LED Off Delay with dimming effect (Help request)

[HTF]

Hello guys,
My idea is to make 1m led strip (the square ultra bright led's) to fade out in a matter of like 3-5 sec after the car door has been opened and closed.

To begin with i though of connecting transistor to the switch that runs the light on when door is open, so it will turn my circuit on without getting the original circuit with its load. The power supply is 13.3v (in car) the led's are separated into 2 stripes each (50cm) and connected between eachother +to+ -to-!

Im asking you guys to help me get the list of the components i will need to use and the circuit sketch!

Information about the strip:
1) Connecting them to positive and negative turns them on, they are connected in a circuit with resistors so they work from 12v.

2) its one strip of 1 metter that you can split each 3 leds, 3 leds are connected into the same circuit and all go + to + - to -, they still gonna work from 12v even if you connect just 3 leds they work from 12 volts.

3) Found a source where leds look just like mine. Comsumption Watts Per Piece: 20ma.
Source: Flexible strip, 60 Warm White SMD LEDs, 1 meter, 60lm, 12V, 120deg products, buy Flexible strip, 60 Warm White SMD LEDs, 1 meter, 60lm, 12V, 120deg products from alibaba.com

4) Nominal voltage supply is 12 volts.

5) No need to connect a protective resistor, there are resistors on the strip.

6) Heres is the picture of 3 leds on the strip: imgur: the simple image sharer


I really appreciate your help!

 

[Diver300]

The light is nearly proportional to the current. The three white LEDs will need about 9 V before they light at all. The best circuit would steadily reduce the current limit from 20 mA (per set of 3) down to zero. The voltage across the LEDs won't change much between 0.1 mA and 20 mA, but the voltage across the 150 ohm resistor that is in series with each group will vary from 0 V at 0 mA to 3 V at 20 mA.

 

[mbarazeen]

rather than changing the current to control the brightness, you better implement PWM with a charging capacitor time constant to change the pulse width from 100% to 0%. you have to modify an existing design to connect RC circuit.

 

[colin55]

Here is a simple circuit to extend the courtesy light in a car and allow it to fade:

Simply replace the interior light with the LED strip (and necessary resistor(s).