Mains voltage detector

It is normally easy to tell when the mains voltage fails because then the radio stops playing or you are in the dark. Joking apart, computers and other electronic equipment may require a special signal which provides a simple mains present/not present indication. Simple as it sounds, there is more to such a detector than you would expect. The main reason is electrical safety, and that is exactly the key feature of this little circuit, which offers total electrical insulation from the mains.