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Comment by binarymax | Hacker News Reader

Does anyone know if these work as advertised? http://www.ebay.com/sch/items/?_nkw=cell+phone+signal+block&...

I rarely receive calls on my mobile - and only really carry one just in case I need to make a call.

How would they not? They're little Faraday cages, and as far as I've heard even Apple has not found a way to violate the laws of electromagnetics ;)

Why don't you just switch off your phone? That would save precious battery time, too…

I know this may sound ultra-paranoid, but I have heard rumors that switching it off may not be enough.

--Edit-- Thanks for all the replies - So does the faraday cage accomplish the same as battery and SIM removal?

Not sure about smartphones, but IIRC, my old cellphone used to ring any alarm that was set even if the phone was completely turned off.

My generic Android phone does this.

Is this common? This is the first I've ever heard of such a feature, ever. Considering that we're talking about the OS being completely shut down, I'm skeptical of this existing in smartphones.

I believe it is normally a separate microcontroller. You generally need something to power up the main phone and to deal with battery charging (thats not usually the main CPU, although my Android phone does display an animated icon on screen when powered off and charging, so unclear whats driving this).

Most computers have a number of extra microcontrollers. You would have to do a teardown to see how they might be wired up.

My nokia smartphone does it too.

I've also heard they can be turned on by the tower, even if the phone is off. The only way to be sure is to keep the battery removed.

It is not ultra-paranoid. The government's own rules regarding cell-phones in certain highly classified areas (SCIFs) require that they be left outside precisely because you can't guarantee that off means off.

I think that has less to do with wireless transmission as it does to recording devices.

Yup. For the ultra concerned, its battery and sim removal.

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