This timer circuit may be used to automatically switch off the interior car ligth. When the doors are closed, the light turns off with a delay of about twenty seconds. In order to protect the car battery, the lamp works for a max time of four minutes.
When you open a door, the IC1 timer turns on. Because of the connection through the D1 diode, the capacitor starts charging once the door is closed. When the IC1 timer backs off, the D1 diode prevents pin 2 from taking the low signal from pin 7 (which is connected to the ground by the internal switch). The Q1 transistor is controlled by the door timer, while the Q5 transistor is controlled by the manual switch. Besides the car light, the Q1 and Q5 transistors also enable the positive supply on the IC2 timer and turn off the reset signal, through the Q3 transistor. Used in astable mode, the IC2 timer sends a clock signal to the CD4017 decade counter. When the last output is activated, the Q4 transistor turns off the car light. Since the input pin 13 takes the high level from the last output, the counter ignores the clock signal. The max time delay is about four minutes with a 120u capacitor (the circuit in the video works with a smaller value). When the Q1 and Q5 transistors are both disabled, the reset signal is activated. But the Q1 transistor would back off twenty seconds after you close the door. In order to reset the counter without delay, the last output of the CD4017 enables the Q6 transistor. This one switches on when the doors are both closed and pulls to ground the Q1 base.