Infrared on a mobile phone?

[android]

I was mucking around capturing infra-red signals from a TV remote control today using a Vishay TSOP4838 IR receiver module. Everything looked normal for a while then, after a break, I came back to it but the logic analyser (Salae Logic) kept triggering as soon as I started the capture, instead of when I pressed a button on the IR remote control.

After much turning off of lights, closing blinds, gnashing of teeth and ruling out other noise sources it finally dawned on me that the problem only existed when I had my mobile phone (a Samsung Galaxy S5) out of my pocket. It seems to be spitting out a 145 Hz IR signal even when it's in standby (black screen).

What's going on? 

I don't have any IR apps on my phone. I checked another phone in the family and it spits out an IR signal too (albeit at 16 Hz).

[SeanB]

That would probably be the face detector, to see if your face ( which is reflective in IR) is close to the phone, to disable the touch screen. It also probably disables the touch sensor if it detects that something is close by, like being in your pocket or in a case, so that the phone stays in standby with the touch facility disabled.

[android]

Sounds reasonable...but it's doing this even when the display is on standby, completely black...possibly in my handbag  .

I would have thought that face detection would only be needed if a call was being made or received. And does it really need to be done 145 times a second. Seems overkill.

[Zero999]

Are you sure it's IR and not some other EM interference? Does it stop if the TSOP4838 IR receiver module is covered?

[rollatorwieltje]

My Samsung S3 mini has this feature as well, but it's only on during a call or when an app enables it, just checked it with a camera. Would be quite a waste of energy if it was permanently on.

[Halcyon]

Most smart phones these days will have an IR emitter with an IR receiver in very close proximity. The idea is that, when you place the phone against your head or in your pocket, the receiver will pick up the pulses from the emitter reflected against the adjoining surface material and trigger certain events (turn off LCD, prevent accidental presses etc...)

I've noticed these on most Android handsets for the past 5 years at least. I know the newer Samsung's have gesture controls, so that might have something to do with why it's active even in standby? (Maybe helping to illuminate the hand in darkness together with the camera perhaps?) You could probably disable it and see if you still get an active IR.

[Mechanical Menace]

I've made little toy apps that play with the proximity sensor, maybe it's something like that causing it to be on all the time?

Funny, I thought all proximity sensors would have been capacitive long before now, I'm surprised an S5 is using what you got on feature phones that were crappy 7 years ago.

[Stonent]

I know the S4 can be used as an IR remote

[DavidDLC]

Most of cell phone proximity sensors are based on IR, and sometimes they have the behavior shown on the l.a. screenshot.

David.

[android]

Apparently, after I did some googling, the Samsung S5 uses a proximity detector from AMS called a TMG399x (I'm not sure exactly which x).

The TMG399x is a pretty interesting unit that can be configured using I2C to emit varying width pulses of IR at about 860 Nm (well, that's the peak "normalized responsivity" of its IR receiver anyway)...and it can detect "gestures" using the built in North/South and East/West IR detectors.

Given that the TSOP4838 expects IR bursts at 38 kHz I thought I wasn't seeing the whole picture, so I hooked up an IR photodiode in parallel with a 4.7 k? resistor and checked it on my scope. The TMG399x was emitting 2 x 620 ?s bursts of 12 pulses at about 19 kHz with a 32% duty cycle every 5.8 ms - CONSTANTLY.

This is kind of what the TSOP4838 was reporting (except it treats a burst as a low and silence as a high), so I guess the TSOP4838 is fairly tolerant of the PWM burst frequency.

So far I haven't found a way of turning this device off on my phone (if only to see whether I'd miss it).

Edit: it would be good if the forum could display unicode characters properly

[android]

Hmmm...just restarted my phone and the problem is gone. The IR only comes on when I make a call. I took a picture with another phone just before and it clearly shows the IR led glowing. My battery might last a bit longer now.