iPhone Proximity Sensor

Question

Can the iPhone SDK take advantage of the iPhone's proximity sensors? If so, why hasn't anyone taken advantage of them? I could picture a few decent uses.

For example, in a racing game, you could put your finger on the proximity sensor to go instead of taking up screen real-estate with your thumb. Of course though, if this was your only option, then iPod touch users wouldn't be able to use the application.

Does the proximity sensor tell how close you are, or just that something is in front of it?

Answer 1

There is no public API for this.

Answer 2

Those proximity sensors are basically a matrix of conductors. The vertical "wires" are tracks on one side of a thin sheet of insulator, the horizontal ones are on the other side. The intersections function as capacitors. Your finger carries an electrostatic charge, so capacitance of each junction varies with proximity. FETs amplify the signal and biasing sets a threshold. In practice the circuit is more complex than that because it has to detect a relative change and reject noise.

But anyway, what the sensor grid tells you is that a field effect has been sensed, and that field effect is characteristic of object about the size of a fingertip and resting on the surface of the display. The centroid of the capacitive disturbance is computed (probably by hardware) and the coordinates are (presumably) reported as numbers on a port most likely brought to the attention of the device OS by an interrupt. In something as sexy as an iPhone there's probably a buffer of the last dozen or so positions so it can work out direction and speed. Probably these are also computed by hardware and presented as numbers on the same port.

Answer 3

Assuming you mean the sensor that shuts off the screen when you hold it to your ear, I'm pretty sure that is just an infrared sensor inside the ear speaker. If you start the phone app (you don't have to be making a call) and hold something to cast a shadow over the ear speaker, you can make the display shut off.

When you asked this question it was not accessible via the public API. You can now access the sensor's state via UIDevice's proximityState property. However, it wouldn't be that useful for games, since it is only an on/off thing, not a near/far measure. Plus, it's only available on the iPhone and not the iPod touch.

Answer 4

Looks like Google is using it but it's not, as benzado pointed out, in the SDK. Go figure.

Answer 5

There is a public API for this. -[UIApplication setProximitySensingEnabled:(BOOL)] will turn the feature on. BTW, it doesn't seem to be using the light sensor, because proximity sensing would tweak out in a dark room.

However, the API call basically blanks the screen when you hold the phone up to your face. Not useful for interaction, sadly.

Answer 6

@Dipak Patel & @Coderer

You can download working code at http://spazout.com/google_cheats_independent_iphone_developers_screwed

It has a working implementation of proximityStateChanged a undocumented method in UIApplication.

Hope this helps.

Answer 7

This is not how the prox sensor works. It's not strictly light-based, as I just tested turning out the lights in a room, and the sensor didn't trip. However, I could make it trip by holding my hand over it about 5-6 inches away. Your description is accurate about how the touch screen works, but the prox sensor works without having to contact it (or even be that near). A friend suggested something ultrasonic. Anyone know for sure?

Answer 8

The proximity sensor works via measuring IR reflectance. If you hold the iPhone up to a webcam, you can see a small, pulsing IR LED.

Answer 9

Interesting possiblities. Would it be possible to register the pulsing ledlight in your home-energymeter using the proximity meter? Some of these lights only send out an IR-signal Does it work in standby-mode as well?

Answer 10

In iPhone 3.0 there is official support for the proximity sensor. Have a look at UIDevice proximityMonitoringEnabled in the docs.

Answer 11

Evidently the proximity sensor will never turn on if the status bar is in landscape orientation.

i.e. if you call:

[UIApplication sharedApplication].statusBarOrientation = UIInterfaceOrientationLandscapeLeft;

You will no longer get proximity:ON notifications.

This definitely happens on OS 3.0, I can't test it on a 2.X device since I don't have one with a proximity sensor.

This seems like a bug.

Answer 12

What does heat have to do with activating the sensor. In hot weather, after I've been holding the phone next to my face to talk, the "end call" button doesn't work! It's as if athe whole screen has been warmed up so that it does not detect the heat of my finger( that I use to touch that button). After about a minute away from my face, the end call button will respond. Same is true for "accept call" and "decline call " ???

Answer 13

There's a lot of confusion between the proximity sensor and the ambient light sensor. The iPhone has both. The Touch does not have a proximity sensor, making it a poor choice for user input. It would be a bad idea anyway since Apple isn't obligated to locate it in the same place in future devices; you aren't supposed to know or care where it is.

The proximity sensor works by pulsing an infrared LED and measuring the amount of reflectance. You can see this using your iSight camera (most digital cameras are sensitive to IR.) Just launch Photo Booth, initiate a call (or play a voicemail) on the phone and point it at your iSight camera. Note the flashing light next to the earpiece; cover it with your finger and the screen will go black.

The ambient light sensor's API is evidently private at this point.

Answer 14

To turn the screen off it's conceivable that more than one sensors is used to figure out if the screen should be turned off or not. The IR proximity sensor described by Cryptognome in conjunction with the Touch screen sensor described by Peter Wone could work out if the iphone is being held close to your face (or something else with a slight electric charge) or if its just very close to something in-animate.