That's just about what I'm going to build - though my single board will have a USB connector attached to it. I'm hoping to keep the price at around $20, but on Monday or Tuesday when I finish my first prototype, I'll be more clear on the cost of the parts I need.
It'll probably be only a bit bigger than that device - I'll be buying printed circuit boards by the square inch, so I want it small
I'm currently working on two different approaches:
Well, all else being equal I'd certainly buy such a thing from you rather than from Tekgear :-)
Just a few questions while you're still in the planning stage though :
- why the USB ? Just for charging the cell(s) (good idea BTW) or also for powering the 770 and peripherals ? That would be less convenient for some...
- why not AAAs ?
- for some DIYers like me a deal on the raw board (like Tekgears's) would probably be best, because we all have our own little schemes. Myself I would like to keep my current setup where my power supply can be connected directly to the keyboard if that's all I use, or to the hub if I want to connect mass storage and need to deploy the whole shebang :-)
I made some research on the subject at wikipedia, thanks fpp. Also, browsing Maxim's site I found this: http://pdfserv.maxim-ic.com/en/ds/MAX1674-MAX1676.pdf
It looks like the perfect regulator for the job, it will work with inputs from 1.1V and up. Maybe one of these is used in the tekgear product, the specs seem similar. It needs a few support components, but it makes for a very compact design. The only thing I couldn't find out is how to select +5V or +3.3V, does anyone know?
Look at page 6, and read about pin 1.
That chip is, indeed, wonderful - and potentially better than the one I am probably going to use.
It has one disadvantage. It is only available in a µMAX package. This is a tiny package with the pins every 0.5mm. An SOIC package has pins at a 1.27mm pitch. A standard DIP package, the old style one with holes in the PCB, is 2.54mm.
In my experience, especially when hand soldering, I generally find larger packages to be more reliable. I immediately ruled out anything that only came in teensy packages - even a DIP package is probably small enough for this project. The battery is the biggest part.
Well, all else being equal I'd certainly buy such a thing from you rather than from Tekgear :-)
The whole point of my device is that it really should be better than what Tekgear offers. If not, then I'm lame
At the very least, I will give you the right connectors and cabling.
Originally Posted by fpp
- why the USB ? Just for charging the cell(s) (good idea BTW) or also for powering the 770 and peripherals ? That would be less convenient for some...
Mainly because: If not USB, then what else? I could try to figure out the most common size for power supply plugs and use it, or I could sell people a power supply as well.
I'm figuring I'll put a mini-B USB connector on the board, which will charge the batteries, and will also run the device at the same time.
I'm still not that sure about the charging option. I'd like to use something like LiIon or LiIonPolymer batteries, but if you don't charge them right, bad things happen. NiMh batteries are much less likely to have problems, but then I'm not sure if there'd be a point. I'm figuring that a charging option would add at least $5 to the cost of the device, plus the battery(ies).
Originally Posted by fpp
- why not AAAs ?
Good question. The BYOB version will probably be available with a AA or AAA battery holder. Mainly battery capacity. I'm actually interested in powering a hub with flash and a USB audio device, so I figure I'll need some capacity.
Originally Posted by fpp
- for some DIYers like me a deal on the raw board (like Tekgears's) would probably be best, because we all have our own little schemes. Myself I would like to keep my current setup where my power supply can be connected directly to the keyboard if that's all I use, or to the hub if I want to connect mass storage and need to deploy the whole shebang :-)
I'm figuring that I'll offer the controller board on its own. It'll have the appropriate solder pads so you could, say, build it into a USB hub if you wanted.
One thing you must be very careful about is that you are not supposed to provide power to a normal USB port. If you modify a USB hub to work with a 770, and then plug it into a normal computer's USB port, the two power supplies will "fight", so to speak. You can mitigate this, and make it less likely to cause damage, but it's still a bad idea.
My design will have the USB cable to the 770 permanently attached - so the worst you can do is make devices get confused - you'll be able to plug a hard drive into a flash card reader if you want. Neither of them will know what to do, but they won't be damaged.
A load of C/100 (9mA on a 900mAh battery) should give you at least the rated output voltage to somwhere past the 60% (remaining) mark, or 40hrs. C/100 is a very small load, so internal resistance should have negligible impact.
Theoretical run time (100%) of 100h (900mAh / 9mA).
Yeah, that's about what I figured. Well, it is fully charging at home so later I will go disconnect the battery leads and measure the actual current and see how long it takes to go flat. I might have a bad cell in my battery perhaps.
Man, you're so right. I never looked at the package. Being sloppy to start with, I find even the DIPs a PITA to solder. I bleed it everywhere.
Then I guess a good candidate would be a MAX757, it needs a diode and is somewhat less eficcient though. Nonetheless, I live in Brasil and I can't seem to find Maxim CIs around here, too bad.
Originally Posted by bhima
Look at page 6, and read about pin 1.
That chip is, indeed, wonderful - and potentially better than the one I am probably going to use.
It has one disadvantage. It is only available in a µMAX package. This is a tiny package with the pins every 0.5mm. An SOIC package has pins at a 1.27mm pitch. A standard DIP package, the old style one with holes in the PCB, is 2.54mm.
In my experience, especially when hand soldering, I generally find larger packages to be more reliable. I immediately ruled out anything that only came in teensy packages - even a DIP package is probably small enough for this project. The battery is the biggest part.
I must say I'm having a hard time picturing what you have in mind. Maybe I'm too formatted by what I've read here and built myself that I can't imagine anything different... guess I'll just have to wait for the first prototype pix :-)
The only scheme I can think of right now that hasn't yet been described in this forum, and would actually improve on all the others from a practical point of view, is this (it was inspired by your "fat cable" remark) :
A very compact assembly built around a short USB cable. At one end the male mini A that plugs into the 770 ; at the other a female A (for a keyboard or hub host cable). In between, the length of an AAA cell plus your board plus a spare inch on either side, and power going to both plugs.
This would really cut down on both the cable spaghetti and the modding crap : no need to power-mod the hub if it receives it on its host port, and you can still plug in only a keyboard if that's all that's needed.
Is that what you're planning ? (if not I get the first proto as a finder's fee :-)
I've been doing some "research" (read: scouring Fry's for goodies) on this very thing, and came up with the perfect solution. The Pelican (no I don't work for them, just nice products) $20 "Power Brick" for the PSP is the perfect solution for power on the go. It features a Li-ion 3600 mAH battery and integrated charge control as well as (drum roll please) a DC-DC converter which puts out 5.25 volts!. It's meant to be plugged into a PSP charger, which will run from $7 for a car adapter to the $15 I paid for a separate AC charger. I used it as a power source for my hub with a 30G hard drive, CF adapter, and network card-- total run time with HD spun up continuously: over four hours! Under normal circumstances I would only plug the HD in as needed to copy stuff over. Run time without the HD is going to be huge since it's the primary power user. Given that the Tekgear is $20+shipping I can't see spending the money. The brick is the same thickness as the 770 minus the case, and about 2.75 inches square.
bhima :
A very compact assembly built around a short USB cable. At one end the male mini A that plugs into the 770 ; at the other a female A (for a keyboard or hub host cable). In between, the length of an AAA cell plus your board plus a spare inch on either side, and power going to both plugs.
My current plan is to have a circuit board with a USB type A socket. This board will be attached to a dual AA battery holder, rigidly. You bring your own AA batteries into this.
I have a mini-B USB cable, going to bare wires. That'll of course hook into the board.
I hope that makes some sense.
I did some testing of my DC/DC conversion circuit on a breadboard and it looks good. I haven't tested it on a 770 yet, but it's behaving properly. I'm going to get some PCBs fabricated and move forward...
