found out yesterday that there’s a shuffle shortage and gadget battery life can only be expected to get worse. bummer. at least you can make the most of your waiting-for-shuffle blues and solder yourself a ‘usb battery’. this little gizmo runs off a 9 volt battery and supplies your favorite little device with the proper 5 volts of mobile power.
in lieu of running an ipod off of it, i tested it with an mpx220 smartphone and it should work fine with any low power mobile device that can charge via usb cable. read more for instructions on how to make your own.
here’s what you need to scrounge up:
5.1v zener diode.
100ohm or slightly greater resistor. all i had was a 330ohm which works. don’t try anything lower than 100ohm.
9v battery connector and 9v battery.
flat female usb connector that mates with your device’s usb cable. this is the hardest part to find. i tore mine off an old usb extension cable.
wire it up
connect everything up as shown. a board makes it easy, but in a pinch you could just wrap ends together. the top two wires go to the battery and the bottom two are connected to the female half of the usb cable i butchered. polarity matters on the diode, so check your work.
connect the battery and test the voltage over the diode. it should be close to 5 volts. if not, make sure you have everything wired correctly (pay special attention to the polarity of the diode).
still waiting for an ipod, i decided to grab my smartphone and kick the tires on the usb battery. as you can see in the image, the device thinks it’s plugged in. you should be able to use any low power device that charges via usb.
how it works
the zener diode conducts in the reverse direction when its breakdown voltage of 5.1 volts is reached, and the voltage across the diode will be that same 5.1 volts, so it acts to limit our 9 volt supply down to roughly 5 volts, which is what usb powers devices at.
the resistor is there to keep the circuit from shorting when the load (your shuffle) is removed. if you use a resistor less than 100 ohms you’ll know it because it will get hot. if you use too large a resistor, you won’t be able to supply enough current to your device.
finish the job
as you can see, i’m pretty nasty at soldering. i’m under strict orders from cl to keep this tiny and pocket sized for the ipod, so i’ve tried to cram everything as tight as possible. as soon as minneapolis warms up enough for the ipods to migrate, i’ll be making a custom case for this.
till then, please send in your comments, ideas, and hacks. i look forward to seeing what you come up with.