The best i know is the one i used in my high school science fair project..
Is just a nail attached one end to metal spring ,as the one used in toggle pens.
then a metal 'O' ring ,close about 1 mm ,and you have a good and sensible
switch ...when it detects a minor movement or vibration nail touch the
ring and close the switch..
I remember an oscillator based on 555 Ic and an output to a buzzer.
( | ) < "O" ring or a little metal hook.______ sw pin 1
|
| < nail or metal wire (uncovered)
0
0 < spring
0__________________________________ sw out 2
Simple Isnt it ?