siri will never come to older Apple hardware. The iPhone 4S, iFixit discovered, has an IR sensor to let Siri know that you have held it up to your face. And it almost never switches off.
Previous iPhones also have an IR sensor up by the ambient light [...]

Source: http://www.wired.com/gadgetlab/2011/11/siris-secret-an-always-on-infrared-sensor/

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