HACK A DAY -- Lee Hosty pointed out Jason Striegel's clever USB power kludge. It gets its juice from a Zener diode that conducts in the reverse direction when it reaches its breakdown voltage of 5.1 volts. Voltage across the diode will be that same 5.1 volts, so it acts to limit the 9 volt battery's power supply down to roughly 5 volts, which is the USB power rating. The resistor in Striegel's setup prevents the circuit from shorting when the load (your iPod or your phone) is removed.
Hot Tip: If you use a resistor less than 100 ohms you’ll know it because it will get hot. Also, if you use too large a resistor, you won’t be able to supply enough current to your device.
Jason Striegel -- "How to make a USB battery"
tags: DIY and Open x_ref101ib