Car Interior Light Extender

Below is a Courtesy Light Extender for cars. When a door is closed in a car, it extends the ON time, so the passenger can see where he/she is sitting. Here is the schematic diagram of the circuit:


The light normally goes off immediately when door switch is opened, but the circuit takes over and allows current to flow because the 22u is not charged and the first BC 547 transistor is not turned ON. To illuminate the interior light, this turns on the second BC547 via the 100k and the BD679 is also turned. The 22u gradually charges via the 1M and the first BC547 turns on, robbing the second BC547 of “turn-on” voltage and it starts to turn off the BD679. When the door is opened, the 1N4148 discharges the 22u. [Circuit diagram source:]