My new Mercedes-Benz B-class electric drive does not have an illuminated charge port, and I was getting tired of fumbling around in the dark trying to plug in the charging cable. For a few bucks and a couple of hours of work I put together this solution: a row of LEDs light up when the charge port door opens. They stay lit for about 20 seconds and then fade out to conserve battery life. You will need:

LED strip with battery pack
SPDT micro switch with roller
4 general purpose transistors (2 NPN and 2 PNP)
About a 10 μF capacitor
about a 500 kΩ resistor
2 10 kΩ resistors
2 1 MΩ resistors
small breadboard, wires, solder, etc.
Mounting tape

Step 1: Test the LEDs

Acquire an LED strip with battery compartment. I chose to buy this preassembled, but you could buy them separately. Insert the batteries and test the LED strip. Check that there is space within the charge port access area to fit the battery box, and check the best position of the LED strip to illuminate the charge port.

Step 2: Build the Timer

I used a timer circuit that I found here posted by Olin Lathrup (Thanks Olin it worked like a charm). Since my LED strip has built in current limiting resistors, I omitted the 270 Ω resistor shown in the schematic, but if you are using bare LEDs you will want a resistor here to limit the current. Once power to this circuit is turned on, it will provide power to the LEDs for a specific amount of time and then turn them off. The length of time (in seconds) is in the ballpark of the RC time constant from from the product of the resistor value (in Ohms) and the capacitor (in Farads). I used a capacitor of 10 μF and a resistance of 560 kΩ to get a delay of about 20 seconds which seems appropriate.

Assemble the timer circuit on a breadboard. I used a mini solderable breadboard from sparkfun after having first prototyped and tested it on a solderless mini breadboard.

Step 3: Add Timer and Switch to the LED Circuit

Now wire the battery pack to the SPDT micro switch with roller. Since my battery box was already connected to the LED strip, I had to cut the leads to insert the switch and timer. The positive lead from the battery box (red wire) should go to the normally closed contact on the switch, and the negative lead (black wire) should go to the normally open contact. The center contact on the switch supplies the voltage to the timer circuit, so it should connect to the node labeled 5V on the schematic in the previous step. Run a wire connecting the negative terminal of the battery pack to the ground on the schematic in the previous step. In the picture shown here you can see this is an additional black wire that comes off the normally open contact on the switch.

When this is assembled you should be able to test the circuit. When you press (and then release) the switch the LEDs should turn on, but then fade out after a short delay.

Step 4: Mount in Your Charge Port

Wrap the circuit in electrical tape to keep it from shorting out, and then mount it and the battery pack using foam mounting tape inside your charge port access area. Mount the switch using more foam tape at such a position that when the charge port door is closed the switch will be pushed in (turning off the LEDs). Mount the LED strip in the charge port access area. My LED strip had an adhesive backing that I used to tape it to the top of the charge port access area. I had to trim it with scissors to make it fit (if you use the same one I used you can cut between the copper contacts between any pair of LEDs). Now when you open your charge port there will be light!