How to use a piezo transducer as a tactile switch
If you ever need a robust button for your next project you should really consider using a piezoelectric crystal as a main component. Piezo crystal has a property to generate voltage when mechanical force is applied by bending it, striking or even touching. This crystal is also used in car alarms as a vibration sensor.
In order to turn on a relay or have a transistor output to control some other electronic device, you need to build a debouncer circuit and also to prolong short bursts of impulse coming from piezo. The crystal is simply connected to a base of NPN transistor that will open when piezo generates enough voltage. This impulse if then fed into the NE555 which is in monostable configuration and it will issue a 1 second pulse after being triggered. How cool is that!
[ Source: awall99 ]