Courtesy light extender

  
Most car door switches are simply single-pole switches, with one side grounded. When the door is opened the switch grounds the other line thus completing the light circuit. In a car where the negative terminal of the battery is connected to the chassis, the negative wire of the unit (emitter of Q2) is connected to chassis the positive wire (case of 2N3055) is connected to the wire going to the switch. In a car having a positive ground system this connection sequence is reversed. When the switch closes (door open), CI is discharged via D1 to zero volts, and when the switch opens, Cl charges up via R1 and R2. Transistors Ql and Q2 are connected as an emitter follower (Q2 just buffers Ql) therefore the voltage across Q2 increases slowly as Cl charges
Courtesy light extender - schematic

Hence Q2 acts like a low resistance in parallel with the switch and keeps the lights on. The value of Cl is chosen such that a useful light level is obtained for about four seconds; therefore the light decreases until in about 10 seconds it is out completely. With different transistor gains and with variation in current drain due to a particular type of car, the timing may vary hut may be simply adjusted by selecting Cl.




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