A USB Charging Port for an Electric Bicycle

This instructable will show how to build a USB charging port for an electric bicycle.  This project is basically a DC-to-DC converter which takes power from the main battery of an electric bicycle (usually 24V, 36V or 48V) and converts it down to 5V needed to power a USB charging port.

Here is the schematic and the parts list with Digi-Key part numbers.
The resistor network is needed for some phones (e.g. the iPhone) that need to see 2V on USB pins 2 and 3.

Here is the printed circuit board layout.  There are parts on both sides of the board in order to create a more compact layout.

I had a prototype board fabricated .. and here are some shots of the bare boards.

Here are some photos of the prototype board with the parts installed and soldered.

Testing just consisted of supplying a 48V input voltage and verifying the output voltages at the USB connector.
Pin 1 should measure 5V and pins 2 and 3 should both measure 2V
After verifying these voltages, the real acid test is being brave enough to plug in your smart phone.
I have tested this charger with a Samsung Galaxy Android phone and an iPhone 5 and both charged just fine.

I used shrink-wrap tubing to enclose the design.
Optionally inject a little hot glue just before shrink-wrapping to make a waterproof casing.