slow on/off fader

In order to provide the optimum conditions for tropical birds and fish in northern climes, artificial lighting is frequently used to extend the hours of `daylight` during the winter months. However, to avoid confusing the animals it is important to simulate sunrise and sunset by slowly fading the lighting up and down rather than switching it on and off abruptly. The slow, on/off fader circuit described in this article will do just this, and can also be used for many other applications. For example, when leaving a room at night it is useful to be able to leave the lighting to fade down slowly to avoid having to make an exit in the dark.