Cell phone giving off NIR bursts of light?

IRDA, perhaps.

Cheers

Phil Hobbs

On Friday, 13 March 2020 11:32:27 UTC-7, Phil Hobbs wrote: ...

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More likely it is the proximity sensor. If so it would probably come from the top front-surface of the phone near the speaker.

On the iPhone we use IR modulated at about 100-300kHz as a short-range sensor.

kw

Probably close to the speaker - many phones use IR proximity sensor to detect they're close to you to disable 'buttons' during a call.

That's probably right. I've never used one myself, but I know they exist.

Cheers

Phil Hobbs

Thanks Keith, yeah front surface near the speaker. I did use it to tweak the PD response. But it was a pain to ID.

George H.

Yeah thanks. It was actually useful... maybe there is some app that lets me control the blink. I'll have to try and measure the edge. (Right now a 1 meg FB resistor, ~1 us, slow)

So what's the detector? There's a front camera too, is that it?

I'll have to search some.

Having a set length/number pulse train is a little like a lockin... but further encoded.

George H.

Perhaps it's the 'ear detector' that determines if your phone is held against your head and in no need of backlight. Irritating little mechanism, when I pull the phone away in order to hit the hangup 'button', it's always dark.

On Friday, 13 March 2020 14:25:37 UTC-7, George Herold wrote: ...

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The detector will be a silicon photodiode - the IR beam will probably be modulated at 100kHz or (it is in iPhones). The camera is too slow.

kw

This is an LG phone from years ago. Once every ~114 ms an 8 bit square wave, ~15 us period.

But good to know if you are ever in a dark room testing some photo sensor.

GH

The proximity detector is usually only active during a call though.

In George's phone it seems to be active when the phone is idle - maybe something to wake up the phone when the user reaches for the phone.

Some new phones have a more sophisticated scheme using time-of-flight measurement with ranges of a couple of feet and resolution of a few mm.

For example:

kw

OK after some f-ing around trying to turn stuff on and off, I found that the light is also active when making a phone call. So here's my story. Some marketing guy asked, "Can we make the battery life of this cheaper phone less?" And the engineer said, "We could leave the proximity LED on when the phone is off." and the marketing guy said "Perfect."

George H.

My Motorola phone shows the lockscreen dimly (battery, time/date, notifications, etc) whenever you put your hand near it. That's clearly a prox sensor, probably IR.

CH

That and more are all part of Motorola "Moto Actions": To the best of my limited knowledge, these features are based on IR emitters/detectors and are always on (unless you deactivate all the Moto Action features). That became a problem for Motorola, so they removed Moto Actions. Someone wrote the Gravity Gestures app that replaces most of the gestures:

Reminder: You can "see" Near IR emissions with most digital cameras. Try the camera with an IR remote control first to make sure the camera works in IR.