Pretty smart. I never thought about using those like that on a car. Spent lots of time adjusting the targets for those on aircraft when I was in the military.It's a sensor used for limit switches (detects when ferris metal passes over it) that I wire into the drivers door lock wiring harness, which allows me to unlock the driver's door with a quarter. Sensor is hidden and only has a range of about 1/4" so if you don't know where to place the quarter or other metal object nothing happens.....LOL I made it when my wife was pregnant and locked her keys inside the car and It took me an hour to get the spare keys and drive to her work, while she stood in the cold. I also hide a spare key inside the vehicle........ so I'll never be stranded due to lost keys etc......... I have them installed on everything I own. Saved my butt twice so far.