There is a saying “dogs have masters, but cats have staff”. However, with the automated pet door created by Instructables user tareker, you can turn the tables on your cats (or mischievous dogs) and control their coming and going very easily. In this example the curfew is enforced by a pet door which has been modified with a variety of different technology.
The brains behind the door is a typical Arduino board which uses an external real-time clock IC for accuracy. You can then program the times that the door is unlocked and locked, allowing or denying the pets access through the door. The locking mechanism is based around a simple servo, and should sustain the most insistent pushing from a small pet. Furthermore the door movements are recorded when unlocked, and therefore the software can determine whether or not the pet is inside or outside. This is also displayed using an LED. For a quick demonstration check out the following video:
This type of project also offers many forms of customisation, for example leaving the door unlocked until entry of a pet – at which point it locks until the next morning. Or perhaps connect the Arduino to the Internet using an Ethernet shield to have it send a tweet every time a pet moves across the border. However to get started, visit the project page here: http://www.instructables.com/id/Pet-Curfew-An-Arduino-Controlled-Pet-Door/?ALLSTEPS