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On Wed, Aug 29, 2007 at 09:10:51PM -0600, Robert Rolf wrote:
> The "tower polling interval" varies by technology. 5 minutes to 60 minutes if
> my memory is correct (read a book about cell tower technologies about a decade ago).
> The newer phones poll less often to prolong battery life.
> The do wake up many times a second to see if there is a page call for them.
> 
> The quick way to find out for yourself is to monitor your phone's battery
> current using a scope and small series resistance. You'll see huge spikes for
> a poll response, and a regular train for the 'listen to polling channel'.

Thanks, I'll try that out.

> Your bigger issue with using RF detection will be false triggers by much
> stronger, but far away transmissions (police radios (5-15W),
> nearby paging transmitters (500W), passing cabs (5W), etc.
> 
> Wi-Fi in a park is highly unlikely. That is way beyond the range limit of most cheap
> systems. And is very hard to detect because of it's short transmission time (milliseconds)
> and low density, not to mention the frequency hopping.

This is an urban park, and next to some apartment towers, so wi-fi is
actually pretty likely. Point taken though on the difficulties of
detecting it though.

> Two way paging? WHY? Then it's a cell phone or blackberry.
> 
> I would just go with KISS and use passive IR. Or possibly microwave door openers
> to sense motion without being easily recognized. If you look at the beat frequency
> by tapping the motion detector internals, you can also get velocity and coming/going.

Oh cool! Didn't think of that possibility.

> You can get both sensing methods in some PIR security sensors.
> They use PIR to confirm the size of a moving object they detect with
> microwave doppler to prevent false alarms. About $40 US. "Pet resistant".

Sounds usefull as well, and the microwave dopplar can probably be tapped
in some way as well.


That said, the project, and it's grant, is more angled to detecting EMF
from portable communications, not people per say. Detecting far away
transmissions is probably going to be ok as well, I'll have to see what
sort of behavior that creates.

I did manage to find a $90 EMF detector claiming to detect microwaves
reliably with adjustable sensitivity:

http://www.detectortechnologies.com/store/detail.aspx?ID=25

I'm going to get one maybe pull it apart to reverse engineer it so I can
make a couple more. They claim it can detect a cell phone tower from
300ft away, and a digital cell phone 20ft, 1uW/cm2.

Hopefully problem solved. :)

- -- 
http://petertodd.org
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