I own a 1988 BMW M5, and this car, like many BMWs of its era, has a key with a button, and when you push that button, it lights a small lamp embedded in the key. Except in the case of my key, the lamp was completely missing. So I set out to fix things. Sure, I could have just bought a replacement lamp, but that would have been too easy. The lamp is lit by a 1.5V battery, and many complain that it's not terribly bright. So here's a guide on to how to upgrade it to use a high-efficiency LED.

This instructable was inspired by:
https://www.instructables.com/id/BMW-LED-Key-Mod/

I wanted to make an Instructable that required less surgery, and provided more detail. So here you go...

Supplies

Required Supplies:

(2) - CR1616 button cell batteries

(1) - 5mm white LED

(1) - small piece of scotch tape

Optional Supplies:

Vinegar, salt, baking soda, water, 2 cups (for copper cleansing)

Tools:

Small flat blade screwdriver (to pry open key assembly)

Needle-nose pliers (to bend LED leads into proper shape)

Optional Tools:

Metal file (to file down LED plastic casing)

Step 1: Separate the Key From the Lamp Assembly

If you push down on the button and slide it to the left, the lamp assembly will separate from the key.

Step 2: Pry Open the Lamp Assembly

There are 2 small notches on the plastic casing of the lamp assembly. Use a small screwdriver to push the tabs and pry open the lamp assembly. It will separate into two halves, one with the battery and push button, and one with the lamp circuitry.

The battery is type V625U, and puts out 1.5V. That's bad news, because LEDs need more than 1.5V to turn on. But we'll deal with that later.

Step 3: Take Apart the Lamp Assembly

The lamp assembly innards contain 3 metal pieces: a spring, a piece of copper that I'll call the curved copper piece, and a piece of copper that I'll call the flat copper piece. The lamp connects the flat copper piece to the curved copper piece. The positive terminal of the battery is the flat part with all the writing on it, as well as most of the outer casing around it. The negative terminal is the small circular island of metal that you see when you flip the battery over. The negative terminal connects to the spring, which connects to the flat piece. When you press the button, the spring compresses, which causes the outer casing (aka the positive terminal) to connect to the curved piece, which then drives voltage through the lamp, to the flat piece and spring to the negative terminal, and voila, light!

The copper pieces on my key looked a little crusty and oxidized, but that can be fixed with an optional step.

Step 4: Clean the Copper (optional)

In the case of my key, the copper pieces looked like they had some oxidation on it, which could potentially impact connection quality. To clean up the copper, do the following:

1. Prepare 1 small cup of vinegar and a teaspoon of salt mixed together

2. Prepare another small cup of water and a teaspoon of baking soda mixed together

3. Put the copper pieces in the vinegar/salt solution for 5 minutes

4. After 5 minutes, remove them and put them in the baking soda/water solution for 30 seconds.

Result: More shiny copper. Yay science!

Step 5: Sourcing an LED

There's a bit of a Goldilocks choice that needs to be made with the LED. The width of the channel that holds the key bulb is about 4mm. LEDs come in 2 standard widths: 3mm and 5mm. If you choose a 3mm LED, it will fit, but will look a little loose in the hole. If you choose a 5mm LED, the fit will be snug, a little too snug. You can address this by using a Dremel tool to widen the hole, or by using a metal file to shave off a little bit of the LED casing. Or you can do neither of these, which is what I did. The lamp assembly casing will be slightly ajar (it's hardly noticeable), but will still work fine. Also, the 5mm LED will be locked into place by the force of the plastic casing.

LEDs are kind of like on/off switches that have a light built into them, the on/off switch being controlled by voltage. The switch won't turn on until you reach a critical voltage, but once the critical voltage is reached the switch is closed and the light shines. Unfortunately, due to the laws of physics, that critical turn-on voltage is more than 1.5V, which is what the existing battery puts out. So we need to retrofit a higher voltage battery.

LEDs will have one metal leg that is longer than the other. The longer leg is the positive terminal. It's important to connect the positive terminal of the LED to the positive terminal of the battery, or it won't work at all.

3mm and 5mm LEDs can be readily found on Amazon and other retailers. Look for one that has an activating voltage of 3V (nearly all of them do).

Step 6: Sourcing a Battery

Unfortunately, the classic CR2032 "button" battery that is pretty commonplace is much too wide for this application. We need a smaller button.

The battery size that works is a CR1616 battery. These batteries put out 3V, so we need to stack two together to create a 6V voltage source. Make sure that both batteries are pointing together in the same direction. Use a small sliver of scotch tape to attach the two batteries together. Use as small a sliver of tape as possible.

(Fun fact: the first two numbers after "CR" denote the battery's width, and the last two numbers indicate the battery's thickness. A CR2032 battery is 20mm wide and 3.2mm thick; a CR1616 battery is 16mm wide and 1.6mm thick.)

Step 7: Trimming/bending the LED Legs to Connect to the Copper

The flat copper piece and curved copper piece have some tabs that are bent in some interesting directions. These bends serve a particular purpose - to act as springs that push against the lamp terminals to make a strong solid electrical connection.

In order to make a solid connection between the LED and these tabs, the LED legs should be trimmed and bent into a U shape with needle-nose pliers to provide a similar spring-like action. The photo above shows how I approached it. For those with the tools, you could also solder these connections together.

Reminder: The longer leg of the LED is the positive terminal, and should be connected to the curved copper piece. The shorter leg of the LED is the negative terminal, and should be connected to the flat copper piece.

Step 8: Putting It All Back Together

With the LED appropriately situated and connected, the lamp assembly can be put back together. Make sure the battery pair is oriented properly; the flat part (the positive terminal) should face the plastic push button, and the curved part (the negative terminal) should connect to the spring.

The lamp assembly may not completely close due to the size of the 5mm LED, but I was able to get it "closed enough". At this point you can push the button to test that the LED lights. If it does, great! You can slide the lamp assembly back into the key and you're done! If not, check the LED connections to the copper pieces.

LEDs are very energy-efficient, so you can expect a long functional life from this project.