A simple alarm can be formed using a series of switches connected to doors and windows throughout a building like this.

How can I design a more sophisticated alarm that allows the houseowner to leave the house after turning it on and re-enter he house and turn it off without the alarm sounding? The univ should arm itself 30 seconds after a switch is closed and allow the user 30 seconds 30 seconds to turn it off before sounding the alarm.

 

The three switches on the left have to be connected in parallel as for the alarm to be activated if one of them is pressed. You can use a keypad to enter a serial number before entering the house as for the system to know that you are the owner. If you cant do this, a more simple way but more easy to hack is to use some switches which you will have to press in a particular sequence as for the alarm to be disarmed.

 

As a guy who works on Alarm System brains, I say, "Get a real panel! Keep me employed!"

You may also wish to look at something called a "keyswitch." Insert the key, turn it in one direction, and the circuit is closed. Turn it in the other direction, and the circuit is opened.

Or, run a separate circuit for the door, which uses a timer. Give the owner 15-30 seconds after opening the door to deactivate the alarm.

--Rich