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The kind Mom had worked through a device attached to the telephone. Periodically someone would come on and ask Mom if she was okay. If she did not respond the service would call the first family member on a designated list. Mom had a necklace to wear with a push button on it. The problem was she did not always wear it in the shower, was not wearing it when the bathroom door handle malfunctioned (but she had a screwdriver handy), and when she had her stroke, the necklace was lying on the bedside table on the other side of the room (although she may not have been able to push the button even if she had been wearing it).
This is the problem with most of the "panic-button" monitoring devices -- seniors will NOT wear them continuously, especially when they are necklaces. I kind of think that the only wearable systems that would really be worthwhile are the sort that are worn by addicts; the biometric bracelets that cannot be removed.
The other alternative is a system that uses motion detectors in the home. They are not cameras, just the sort of beam sensor that lets a store know when a customer has come in the door. The usual spots for the beams are the doors to the kitchen and bathroom, the entry doors to the home, the staircases, and the medicine cabinet (or wherever the person keeps their meds.) Essentially, they monitor the person's routine, and when no movement is detected for an unusually long time during the day (if the exit doors have not been opened), then they call on the phone to check. If the person doesn't answer, it notifies someone to go and physically check on them. (I'm simplifying; there is a bit more to it than that.) Examples of this kind of system are BeClose, QuietCare, or GrandCare. I haven't been in a situation to use them myself, but we may be looking into them soon for MIL.
PS: Conditions that cause blackouts are very tricky, because they can be very dangerous if the person still drives. A few weeks ago my MIL noticed that she lost her rearview mirror somewhere between leaving her house and leaving the post office, but she couldn't figure out how. Obviously, she blacked out while driving and sideswiped something, but she has no idea what she might have hit. Luckily it scared her badly, and she has permanently parked her car. (We're going to give it a little time and then offer to buy it off her for DS, who won't be old enough to drive for another year. Once he gets his license, we're planning on having him use the car to drive Grandma on her errands.)