Just for fun I created a burglar alarm circuit using NE555 and
logic gates. Two door sensors can be connected to the circuit in NO or NC configuration.
When the ON/OFF switch is open the device does not work.
When you close the ON/Off switch the burglar alarm starts working with a time dalay given by the C5 capacitor.
If the circuit detects a change in the condition of a sensor the alarm sounds with a time delay related to the C3 capacitor.
The C4 capacitor turns off the alarm, then the circuit backs to the start condition.
The bell works even if the sensor backs to the start condition in a very small instant, so the circuit can be used to detect vibrations.
Although this burglar alarm has been tested successfully, it is just an hobby project. In the link you can find more info and a pdf version of the circuit.