the most simple one as i happened to stumbled upon and modify a little, was the one that came from a removed door. its door stopper. It came from Yale BKS and was made in Germany. it works fine with some practice of course. i made the mark line to determine which distance from the key is enough when bumped. please see pic. it might be useful to you too. you dont need to modify the spring, it is enough strenght, it is the distance and the timing you need to practice with. still ,the bumpkey hammer is irreplaceable.
http://i302.photobucket.com/albums/nn11 ... 040858.jpg
http://i302.photobucket.com/albums/nn11 ... 040859.jpg
http://i302.photobucket.com/albums/nn11 ... 040860.jpg
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