Apple's new Siri personal assistant feature on the iPhone 4S includes a "raise to speak" feature that allows Siri to be activated whenever an active iPhone is raised to the user's ear. The device is able to sense the raising action using a proximity sensor capable of detecting how close the user's face is to the screen. The iPhone has always incorporated proximity sensors in order to turn off the device's display during phone calls to prevent accidental input, but Siri's raise to speak feature requires that the device be ready to sense proximity to the user's face at all times.
Teardown firm iFixit has gone back to its original teardown of the iPhone 4S to highlight the sensor involved and show how it emits infrared light at all times to support Siri's raise to speak function.
During our iPhone 4S teardown, iFixit buddy Markus noted that the new iPhone had a rather unusual-looking black component next to the ambient light sensor. We didn’t make much of a fuss about it since we were knee-deep in disassembly pictures, but the little black box certainly piqued our curiosity.
Now that the teardown is wrapped up, we’ve re-opened the mystery and made a neat discovery about the 4S: that black component is an infrared LED, and the little bugger almost always wants to know if you’re nearby.
In a video, iFixit demonstrates how the iPhone 4's proximity sensor is only activated while on a phone call, while the sensor on the iPhone 4S is active whenever the device's display is on.
