Switches with LEDs

The XM Series 1 was born in the era of automotive bulbs. When it came to the Series 2, LEDs were already entering the scene. New switches like those on the dashboard started to come with LED illumination but earlier switches retained (and shared with other Citroëns of the time) remained with plain old bulbs in many switches of the door windows, electric seats or some center console switches. Retrofitting LEDs to these is a simple process with huge benefits: longevity (practically, eternity) of the LED, cool operation temperatures and lower power consumption are all welcome changes.

Window and electric seat switches

The bulb holders inside the switches are not huge, they are only large enough to house a small socketless bulb:

Using traditional (through hole) electronic components, although possible, is a very tight fit. The LED itself can take the place of the bulb but the compulsory series resistor has to be housed outside the holder, with care taken not to obstruct the operation of the switch. And even then, the positioning of the LED is not optimal: bulbs are not directional but LEDs are and placing them horizontally, as the holder recess allows it, will not direct its light at the switch surface. Note that you need green light LEDs, not white. White will illuminate, too, but the colors will be washed out, green is much better.

Smaller, SMD components are better here, but at the expense of more skill required to handle them. It really helps if you have a helping hand stand with a magnifying glass.

The SMD components should be of size 1206 imperial (3216 metric, this actually denotes the physical size, 3.2 by 1.6 mm). Window switches have a separating plate in the middle of the cap, shadowing the illumination of the opposite arrow, so it's best to place an even number of LEDs into the holder. The 1206 components fit easily into the holder, with some room to spare but a resistor of this size can only handle 0.25 Watts and we would need slightly more. The first thought would be to use a 1210 format resistor but there is a better solution: two resistors in parallel. The main advantage is that resistors come with resistance in specific, standarized steps and using two makes it easier to fine tune the necessary resistance still using common, run-of-the-mill values.

The original version I suggested earlier had the components soldered together, with wires attached to the ends:

Not bad but we can do better than that. With all the experience gained in LEDs Everywhere, I came up with a better idea. Instead of attaching these components end to end with tedious work, a small PCB can hold them much better, is easier to solder and with careful packing, has room for twice as many LEDs:

But first, you have to know the electronic details of the LED you use in order to calculate the resistor values. Two pieces of information are important from the supplier catalog or the component datasheet: forward voltage (Uf) and forward current (If). The resistor value is easy to calculate:

R = (Us – 2Uf) / If

where Us is the board voltage, 14 V to be safe. If we take an example of U= 3.1 V and I= 20 mA (the Foryard FYLS-1206PGC LED I used) the resistor will be (14 V – 6.2 V) / 20 mA = 390 Ω.

This is where the parallel resistors come in handy. Although you could buy 390 Ω SMD resistors for sure, such uncommon values might not be that readily available. These SMD components leave the factory on reels of 4,000 components. Just as an example, in the store I buy my components from, bog standard values can be bought in any quantity, they always have reels available and cut off as many components as you wish. With uncommon values, you might be forced to buy in larger quantities or maybe per reel because they probably won't be able to sell the rest that easily.

Just a reminder, you calculate the common resistance as:

R = (R1· R2) / (R1 + R2)

Two very common usual standard resistor values, 1 kΩ and 620 Ω give us just what we need (differences of a few Ohms aren't important): (1000 · 620) / (1000 + 620) = 382.7 Ω.


A schematic diagram isn't necessary, all we need is two serial LEDs fed from two parallel resistors. We have two identical circuits in the window switches but only one in the seat switches. The board layout:

So, if you use the same (or equivalent) LEDs as I did, the components needed will be as follows. With different chips, modify the resistors accordingly:

Position number Window switch Seat switch
R1 1K0 1K0
R2 620R 620R
R3 1K0
R4 620R


Soldering SMD components is a challenge, especially if you only have a regular soldering iron and eyesight. Using a helping hand stand with magnifying glass helps a lot. The usual way would be to use hot air rather than a conventional iron and a soldering paste that is tacky enough to hold the small components in place until you solder them. But you can still do it if you only have the regular equipment. Have a pointy iron tip, and buy two extra items: a flux pen and a desoldering wick. Flux pens are made under many brands, they look like a thick marker pen, have an alcohol or water soluble flux fluid coming through the tip. Don't think you can skimp on this; with through hole components, it was OK to use the flux (usually resin) in the solder core. But you won't solder SMD parts without a flux, believe me. This is what makes it work.

The desoldering wick is a simple braided copper wire that, due to its capillary action, sucks up unused solder like a sponge. Because you will add more solder than needed in many spots, the wick makes it possible to remove the unnecessary extra. Find a thin one (1 mm or less is diameter) because when you use it, you need to heat the wick and a thicker version will require too much heat.

The best way to solder SMD components that I could find on the Net and can verify by my own experience is as follows:

1. Tin one pad of the component you want to solder. If the PCB has been tinned in manufacture, there is no need for flux here. Just make sure there is a small bump of tin on that pad (practice will teach you how much but really not much).

2. Apply flux onto that tin now.

3. Place the component on its pads. It won't lie completely flat because of the tin on one pad. Clean the tip of you iron: there's no need for extra solder at all. Carefully pushing the component down in place with a forceps or needle-nose pliers (as you prefer) heat the end lying on the tinned pad. As soon as the pad gets hot enough, the solder will melt, the flux will do its job, the component will sink into its flat position and it will be soldered in place all right.

4. Now turn over, apply flux to the other pad and component end.

5. With a very small amount of solder on the tip of your iron (practice will teach you how much, really small), heat the other end. The flux will do its job once more, this end will be soldered in place, too.

With enough practice, that's all there is to it. If the blob of solder is too large, remove it with the wick, apply flux and heat again with a smaller amount of solder (if any) on your tip than before. Use a strong magnifying glass (25x to 30x) to look around closely.

The PCB itself can be held still with Scotch or applying double-sided adhesive to your work area and pushing it into the adhesive (you can also use a stretch of Scotch tape upside down, fixed in the end with normally positioned tape). A piece of glass can serve as a perfect work area (I always use an old BX mirror plate). Solder doesn't stick to it and you can apply and removes adhesive tapes with ease.

Always think about the order of components before you start. In our case, the R2 resistor needs to be soldered first because once its neighbors are in place, it would be much harder to reach its terminals with the iron. The rest isn't important with this particular PCB. Follow the markings on the board with polarized components like LEDs.

You'll also need wire connections (use a multi-stranded wire, remove the insulation and use the individual strands):

1. Tin the pad you want to connect to. Tin the end of the wire.

2. Apply flux.

3. Solder the wire end to the pad in one go.


As soon as you have all components and the two connecting wires (length: 38 mm, diameter: 0.3 mm or similar) soldered, you can start assembling. Hold the board with the black resistors facing upwards, the wires to the left:

Have your green bulb holder with the opening facing up, flat part to the right and push the two wires through the holes in the left side wall of the holder. Yes, I know, it's not that easy; still, it can be done. If you're desperate, enlarge the slots leading to the holes (I don't recommend forcing the wires down the original slots, the wires are thin and the soldering might also let go) with an Exacto knife or similar:

At any rate, you need the board inside the holder, LEDs facing down (into the closed part of the holder), wires sticking out from the left wall holes. Route the wires to the sides and wrap them around the pegs twice. Cut the excess wire. Although the key in the green holder and the notch in the PCB, as well as its wire will probably keep the board in place, you might want to add just a blob of hotmelt glue in the end to make sure the board never wants to peek outside any more.

With the switch (be it either window or electric seat switch) in your hand, hold it in the way shown on the picture, the separated wall on the bottom connector upwards:

While you do so, note which side of the switch is to your left:

On the seat switch it will be obvious because this is where the symbol is but the window switch is two-sided. In either case, make sure you put the green holder with its non-flat, illuminated side rotated to the left side of the switch as you determined:

If you fail to do so, there will be no light: the LEDs are sensitive to polarity by their very nature.

One-touch driver's window

This switch requires a different angle of attack. Remove the caps (the inner part is just like the regular window switches, the outer parts can be prised off with a small screwdriver; keep it parallel to the switch surface and try to prise the side closer to the center of the switch). The center operating flaps just slide out and remove the rubber seal carefully. The bulbs are wedged into the slots of the connector plate. Cut out the bulb, leaving as much of the wires as possible. If you happen to remove the wires, there is no way to push new ones into their places (in this case, the only alternative is to solder new wires onto the connector plates. Not impossible but requires a relative hot iron (around 450 degrees Celsius) and, consequently, very skilled, very quick and very careful soldering so that nothing plastic will be damaged). Leave the wires, it's better that way.

Form a resistor-LED combo with one LED, just as with the seat switch. The polarity should be same as on the picture above (the resistor is on the cathode side).

Now hold the switch so that the plastic frame around the pins has the missing side at the bottom:

If you turn the switch around, still the missing side at the bottom, the LEDs will be connected to the bottom connectors, the resistors to the upper ones. Make sure the LEDs face outwards.

Careful with the iron, try not to melt the plastic of the switch.

End results

These pictures were taken an hour before sunset, the switches jut shaded by hand. They start to become visible to the naked eye around this time of day, the window switches being close to the window are not yet really glowing but the seat switches are nice green already:

In pitch darkness:

As you can see from the switch photo above, the green holder isn't symmetrical and the lighting will be offset to one side, hence the difference in the two arrows. A solution could be to modify the holder a bit: namely, to file or cut half of the green material covering the side farther from its arrow to illuminate. This might result in a slightly different tint for the two arrows but our eyes are more sensitive to the brightness here than the colour difference. But if you don't mind the extra light, you can remove the green material completely, leaving all LEDs exposed. I tried both and I like my switches not only to faintly glow in the dark but to give a nice green light, so I decided on complete removal but you have to experiment for yourself. The holder material is much tougher than one would think but with a good Stanley knife or a coarse file and with a little perseverence it can be done.