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.
If the LED lights up, the LDR resistance decreases causing the SCR to conduct periodically earlier. This tehnique causes the lighting system to turn on slowly.
On the other hand if switch S1 is turned off, meaning the switch is opened, the LED does not immediately turn off since the capacitor voltage at the base of T1 discharges slowly. The LED slowly dims until finally turns off. This causes the lighting dim out before it finally turns off. Potentiometer P2 must be set so that the anode voltage of D1 is about 0.7 volts. If this is done, the capacitor voltage will be around 0.5 volts during standby, meaning lights off.
Never use replacement diode types for diode D1. Be sure to use an originally marked 1N4148 signal switching diode.
Caution! Danger of electrocution!
You are working with a line voltage of 220 Volts AC. Extreme shock hazard.