Electronically deter stray moggies from invading your cat’s private door

Cats are not very brave. The author’s work-desk is situated next to a window where the household cat jumps in and out. Occasionally, working late at night, a foreign cat will poke its nose through the window. On noticing the author (before the author has noticed the cat) the cat will sometimes retreat in such panic that it strikes the window with a bang. It is this characteristic of the cat – its extreme nervousness on entering unfamiliar terrain – that is exploited in this design.

The project described here is a Cat Flap with a difference, both conceptually and electronically. The flap itself swings freely, and is not released with a catch, as is usually the case. This simplifies the design a great deal.

An “authorised” cat may walk freely in and out of the flap. However, if a foreign cat (or perhaps a rat or a mongoose – depending on where you live!) opens the flap, even to a crack, it is surprised by whatever might meet the constructor’s fancy – perhaps an ultrasonic shriek, or a spotlight – but nothing that might harm the cat. Bearing in mind a cat’s timidity, this is bound to keep a foreign cat at bay. Having said this, the circuit has been so designed that it may also be used as a “stock standard” cat flap, whereby a flap releases when the "authorised" cat comes near, yet remains locked to foreign cats.