Simon Ludborzs has posted an interesting look at a project from 10 years ago: a pair of LED lamps he created for his father-in-law's trailer. As well as having created a long-lasting and useful end-product - having survived a decade of use, until the plastic lenses became opaque from weathering - Simon's project offers a fascinating glimpse at changes in design and manufacturing techniques available to makers over the years, including the shift away from hot-air surface levelling (HASL) to immersion silver or tin and electroless nickel immersion gold (ENIG) to meet lead-free PCB requirements and the ever-dropping cost of high-brightness LEDs.
About 10 years ago I built a pair of LED trailer lamps for my Father In Law. This was back when LED lights were stupidly expensive - and as luck would have it I had access to some high brightness samples. Anyway, the lights served their time in the harsh conditions of the Riverland and the plastic lenses finally turned more opaque than useful.You can read more about Simon's design and see more images of the LED lamp boards, including the HASL-finish PCB which was designed without solder mask, over on his blog.
These LED lamps were a very simple design - each string of three LEDs gets its own current limiting resistor. I popped open the [replacement] eBay trailer lights - same deal inside of them there. I've read some forums that this is a crap way to go due to voltage variation in cars/trailers but if the LEDs all have around 2V across each, then there's about 6V across the resistors - a volt or two more or less around 12V won't have a big effect on the LED brightness.