Custom blind spot monitor LEDs
The GTI, like many cars, has a blind spot monitor feature that helps the driver know if someone is right next to them - for example, when changing lanes on the highway.
There are a couple common designs for the LED light in the mirror. Either the LED is in the mirror glass with a small picture of a car, or it’s part of the mirror surrounding the glass and faces towards the inside of the car.
Example:
The GTI has the design on the left. But the one on the right looks a lot sleeker and is also easier to see, in my opinion.
Could I use some custom LEDs to make my own upgrade? Sounds like a project!
Reversing the circuit
If I wanted to use my own LEDs that plug into the car wiring, I had to make sure that my new circuit matches the factory lights as well as possible. The blind spot control system is always checking to see if the LEDs are plugged in and working properly. If there is no current or too much current flowing, the blind spot system disables itself and shows an error to the driver.
After removing the glass from the mirror, I separated the glass from the plastic backing to get to the factory blind spot LEDs, which were sandwiched in between.
LED circuit board removed from mirror
A capacitor, 3 resistors, a diode, and 3 LEDs. The diode I could guess was to protect against reverse polarity and the capacitor for decoupling. However, it wasn’t clear to me what the 3 resistors were doing in the circuit other than to limit the current to the LEDs. And if that was the case, why were there 3?
In order to see what was really going on, I decided to de-solder all the components off the board to get a better view of the traces on the PCB.
Using the above two pictures of the board, I started piecing together how the components were connected. It really helped that the board was translucent enough such that a flashlight could show all the traces.
This schematic is what I came up with after tracing the wires around on the board:
Blind spot light schematic
The numbers in blue, green, and red are voltages and currents at different points of the circuit depending on 3 different input voltages: 13.7, 9.3, and 6.3 volts.
Most of the circuit is pretty straightforward. There is the DC input voltage on the left, the capacitor C1 which I’m guessing is for smoothing out the input signal, then the diode D1 which prevents current from flowing in reverse if positive and negative are plugged in backwards.
On the right, there are the three LEDs, D2, D3, and D4, and at the bottom there are two resistors R2 and R3 for limiting the current through the LEDs. The only mystery that remained (and still remains) is the purpose of the 714 Ω resistor R1. My thought is that it allows some current to flow for any input voltage, no matter how small, and maybe that’s what the blind spot monitor system expects. Not totally sure.
Then, trying to remember what I learned back in circuits class, I calculated the voltage and current through the two paths of the circuit, given an input voltage. If the input voltage is 13.7V — maximum brightness — the current flowing through the resistor R1 is about 18 mA and the current through the three LEDs and other two resistors is about 50 mA. This is definitely a healthy current, which makes sense - the LEDs are pretty bright!
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You may be thinking—why go to all this trouble when you could just drill a few holes in the mirror plastic, glue or tape the circuit board that I took out of the mirror glass to the inside, and call it a day?
It’s certainly an option. But what’s the fun in that :)
Making the new circuit
It took some time for me to find the right LED to order since, as mentioned earlier, they had to be pretty bright. I first tried some that had a maximum current of 20 mA and these ended up being way too dim.
What ended up being the best for this application were surface mount 5730 LEDs, 1/2 watt, 150 mA maximum. They are pretty much on par with the brightness of the factory original LEDs.
All that was left to do was re-construct the factory circuit with the three new LEDs. I used 1/2 watt resistors when making the new circuit to accommodate the higher wattage. The final circuit ended up pulling around 70 mA through the LEDs, which ended up being close enough and did not cause any issues when plugged into the car.
LEDs and circuit board soldered together
In this first iteration of the lights, I spread epoxy over both sides of the circuit board and also surrounded each LED with epoxy. I soldered wires directly to the pads on the LEDs, which ended up not working out in the long term (more on this below).
Modding the mirror
The last step of the process was actually getting the lights into the plastic cap that goes on the mirror. Going back to the first section, I wanted the lights to shine out of the side of the mirror, facing the inside of the car. I drilled 3 holes with equal spacing between them and made sure they followed the curve of the mirror cap.
Mirror cap prototype with holes drilled and LEDs
To seal the holes, I stuck electrical tape on one side and filled the other side with clear epoxy. Once the epoxy dried, I pulled off the tape and installed some dark tint film over the 3 holes. The tint makes the holes practically invisible when the LEDs are off and doesn’t really affect the visibility when they’re on.
Finally, I attached the LEDs and their circuit board to the inside of the mirror cap with double-sided tape. With that, the project was done! (At least, for a while)
Final product installed on car
Revisiting the project
Remember when I mentioned above that I soldered wires directly to the LEDs? This surprisingly held up for nearly 3 years but started breaking down recently (summer 2022). The lights on the driver side first started flashing, then flickering, then finally died completely. Sure enough, when I took the mirror apart and squeezed one of the LEDs, the circuit started working again.
I decided to use the PCB (printed circuit board) skills I learned during my other big project, the RGB ambient lights retrofit, to make a small board for the LEDs. I also ordered some red LEDs instead of the amber ones I used originally just to try them out.
The PCB made it so much simpler and quicker to solder the LEDs. Now, the lights should last for hopefully a very long time - maybe even the life of the car!