I'm fond of electronics that are powered (or recharged) by USB. So I've sort of been on a quest for that 'perfect' USB power source.
I've got one of those 4 battery jobs... (very much like this: http://lygte-info.dk/review/Review%20USB%20battery%20box%204x18650%20UK.html )... lots of juice but too bulky/heavy for normal use. Plus (first-world-problem-warning), it has screws holding the case shut, so is bothersome to change batteries.
I've also got a single cell unit (like this: http://www.fasttech.com/products/1163402 ), but the battery is soldered in and I'm not really sure I trust it for safety/reliability. It's only one step up from aluminum beer can in sturdiness.
I've got a ML-102, but I'd like something that stays together a little more convincingly and won't short out when it mixes with my keys.
This (http://www.haipaitech.com/haipaite-solar-ipad-charger-008-portable-charger/solar-mobile-charger/haipaite.aspx
) looks really promising, as it's steel, not flimsy aluminum, and it takes a separate battery. Sadly, I haven't found it anywhere but alibaba or tae-bo or whatever those dodgy sites are called.
So, what's a man to do? DIY, of course.
I present to you the ubiquitous craptastic 501b. (Oh, and Tuna in the background.)
And for those that can't be bothered with 3 chapters of blah-blah-blah, the USB mod.
Parts
- a donor 501b plus the switch and switch retaining ring from another 501b
- usb board, "3V to 5V 1A Adjustable Step-up Boost Power Supply Module USB DC-DC Converter" from ebay seller hotsale-eshop. A hair over 3 bucks. http://www.ebay.com/itm/150963216664 . This one charges my iphone 4; it should work with other small apple devices, but ymmv. They took forever to ship (got caught up in CNY even though I ordered way before then), so I wouldn't exactly recommend the seller, but they came through in the end.
- a chunk of plastic that might be delrin. (I can't remember.)
Building
Make something that looks like a P60 dropin out of plastic. Mine was 22mm thick, and that seemed to work fairly well, though it meant that I had to work from the bezel end, dimension-wise. I don't think thickness is critical, as the (+ve) board pushes the usb board toward the front of the host.
For the lathe-less masses, a 32mm holesaw leaves a plug of about 28mm, which is good for the front of the dropin. A 25mm holesaw gives a plug of about 22mm for the back. I drilled the big plug first, then using the existing pilot hole, did the smaller back. When I got close to the transition, it was just a matter of slicing the angle with a utility knife.
Once you've got a reasonably credible dropin, cut a notch for the whole USB assembly and then cut through the front for where the USB port sits. This part is, I'm afraid to say, the most fiddly. My life was made a bit easier by having a 1/8" chisel. (These are a bit hard to find, but it's pretty easy to grind down a 1/4" chisel.) It's pretty obvious when you get into it, but one of the critical fit elements is that the front of the circuit board butts up against the bottom of the notch in the dropin. Too thick and the USB port won't come out far enough; too thin and the port will stick out proud of the face of the dropin. 3mm was the magic thickness for my board.
To make the (+ve) battery contact, I plundered the switch from another 501b. Desolder the clicky part and you're left with a pcb and a spring. Solder a couple of wires onto the pcb and you're good to go. A bit of kapton tape on the wire side covers up all that pesky real estate that wants to short circuit when you assemble everything. The retaining ring zips everything down tight. Don't overdo the tightening, as it seems to push the kapton tape and maybe overstress the USB circuit board. (Mouse over for the back-side.)
Ta-da!
If I was to do it again, I probably wouldn't. I enjoy working with hand tools and fiddling around, but this was time-consuming. I'd also like to have a status led on the front to remind me when the switch is live. I haven't had it long enough to know what the parasitic drain is going to be like. When I was dreaming this up, I was hoping I could have the switch casing at both ends, but there simply wasn't enough room. I guess I could go down to an 18350 or something...
Questions and comments welcome.
