So, what did my 640-pixel Technocolour Dreamcoat’s shipment of fail arrive in at Burning Man this year? I’m glad you asked!
There were two main problems, plus a couple of annoyances.
Due to the PITA (and expense) of shipping lithium batteries around, I had four Limefuel L130X batteries delivered to meet me at my friends’ place in Seattle, so I’d never hooked it all together on the eventual power source (I’d been running off a mains-wired 100W PSU during development.) The special thing about the Limefuels is that they promise 2.1A on both of their USB ports—all others I’d seen say 2.1A on one port, 1A on the other. So over four batteries, I ought to have got a total of 16.8A, or 84W @ 5V.
Problem one is that I misread the spec and thought it was 2.4A from both ports simultaneously, where it’s actually 4.2A from both simultaneously, but 2.4A max from one port (if you’re pulling only 1.8A or less from the other.) So using FastLED’s dynamic power management to cap my draw at 80W, I had way way less headroom than I thought.
But even so, I should have had some headroom: maybe the the Limefuels aren’t actually capable of delivering the current advertised on a sustained basis, OR I may have had an intermittent short somewhere? In any case, after running my rainbow plasma for a while (draw of about 30W), the batteries’ protection circuit kicked in and they all shut off. Briefly plugging them back into the charger reset them, and I tried again. This time not so lucky. All four tripped again, but one wouldn’t turn back on. The sniff test revealed I’d let the Magic Smoke leak out.
Not good. Fortunately one of the awesome people on the FastLED Google Group was leaving for the Playa after me and had time to Amazon Prime a new battery that I could pick up from him when I got there. So I lowered the max draw to 50W (giving me a whole battery’s worth of headroom) and crossed my fingers.
It didn’t work out. Even with a max draw that should have been within the current delivery capabilities of three of the four batteries I had connected, their over-current cut-outs kept tripping, and I eventually fried another battery. By now I was pretty sure that running these packs wired in parallel was not a good idea (I had been warned.) My best guess as was that one battery tripped, then the other three had to suddenly take up the load, one of those tripped, then it was two… and all of a sudden one poor unit is left holding the (twenty amp) bag and pop.
I did take the jacket out another couple of days, but made sure to only run it on the less dense, lower power patterns.
This was another issue. The spade lug connectors I had soldered into the bottom of the strips to connect to the power bus were on short, stiff wires without enough strain relief (despite me trying to pot the ends of the strips in heat shrink filled with E6000 (aka “Our Lady of Perpetual Adhesion”—it’s going to be a bastard to undo so I can fix it.) Quite a few of the wires just snapped off, but funnily enough, not enough of them to make it an actual problem! I had enough redundancy that despite both the +5V and ground connectors coming off the bottom of at least one strip, the circuit was maintained by the connection to the adjacent strips at the top.
I am going to fix this by changing how my power bus is mounted: I’ll sew two parallel pockets vertically adjacent to each other around the bottom jacket for the +5V and ground bus braids, and have longer wires coming off the end of the strips to connect to them (instead of short wires off which the power bus braids hung.) This will allow me to pot the connection into the bottom of the strips properly and will be much more reliable, with the added benefit that there won’t be such a big “bulge” at the bottom, leading to less bunching and a flatter profile.
I think I have a dodgy signal wire somewhere, but that should be easy to fix. More annoying was the touchiness of the cheap-arse rotary encoder I was using to switch patterns. It was registering a count before settling into a detent; I thought I’d fixed it in software but it was still touchy as hell. I’ll get a (stiff) rocker switch for up/down next time, and also a separate (analogue) pot for brightness control, and a separate button whose use would be up to the currently selected effect.
Until I realise my dreams of doing stupid shit like running 40A across my torso via explosive lithium chemistry strapped to my back, I’m going to try splitting the power bus in two and running each half off separate Limefuels to see if this fixes the “last guy standing cops it” effect. I need to get the new power bus pockets sewn in, and strip down and rewire the power bus ends of the strips. I’ll also rebuild the control box with independent controls, and see if I can find a better way of attaching the strips themselves to the velcro that holds them in place on the jacket (the adhesive that comes on the strips isn’t worth a damn, and the velcro expands in the heat at a different rate to the strips, so it has the tendency to all come undone.)
Hopefully I can get that lot seen to before London Decom at the end of October? Yipe!