A tidy Citroën door projector mod by maker [OrangeTungsten] has replaced an unreliable Hall effect sensor with a straightforward mechanical button, getting a pair of logo-casting projectors back into reliable service and documenting every step along the way.
The projectors in question cast the Citroën logo onto the pavement when a car door is opened, and by all accounts the visual effect is genuinely pleasing. The trouble was never with the optics. The original activation mechanism relied on a small magnet affixed to the door frame, which a Hall effect sensor inside the projector would detect and use to trigger the light. On paper, a contactless magnetic switch is an elegant idea: no moving parts to wear out, no physical connection to corrode.
In practice, the retrofitted magnet had a frustrating habit of working itself loose and falling off entirely, leaving the projector sitting dark and inoperative. The elegance of the solution counted for nothing if the magnet was not reliably present.
Inside the Citroën Door Projector: A Microcontroller Doing Very Little
Before settling on a fix, [OrangeTungsten] took the time to trace out how the circuit actually worked, and what they found inside is the part of this story that will raise an eyebrow or two. Tucked away in the projector housing is an 8-pin microcontroller, whose sole apparent job is to poll the Hall effect sensor and use its output to switch a transistor, which in turn powers the LED that does the actual projecting.
That is a microcontroller being deployed to turn an LED on and off. There is some chance the original designers included it to enable fading or flashing effects, and a small MCU is certainly cheap enough that the bill-of-materials argument might just about hold. But [OrangeTungsten] reports the projectors do nothing more elaborate than switch on and switch off, which leaves the processor looking rather over-qualified for its role. It is the kind of design decision that makes you wonder what the engineering meetings looked like.
Finding a microcontroller handling what a single transistor and a reed switch could manage is not entirely surprising in modern consumer electronics, where software-defined behaviour is often baked in during design even when the final product never uses it. Whether these units were intended for a broader product family with more features, or whether the MCU simply made the firmware easier to iterate during development, the finished article ended up as a very small computer devoted entirely to watching a magnet.
The Fix: Parts Bin Hardware and a Physical Button
The repair itself is satisfyingly direct. [OrangeTungsten] removed the Hall effect sensor from the circuit and wired in a simple push button in its place, positioned so that the button makes physical contact with the door frame when the door closes. Open the door, the button releases, the projector lights up. Close the door, the button depresses, the projector cuts out. No magnets required, nothing to fall off, nothing to lose.
The entire process has been documented on Hackaday, meticulously enough that anyone with a similar projector and a similar problem could follow along without much difficulty. That thoroughness matters. Half the value of a repair like this is in the doing; the other half is in leaving a clear record so someone else does not have to start from scratch.
There is something deeply satisfying about a fix that strips away unnecessary complexity rather than adding to it. A wayward magnet, a redundant microcontroller, and a handful of components from the parts bin: the Citroën door projector mod is a small job, but it is exactly the kind of documented, thoughtful repair that keeps hardware running rather than heading to landfill. The projectors are working again, the pavement logo is back, and the write-up means the next person with the same fault has a map to follow.
