When reed switch is opened, pin 2 of Arduino goes high with the help of the pull-up resistor R2, and the alert signal is available through the piezo-speaker PZ1 (not an active piezo-buzzer). At the ...

Some uses of Piezo Buzzer: e.g clock alarm, alarms, car/truck reversing indicator, bells, timers etc. So we see that this element is used on a wide area of electronic devices.

You'll need an Arduino along with a bunch of various sensors to get the project rolling, and once it's all set up you'll have a simple, but cheap system to monitor your home.