I've been searching for a new project to do for this season's holiday lights. I typically have four or five ideas floating around my head, and this year is no different.
Lots of choices, so I've had to come up with a "rule" about new projects. The rule is that the project has to be in the neighborhood of effort-neutral. It already takes too long to put up (and worse, take down) the displays we already have, and I don't want to add anything that makes that worse. Oh, and they can't take too much power, because I'm already on a power budget.
Unless, it's, like, especially cool.
I had an idea that met all my criteria. It was small - small enough to be battery powered, if I did my power calculations properly, and was going to be pretty cool.
It was, unfortunately, going to be a fair pain-in-the-butt to build - the fabrication was a bit complex, and the plan was to build a number of identical pieces. Oh, and it required me to choose the perfect LEDs from the 15 thousand that Mouser carries.
So, I hadn't made much progress.
Then, one day I was waiting for some paint to be tinted at my local home store, and I came across these.
They're holiday lights. Jumbo-sized holiday lights. The bulb part is made of colored plastic, and measures about 7" high. At the bottom there is a large fake lamp socket. Inside of all of it is a genuine C7 bulb of the appropriate color.
I bought 3 sets, 15 in all.
To be different, I wanted to build these as self-contained devices, with a separate microcontroller in each of the light bases. The microcontrollers I'm using cost about $1 each, so there isn't too much cost there, but the big challenge is a power supply. Generally, I build a linear power supply, which is simple and performs well, but you need an expensive and bulky transformer.
There is a way around that, with the reasonably named "transformerless power supply". Realistically, a better name would be the "high-voltage shock-o-matic", because it involves hooking things directly to the AC line, can only supply a small amount of current, is inefficient, and is hard to troubleshoot. Oh, and if one component fails you get 150 volts instead of the 5 volts you were expecting.
I decided to build one of these, so I ordered up some parts, wired it up, plugged it in, and immediately lost the magic smoke from one of the resistors. Turns out I miscalculated, and I needed a much-more-expensive power resister.
Thinking about it some more, I decided that since I still needed power to each bulb - and therefore a wire to each bulb - it was simpler to just build a simple system with one microcontroller.