Children love noise. The more the better. In order to introduce a practical application of a simple electrical circuit, I decided to show them how to build their own alarm. One requirement is that the alarm should stay running even if the person who enters closes the door.

The basic wiring diagram for any circuit is shown above. The little electric motor is just allowed to lie in the tin. When the battery is connected the motor rattles. The tin makes noise

A switch can be included, by simply breaking the wire, baring the ends and joining them or not.

A peg can easily be converted into a spring switch. The bared ends of the wire are wrapped around the peg as shown in the diagram. When the peg is open, the circuit is broken. When it is closed, the circuit is made. Tinfoil be used as well as the copper wire. This improves the contacts.

The peg can be held in the open position by placing a lollypop stick in the open end.

One end of the lollypop stick is tied to the door using a thumb tack at the other end of the string.

The peg itself is tied to the door frame.

When the door is opened, the stick is jerked from the peg. The peg closes, the circuit is made and the motor makes plenty of noise in the tin.

The peg remains closed even if the person who enters closes the door.