This device is designed for smooth damping of light in the cabin auto when closing the door. A very useful thing, especially in the dark, when you need a little light when leaving the car.
The circuit diagram of the device is quite simple. In the event that (circuit SA1) the switch is open, then the light does not light, due to the closed transistors KT817 and KT361 (VT2 and VT1). Based on this, when the door is opened, these transistors will open and therefore the interior lighting will turn on.
The transistor (KT817) is recommended to be placed on the heat sink, since heating occurs when the brightness of the light decreases. Radiator for heat sink is better to use with an area of 4 to 6 cm2. The capacitor (C1) is used for the effect of smooth damping of light. Therefore, when the door is closed, the switch closes and the light continues to glow due to the remaining discharge of the capacitor.
This circuit is powered from a 12-volt network of the car. You can also connect in parallel other devices that will not interfere with each other. If you put this scheme on each door of the car, then when you open and close each of them, the light will also turn on and off smoothly.
A resistor (R3) is used for a better lamp off speed. In order to change the adjustment limit, the value of the resistor (R6) must be changed from 270 to 430 Ohm. Devices are placed under the interior light.