This automatic light dimmer circuit makes it possible to control a lighting system so that it turns on or off slowly. The circuit works this way: when switch S1 is closed, the capacitor C1 is slowly charged. Once the voltage at C1 reaches 0.6, transistor T1 begins to conduct and the LED also begins to light. If the capacitor voltage increases further, then transistor T1 conducts more current and in return the LED lights brighter.
