What about a mouse trap?
I have another idea that would not be very expensive and with a little tinkering could work and that is to use a mouse trap. The tension on the spring would not have to be as much as is needed to kill a mouse, so perhaps the spring could be cut down or replaced with a less powerful one. Then a string is tied to the tripping device and could then be any length. This is similar to the weight being released as one person suggested, but the advantage is that no weighty object is needed. The wood base of the mouse trap would only need a small hole drilled through it where the bar comes down that kills the mouse, and that could be padded a little so as to not be like a hammer coming down on a nail. Another way of doing this would be to use a spring loaded clothes pin with a piece of wood or metal in the jaws of the pin. A metal piece would be glued to the top of the clothes pin which would come down on a cable release where all this is mounted on a board. The only problem with any of these is the string would have to be kept out of the picture. That might required a pulley system. I'll try to come up with something else too. Ric.