Well, this is my super simple solution to this all too common issue.
The idea here is put some rechargeables in a project box, or get a battery case or a sealed lead acid rechargeable or whatever - the circuit is the key.
Build it into a flashlight, AM/FM Radio / ghetto blaster, or other battery operated device that you might take camping or anywhere you may not have a power source.
The circuit itself is where the genius is. To my knowledge, NOBODY else has made the circuit this way.
The video of the final product I made is below. I decided not to edit out the initial OOPS where I didn't quite get the switch fully flipped...LOL. Stuff happens!
Step 1: Parts List:
We don't really care much what value they are. Put away your calculator - we don't need it.
I have tried from about 100 ohms to about 5K and they all work, as long as all 5 are the same value.
Using 5 resistors means no calculations of ratios, and chances are, the last time you bought an assorted pack of resistors, there was a value you didn't use - or if you bought 100 of some value of resistor, chances are, there's leftovers.
This reduces cost, as we don't have to buy them if we already have them.
Also, if you are buying new resistors, you don't have to get 2 different values.
It's cheaper to get one value, as you usually have to buy a minimum quantity anyway.
A 5V DC power supply.
If you want to use a battery pack, then we are going to also need a 5V regulator.
A small amount of wire, solder, and soldering skill
A female USB type A socket.
A scrap piece of perfboard
Step 2: The Hookup
Connect all the resistors in series.
connect one end to the GND of the USB, and the other to the 5V.
Connect the junction between resistor 2 and resistor 3 (counting from GND) to both the D+ and D- of the USB.
That's it. If you put 5V into the USB, your apple or other device will now see it as a valid charger.
If your source is a battery, you will need to put it through a 5V regulator first, and put the output from the regulator to the 5V on the USB plug.
If you want, you can put separate inputs for 5V and 6+V (the 5V input bypassing the regulator).
Step 3: That's IT - Except, We Can Make It Simpler Still
I made a quick and dirty one to prove the concept, and it worked on my iPod and an iPad I have in for repairs.
I didn't have a 7805, so I used a 5V 1117 SMT regulator.
This lets me charge or run a USB device portably off a battery pack (rechargeable or otherwise).
Better yet, mount it to a 9V battery clip!
Going out to the woods for the whole weekend? Get a 6V lantern battery or Sealed Lead Acid rechargeable.
I actually built this into a $5.00 6V flashlight. You have to take the flashlight anyways, may as well make it all 1 device!
You'll have power for everything you need most all weekend long! Similar devices start around $100 and go up from there, and don't last nearly as long.
JUST REMEMBER TO SWITCH IT OFF OR UNCLIP THE BATTERY PACK WHEN NOT IN USE - There will ALWAYS be SOME current flow because of the resistors across the power supply and/or the regulator. Higher value resistors = less current drain when idle. 5KΩ each works well.
Don't forget that you WILL get that error message about not being able to charge off your source once your batteries or source voltage drop below a certain point!
But guess what? We can make it EVEN SIMPLER!!!
Remove the 5 resistors, and put in an adjustable potentiometer (a trim pot).
Step 4: The Simplest Version
Put either end of the trim pot across the USB Power (5V and GND) and trim it until the wiper (center) voltage is 2V.
Connect the wiper of the trim pot to the D+ and D- of the USB connector.
That's it. That's as simple as we can make it!
If you have a 5V source, putting this trim pot on the connector will allow charging the device.
If your source is more than 5V, you will require a regulator, just as before.
Above is the basic schematic and examples of adjustable trim pots.
Have fun charging or running your favorite devices from your favourite alternate sources!
Just remember that 5V regulators produce more and more heat, the more your source is above 5V
I usually don't recommend going above 9V (12V with a GOOD heat sink) into the regulator.
Also don't forget that you WILL get that error message about not being able to charge off your source once your batteries or source voltage drop below a certain point!