To be honest, when we moved into our last home the garage door opener was a gigantic two-pound, nine inches square monstrousity the everyone in my car pool poked fun at. The was an electrical engineer in the car and he suggested we punch the button on the opener as we drove down the street (something about older models have a wider bandwidth). To my surprise we opened three garage doors and a car alarm went off (although, I don't know if that was attributed to the opener).
I think the problem with adding technology to simple tools should be apparent. We would be adding unnecessary steps to a self-defense situation where fractions of a second may mean actual life or death.
Here is a quote from a law enforcement officer mentioning the video, "Surviving Edged Weapons," which is basically a law enforcement video. The reason I mention it is the simple fact that people (especially law enforcement officers) often underestimate the quick lethality of a knife in a close combat encounter.
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"Law enforcement has conducted many studies on this and developed some guide lines to follow about being attacked with such a weapon. The first video I saw was one called, "Surviving Edged Weapons." In this video they showed how if someone with a knife was within 21 feet of you they could get to you and stab or cut you before you could draw your firearm and shoot that person. The other point that is very important is that the vests that police officers wear will do nothing to stop a knife from cutting right through them."
http://www.sierratimes.com/05/04/06/sheriff.htm
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Here is a synopsis of the video that is fairly accurate. Please note that this video isn't for the faint-hearted as there are several autopsy and morgue photos as well as crime scene photos and reenactments.
My point being, it's much easier for a person to use a knife effectively than it is a gun, BUT most knife-wielding criminals will be detered by a firearm even in the hands of an elderly lady or a young child. Certainly much has to be said about the added preparations a homeowner would have to take in this stressful situation (donning the correct rings or bracelets, loading the firearm, preparing it to fire, etc.) while nothing has been done to slow the attacker.
WAIT! I've got it! Knives that only work when . . .