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?

Bandoleer

on a touch screen device how can one implement mouseover without proximity sensor? – Ricardoricca

Answer 1

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.

Kattie

The iPod touch actually does have a light sensor. I can see it on mine near the top-left corner, above the screen, in direct sunlight. The screen brightness can auto-adjust to ambient lighting conditions. – Wilbertwilborn

Uh... I didn't say the iPod doesn't have an ambient light sensor. We were discussing the proximity sensor, which as far as I can tell is implemented by detecting light. – Kattie

Oh so in the iPhone it's 2 different sensors? – Wilbertwilborn

if it was a light sensor then the display would go off during a call in a dark room? – Granth

This is technically untrue. There is a light sensor to automatically adjust screen brightness and a second infrared LED / sensor pair to detect the proximity of your face. (Holding a white sheet of paper an inch or two above the speaker end of the phone will trigger the proximity sensor, yet the display still works in a dark room.) – Ideology

Answer 2

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.

Ideology

How about a hide&seek game :) – Superscription

The UIApplication method is deprecated. In 3.0 you enable it with: [[UIDevice currentDevice] setProximityMonitoringEnabled:YES]; Also, it is possible to check the property proximityState, so if you poll that value, it could be a primitive input method. – Hippogriff

There was no public API at the time this question was asked; I've updated my answer (the accepted one) to reflect the change. – Kattie

Actually, no need to poll the value of proximityState. Just observe the notification: UIDeviceProximityStateDidChangeNotification – Hippogriff

Answer 3

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 the 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.

Stablish

Answer 4

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.

Kakaaba

Answer 5

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.

Rationale

I would disagree with the proximity sensor being IR based. I can cover the top of the phone with a solid object, like a note pad, and the sensor is not triggered. However, I can touch the top of the phone with my chin and the sensor is triggered. I tested this with the call dialing app. – Delete

The experiment I suggested, which makes the IR transmitter clearly visible, should remove any doubt. It is only active when proximity sensing is enabled, such as on a call or playing a voicemail. I don't know why your notepad failed to trigger it, but it worked for me using a greeting card, notepad and a paper towel. – Rationale

Answer 6

Just to update, this is possible.

device = [UIDevice currentDevice];

// Turn on proximity monitoring
[device setProximityMonitoringEnabled:YES];

// To determine if proximity monitoring is available, attempt to enable it.
// If the value of the proximityMonitoringEnabled property remains NO, proximity
// monitoring is not available.

// Detect whether device supports proximity monitoring
proxySupported = [device isProximityMonitoringEnabled];

// Register for proximity notifications
[notificationCenter addObserver:self selector:@selector(proximityChanged:) name:UIDeviceProximityStateDidChangeNotification object:device];

As benzado points out, you can use:

// Returns a BOOL, YES if device is proximate
[device proximityState];

Rancell

Answer 7

There is no public API for this.

Wilbertwilborn

Answer 8

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

Fairfax

Answer 9

If you aren't aiming for the AppStore, you can read my articles here on getting access to those:

Proximity Sensor: http://iphonedevwiki.net/index.php/AppleProxShim

Ambient Light Sensor: http://iphonedevwiki.net/index.php/AppleISL29003

Soudan

Answer 10

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. answered Jul 22 '09 at 5:49 Kevin Lambert

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I've encoutered this problem too. It took me a long time to figure out the real reason of why the proximity sensor is not working. When orientation is UIInterfaceOrientationLandscapeLeft or UIInterfaceOrientationLandscapeRight, proximity sensor does not work; while in portrait mode it works well. My iPhone is iPhone 4S (iOS SDK 5.0).

Pennant

Answer 11

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.

Densitometer

It sounds like you are describing the touch screen, not the proximity sensor (which shuts off the screen when you hold the phone to your ear). As far as I can tell, that's just a light sensor. – Kattie

I am, a touch screen is a proximity sensor. I didn't realise there was an ear-detection mechanism. – Densitometer

Answer 12

@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.

Reamer

Answer 13

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.

Murrell