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Switching 12V circuit using power from USB port Answered

Hello friends,

My car's cigarette lighter/charging socket (12V) always remains ON, even after locking the car. But the pen-drive attached to its music system gets activated only when car is in accessory mode or its ignition is turned ON. What basically I want is to turn charging socket (12V) ON as soon as music system gets activated and should be turned OFF as soon as music system gets deactivated(Upon exiting from car).With a little knowledge of electronics I know that USB's are powered with 5V DC. Can someone help me to use this USB power to trigger 12V lighter circuit? I feel this can be done using some Transistor (probably NPN). Which transistor should be used and what will be its circuit diagram?