How Tech Works

Little Sister Siri

Little Sister Siri meme with infrared LED sensor

Every smartphone has its share of sensors—little doodads that know when you’re “steering” that awesome car in that awesome game, or making sure you come back home safe by knowing which way is North—and the iPhone 4S is no different in this regard.

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.

It’s no innovation for the iPhone to integrate an infrared proximity sensor. In iPhones past, the IR proximity sensors would only switch on when a phone or Skype call was initiated. The sensor could detect that a face was near and then smartly dim the display / kill the keyboard, and then resume normal activity once it detected the face was not in close proximity anymore.

Infrared sensor in the iPhone 4S

The 4S, however, has a neurotic tendency of always wondering how close your face is. As long as the screen is activated, that IR sensor will be shining brightly (though you wouldn’t know it, unless you’re a snake, fish, mosquito, or cheap digital camera).

Don’t believe us? Check out MJ’s demonstration:

So we began to ponder: why is the 4S so interested in our faces’ whereabouts? Well, we believe the answer lies with our new search-servant Siri.

Siri is ready and waiting to answer her master’s beck and call at any time. And in order to be as attentive as a personal assistant ought to be, Apple had to design the proximity sensor to be as vigilant as Big Brother, but as cute as Little Sister. So whenever the screen is active, the proximity sensor is active too. Thus, whenever you raise the iPhone 4S to your face, Siri is ready to take orders.

Of course, everyone’s going gaga for Siri’s accommodating attitude, but in light of this new discovery, we’re curious if we should start wearing tin foil hats. Many think that a constant infrared radiation beam is no big deal, but there are some of us at iFixit who aren’t above being extra-cautious.

All kidding aside, the fact that the infrared light is abnormally active isn’t a cause for panic. We come into contact with infrared radiation all the time (every time we venture forth from our caves and step out into the sunlight), and just because the iPhone 4S’s proximity sensor is emitting infrared radiation whenever your screen is active isn’t a big deal. Infrared light is non-ionizing, meaning it is a low-frequency radiation, lacking the energy to influence changes in DNA.

But we recommend keeping your iPhone 4S face-down in a drawer while you sleep. Just in case.