So dark, much seeking!

We have a new addition to the family: bee-mobile - a Smart fortwo in black and yellow livery. And bee-mobile is lovely, but suffers from an inordinately dark cargo space. There are no lights to guide the fumbling hand, and for such a friendly car this seemed to me like an oversight.

First remove the plastic liner of the cargo area - two screws hold it in, and a couple of snap-fit clasps have to be freed before it can slide forward. Admire the liner - and pause in wonderment at the shape of the mold that was needed to form this piece.

I bought five CREE 'star' LEDs from eBay, and a constant-current DC-DC switching power supply for a total cost of 15 dollars. I didn't spend a lot of time trying to find a certain grade of LED, I knew that even mediocre luminous efficiency would be adequate.

eBay is awash with cheap converters, the PCB colours may change, but what you want is a module using the LM2596. The controls are as shown below, and the only one of note is the 'Out current adjustment' - tweak that till good.

I found a biro pen with an internal diameter comparable to that of the collar of the LED and made four short tubes, about 6mm tall, so that I could recess the LED somewhat.

I drilled four holes in the liner, epoxied the collars in place, and then painted the visible parts of the collars to match the liner (it's not a neutral gray, there's a hint of red in it).

The finished article is shown below (click to enlarge):

The LEDs turn on at 2.25V, draw 100mA at 2.75V, 200mA at 2.9V and top-out at about 300mA. Sure, you can drive them harder, but diminishing returns kick in, and all you do is make a heater that incidentally produces light.

I thought that it would be nice to have the lights come on automatically when the rear window is opened. This is acheived by powering the LEDs from the 'automatic delay' wire in the main dome light housing. But equally, I wanted the cargo light to extinguish immediately once the rear window is closed - it's meaningless to have a courtesy delay for a suitcase.

If you're stealing power from the dome light, you'll need to remove the liners over the door and the windscreen. Pop out the dome light and use some phosphoric acid to attack the zinc-plated steel, and then go ahead and solder with plain Pb/Sn 60:40.
Click to enlarge.

A reed switch and magnet combination was going to solve that nicely. But I noticed that the PSU had a fairly high inrush current - I got a decent spark when I made a flying lead contact to it. This was enough to weld one reed switch closed (oops). So I found a beefier reed switch (NC and NO pins no less!) in my boxes of bits, and then for good measure put in a choke to slow the inrush. And the sparks went away.

It is then just a matter of laying things out nicely. I had some 'U' channel aluminium extrusion (actually, my old name plate from one of my post-doc positions - thanks Martin!) that I used as a heatsink/bracket for the LEDs, and then made offerings to the god of Hot Glue.
Click for big.

Here's the rig in action:

It is then an exercise for the student to find a magnet - I had some scavanged from old hard drives - and find a cunning spot inside the rear-window windscreen wiper housing to mount said magnet.

In the end, I used a short stack of 1/8" by 1" rare-earth magnets from Lee Valley Tools as the actuating magnet. I stuck them to the gearbox housing of the windscreen wiper, with a dollop of impact adhesvie. Not that they would shake free, but y'know, belt and braces.