Warning: wall of text introduction to my problem!
I saw a deal on Google Offers for an SOSCharger, which is a portable flashlight with a barrel plug power output that charges via an onboard dynamo. I figured it wouldn't be too bad of an idea to get, seeing as it has that extra power output. The site did say it could not charge Iphones (and presumably other smartphones), so that means that from the getgo I was looking at playing with this thing. It arrived in the mail with a bunch of adapters, several of which were USB, which was a little disappointing (but unsurprising) when none of the USB adapters had a built in voltage divider circuit.
I also found out that the barrel plug would only be energized when the dynamo was active, which is all well and good, but it is a pain in the rear to charge your phone that way continuously. I wanted to be able to plug any USB device into the SOSCharger and have it work, while only needing to rev up the Dynamo every couple of minutes or so, allowing the onboard battery used for the LEDs to provide steady power output in between charging spasms.
I did already have some booster circuits from previous builds lying around, I picked them up off Ebay for about 3-4 dollars apiece, and I had a pushbutton switch that I salvaged from a LED bumplight, so I figured I could also make an adapter that converted the standard DC output into USB output that could be used by all devices.
You will need the following:
1 DC-DC Converter 3V to 5V 1A USB Charging Circuit (lesser amperage is fine, nothing under 250 mA)
1 Pushbutton switch
Resistors in some way that will total:
and 27000 OHMs
Soldering Iron / Solder
USB Male connector
Drill or rotary tool
We need all of the extra items in order to restore the unit its water-resistant qualities.
Step 1: Crack It Open
Once inside, unscrew all screws (don't lose any!) and pull out the barrel plug connector for easier access. You'll see that the red wire is at the back of the barrel plug, with the black wire at the bottom. Looking inside, you'll see that only the black and red connect to the inner leads. Hmm...
Step 2: Wiring to the Battery
This makes our job a little simpler, because we don't have to worry about frying anything. Fire up your soldering iron desolder and then switch the black and yellow wires.
Step 4: Booster Circuit Adapter
There will be four leads on the USB female output. On the back of them, solder a black to the ground, red to the +5v, white to the D- and green to the D+.
Have two diodes coming from the red wire. Between the red and the green wire, solder in a 29kR resistor. Between the green and black, solder a 20KR. Between red and white (ON THE SECOND DIODE), solder a 22kR resistor. Between white and black, solder the 27kR. (remember, any combination of resistors in series that add up to these values will be what you need in those spots, but the fewer resistors the better because it takes up less space).
Here is the voltage divider calculator I used.
Solder the leads from the Male USB adapter for ground and 5v into the appropriate input leads on the booster circuit chip. Finally, wrap the entire assembly in electrical tape, insulating everything. Keep it compact.
Step 5: Final Housing for Adapter and You're Done!
Heat the plastic in the hot water, then mold it around your booster circuit adapter assembly, make it as compact as possible, and let it cool.
Plug it all in, press the switch, and you're done!
Some things to note: the battery has a low capacity. You can't recharge a cellphone fully off of it, really all this does is let you set the thing down for a few minutes between furious cranking. It is likely overdischarging this battery could really hurt it, which means you need to keep an eye on your charger, and crank often.
Thanks for reading!