Quote Originally Posted by Melanie View Post
First make sure it's the Piezo you've got - and not the Sensor.

The three wire devices usually employ the third wire to change the tone. When connected to Vdd you'll get one tone, and when connected to 0v you get another. That's the way they get the 'warbling' effect for the Alarm sound.

Other more sophisticated Piezo's allow the sychronisation of several sounders - and they use the third wire for that purpose. You'll know that is the type if the original smoke alarm had an additional conector for connecting to additional alarms.

Experiment and see what you get...

Maybe... I'm guessing it's a "self-drive" piezo buzzer. It needs a transistor and a few resistors to make it work...