I've tried a lot with piezo-summers... (without any circuit in them!)!

Forget it !

Buy readymade Piezo-beepers for intrusion-detection !

Piezo-beepers are allowed to have 24 Volt accross them. So I have used a circuit to double the power (5V -> 10V) with one pin of the PIC and loaded a 1000uF condensator. Then I use a MOSFET-driver ICL7667 to drive the beeper from this 10V. It is loader, but using some cheap magnetic beepers from digisound is much more easy....

;-)