That would be less obtrusive than a webcam, and less annoying than robo calls.

Of course, it requires access to (very) low bandwidth Internet access.

This would be trivial to do with the Raspberry Pi and Perl. Most motion sensors are single-wire (well, V+, V- and feed), so just a single GPIO pin. Here's a quick example with a mix of real and pseudo:

use warnings;
use strict;

use RPi::WiringPi;

my $pi = RPi::WiringPi->new;

my $pin = $pi->pin(27); # GPIO pin 27

my $count;

while (1){

    # check if time >= 0900 here
    # ...if not, sleep and next

    my $motion = 0;

    for (0..10){
        if ($pin->read == 1){
            $motion = 1;
            last;
        }
        sleep 5;
    }
    
    $count++ if ! $motion;

    if ($count >= 3){ # 0915 hrs and no movement
        # send email, sound an alarm etc
        ...
    }

    sleep 300;
}
[download]

If i were going in this direction id give her an actual button, im thinking of something like a big staples-easy button. Maybe with an led to remind her it is check in time. A positive action rather than a lack of action.

Calling your mother every day at 9 o'clock and being called by your son every day at 9 o'clock will become very tiresome and annoying on the long run. For the caller it is a duty, which may be impossible to accomplish given personal conditions and schedule; for the callee, it can lead to a debt of gratitude knowing that every single time there's somebody over there doing the check. IMHO calls to one's mother should not be those of type alarm call, i.e. call personally if you've got something to say.

There's enough concern visible by the OP in wanting to implement such a dead-man-button, and I'd guess that it is based on mutual consent, otherwise it just doesn't make sense: then it would be better to just delegate the job to the other number - which might have been discussed also, but have turned out as not feasible.

perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'

@shmem:

The point of the dead-man-button is to allow her to live independently while assuring that she won't lie unconscious for more than 12 hours after a stroke or an incapacitating fall. She's more than willing to pick up the phone every day at 9am and 9pm if it means not having to go into assisted living.

I was thinking the same thing. Even a one minute call if it's at the same time every morning to simply say "Good Morning".

@Anonymous and stevieb:

Because of my job, I can't always be at my computer at 6pm Barcelona time. But for her convenience, it's better that the call be every day at precisely 9am (Seattle time) so she can plan her morning around it.

Perhaps (along the lines of the "easy button" from above) something that sends an IM or other messaging service notification. Doesn't necessarily need to be telephone to get the signal transmitted.