> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of Peter
> Sent: Tuesday, September 27, 2005 2:16 PM
> To: Microcontroller discussion list - Public.
> Subject: RE: [PIC] DTMF encoding/decoding
> 
> 
> 1) Program the microncontroller on the board to pick up the phone and
> dial a number from time to time. The number beed not exist (or it may
be
> a line that you own and never pick up. During the time the device is
off
> hook you can charge the battery with the line current. The power
> available will be low. Say 10mA x 5 minutes x 12V ~= 1.6mAh 

Umm.. Where did that 12V come from?  You aren't guaranteed more than a
few volts.  It's 600 ohm impedance, not DC resistance.
You are free to bank any power that you don't need, and you are welcome
to draw as much DC current as you can, but you will crash the voltage
when you do that.

There's a good book on this called "Understanding Telephone
Electronics".
Used to come from Radio Shack, for $5, now it's thicker, and $30.

> 2) Program a server to call the device's line once a day. The device
> will not answer this call. This time you have a lot more power. 10
rings
> 2 seconds on each at 90Vac give 20 seconds of 0.05A which is
equivalent
> to about 0.8A at 5V for 20 seconds, which is about 4.4mAh and will run
a
> 60uA device for 24 hours with a 50% efficient battery.

Sort of works, but only if you don't draw enough current to loop the
line.
Not anything you can count on.

> 3) The off hook current must be under 500uA (better 100uA) or it will
be
> flagged as a leaky line eventually. 100uA @ 40V gives 800uA of charge
at
> 5V, 24/7. But you will get caught.

I don't remember the number offhand, but you are allowed a few uA to
power your device's memory. 

 
> 4) If the phone company objects to this tell them to change their
> tariff. Currently a dialout that does not connect is not charged in
most
> countries. This applies to 1) and 2).

About the time we get permanent colonies on Neptune...




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