Cell phone giving off NIR bursts of light?

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So I'm building this photodiode/ noise electronics  
I'm just bringing up the system and testing things out.  
And I've got this really weak light signal I can't trace  
down.  Every ~114 ms there is square wave burst...  
eight steps ~15 us period.  I can block it by waving  
pieces of card board around... anyway long story short.
My cell phone in the off state is sending out these bursts
and they were bouncing off the ceiling and getting into my detector.  

I'd post a pick but there's a little gain peaking in my
PD TIA that I have to clean up first. :^)  

Hey, maybe I can use this for testing... I wonder how sharp the  
edge is?  

George H.  

Re: Cell phone giving off NIR bursts of light?
On 2020-03-13 14:22, George Herold wrote:
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IRDA, perhaps.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Cell phone giving off NIR bursts of light?
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

Re: Cell phone giving off NIR bursts of light?
On 2020-03-13 15:15, keith wright wrote:
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That's probably right.  I've never used one myself, but I know they exist.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Cell phone giving off NIR bursts of light?
On Friday, March 13, 2020 at 3:16:00 PM UTC-4, keith wright wrote:
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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.  

Re: Cell phone giving off NIR bursts of light?

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

--
mikko

Re: Cell phone giving off NIR bursts of light?
On Friday, March 13, 2020 at 3:18:43 PM UTC-4, Mikko OH2HVJ wrote:
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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.

Re: Cell phone giving off NIR bursts of light?
On Friday, March 13, 2020 at 2:25:37 PM UTC-7, George Herold wrote:

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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.

Re: Cell phone giving off NIR bursts of light?
On Friday, 13 March 2020 16:14:17 UTC-7, whit3rd  wrote:
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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: https://ams.com/-/ams-introduces-world-s-smallest-1d-time-of-flight-sen-sor-for-accurate-proximity-sensing-and-distance-measurement-in-smartphones

kw

Re: Cell phone giving off NIR bursts of light?
On Friday, March 13, 2020 at 8:26:03 PM UTC-4, keith wright wrote:
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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.    


Re: Cell phone giving off NIR bursts of light?
On 14/3/20 12:10 pm, George Herold wrote:
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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

Re: Cell phone giving off NIR bursts of light?
On Sat, 14 Mar 2020 12:18:20 +1100, Clifford Heath

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That and more are all part of Motorola "Moto Actions":
<https://motorola-global-portal.custhelp.com/app/answers/indevice_detail/a_id/112154/p/30,6720,9833
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:
<https://play.google.com/store/apps/details?id=com.tinoooapp.gravitygestures

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.  


--  
Jeff Liebermann     snipped-for-privacy@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
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Re: Cell phone giving off NIR bursts of light?
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


Re: Cell phone giving off NIR bursts of light?
On Friday, March 13, 2020 at 7:55:48 PM UTC-4, keith wright wrote:
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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

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