USB power "injector" for USB host mode

Reminder: I TAKE NO RESPONSIBILITY IF YOU COOK YOUR DEVICE OR YOURSELF! My blog posts are about my experiences and projects and are documented for entertainment purposes. Only experienced professionals should attempt to duplicate any of these procedures. Note: I realize I spelled "injector" wrong as "injecter" earlier. Oops.

*UPDATE: I made a new version of this - more stable and much prettier! If you link to this post, see this one instead.*

I decided that my previous USB host rig was too cumbersome for road use so I designed a USB "power injector" that passes data but supplies +5V to both the 770 (to put it into USB Host mode) and the USB device.

Parts: (Radio Shack part number)

• (4) AA batteries (2300873)
  
• (1) battery holder with an on/off switch and leads (2700409)
  
• (1) 5.1V zener diode (2760565 for a pack of 2)
  
• (1) 100 ohm resistor (2711108 for a pack of 5)
  
• (2) USB female cable ends. (I dissected these from USB extension cords)
• Solder, soldering iron, multimeter, heat-shrink tube, etc. (for assembly)

I spent under $10 at Radio Shack. The USB cables can also be "borrowed" from a motherboard's USB Riser if you've got them laying around.


I got it to work. The diode and resistor go together like this. The the small red wire is the positive lead from the battery pack and the black wire is the negative lead. A picture will explain this better than a verbal explanation. The red alligator clip goes to the positive channel on the multimeter and the black goes to the negative. We're testing voltage:


The voltage that came out of this was 4.94 volts. VERY close to spec. with this device.


From there, I needed to solder on the female USB cables. I discovered that it's very simple: Both sides go on the same way. Voltage to voltage, ground to ground, +DATA to +DATA (green on my cables, I think) and -DATA to -DATA (white on my cables.) In essence: it's all "straight through."

I then attached a USB keyboard light (simple LEDs) to test that the wiring was correct.



It was. I then got bold and attached my USB keyboard to one female lead and my Nokia 770 tablet to the male lead. It worked like a charm!

Bet you're wondering where the "mighty success" picture is huh? Well tough luck: On the next to last solder point, I broke my ColdHeat soldering iron tip! So until I can find another one (and maybe a project enclosure for this beast) the whole mess is remaining on my dining room table.

I didn't try this with other gadgets yet (like my iPod nano) and won't until I have all clean, insulated solder points and have it all in a nice project enclosure. I hope to keep it smaller than an iPod. The beauty of this design is that you can plug in the device or the tablet into EITHER port and it will work properly.

Someone on internettablettalk.com forums asked how much power draw this would take. This is what I found out: The contraption simply "switched on" shows 14.3 mA. My USB keyboard takes 14.73 mA plugged in and then when I plugged in the 770 tablet it went up to 14.84 mA. With the caps lock light on and while typing it got up to 18.05. I assume most of that was the caps lock light. I don't understand why this would have a current draw without anything plugged in since I felt no heat from the diode or resistor. Even so: cheap alkaline AA's have about 1800 mAh so if my assumption is correct this will power it for up to 100 hours. I can't say for sure, though, because I don't know what effect the dropped voltage over time will have on the tablet or on the keyboard or at what voltage either cuts out.

Edit: It's done. Here are the pics.

Special thanks:
HackADay for this post.
Mom for giving me a ColdHeat soldering iron for my birthday last year.