Switching applications involving controlling devices or appliances using digital-I/O lines through a relay often need to indicate the change of state of the I/O line and, hence, the connected device.

This circuit simulates an alarm to discourage thieves and it is originally designed to be used on bicycles but can be used for cars too. It blinks a LED on and off thus simulating a real alarm circuit ...

This alarm system was originally designed to protect the contents of a garden shed, although it would also be ideal for other outside buildings. In addition it would offer a good basic security system ...