UPS-connected 12-Volt DC car jack to 5-Volt DC USB power adapter

It all started when a friend of mine was going to throw out the old uninterrupted power supply (UPS) that came with her previous Internet set-top box.  All it needed was a new lead-acid battery, but she did not know that until she had two of them.  The newer one did not cost her anything, but she had no use for it and was ready to trash it.

After saving the old UPS from the trash heap I made sure to dispose of the old lead-acid battery responsibly by taking it to a toxic waste disposal facility.  Next I purchased a new lead-acid battery for $30 at my favorite home improvement store.  In no time at all, I had a good-as-new 120VAC to 12VDC battery back-up power supply on my hands.  It did not take me long to figure out what to do with it...

Now, I am using it to keep some "mission critical" 5V-powered home electronics up and running, even when the power goes down.  (FYI: those 12VDC AutoPlug-to-USB converters can be bought really cheap, under $10.)  And in an emergency, I can use this setup to charge our mobile phones.  (I love USB.)

Arduino UNO as USB Keyboard "Emulator"

I started out by searching the online Arduino help web pages and discovered that native Arduino software libraries exist to support creating a USB Human Interface Devices (HID) such as a keyboard, mouse or gaming joystick.  Unfortunately, this support does not extend to the Arduino UNO boards.

The root of the problem, is that the Arduino UNO board cannot behave like a USB HID (e.g. keyboard) while supporting uploading compiled sketches to the microprocessor.  
 
Fortunately, I am not the first Arduino UNO user who has run across this limitation.  Better yet, there are several Arduino-savvy power users who have figured out and shared their knowledge with the rest of the Arduino user community.

Here's the gist of the solution: The Arduino UNO board needs to be put into Device Firmware Update (DFU) Mode.  While in DFU mode, the Arduino board's USB interface firmware is changed from a USB Arduino programming device to a USB keyboard interface device.  While a keyboard device, an Adruino sketch can include sending "keystrokes" to the USB host device such as a PC or Mac. 


In the following discussion, I am going to refer to "Programming Mode (PM)" and "Keyboard Mode (KM).  These are just terms I have created for the purpose of this description.  PM refers to the "mode" where the standard Arduino USB firmware is installed to the Arduino UNO board.  PM supports uploading Arduino sketches.  KM is the "mode" where your sketch can interface with you computer as if it were a standard USB keyboard.  The fore-mentioned DFU mode is a more commonly used term that we will recognize as an intermediate state required to "toggle" between these other two states.

So the programming sequence goes like this:

1. Connect a USB cable to the Arduino UNO board.
2. The original Adruino UNO board starts in "Programming Mode (PM)".  So, to start, compile and upload your "Keyboard Mode (KM)" sketch the same way you would normally for non-KM sketches.
3. Put the Adruino board into DFU Mode.  This is a 2-step process:
1. First, you must connect two points of the Adruino UNO board to ground.
2. Next, use the "dfu-programmer" command to set the Adruino into DFU mode and upload the keyboard firmware (Arduino-keyboard-0.3.hex) file.
4. Unplug the USB cable to remove power.
5. Plug-in the USB cable.  The KM sketch will begin to run.

Done!

For details, please refer to [solved] DFU Mode Arduino UNO without soldering.

IMPORTANT:  While the Arduino UNO board is in keyboard mode, you can no longer upload new sketches.  So, when you are finished running the KM sketch, you need to use DFU mode to upload the original PM firmware (Arduino-usbserial-uno.hex).

The process is similar to going from PM to KM,except that this time the "dfu-programer" command is used to upload the PM firmware (.hex) file:

1. Connect a USB cable to the Arduino UNO board.
2. Put the Adruino board into DFU Mode.  This is a 2-step process:
1. First, you must connect two points of the Adruino UNO board to ground.  (Same as before.)
2. Next, use the "dfu-programmer" command to set the Adruino into DFU mode and upload the PM firmware (Arduino-usbserial-uno.hex).
3. Unplug the USB cable to remove power.
4. Plug-in the USB cable.  The KM sketch may begin to run, but no keystrokes will be sent to PC or Mac.

After that the trick was toggling through the modes to write the sketch.

EDIT (Months later):  I never quite finished this blog entry.  Sorry dear reader, but I moved on before really putting together the kick-ass USB game controller that I'd originally intended.  Brainstorms sometimes do that.  They come, then go.  Despite this, I've decided to publish what I figured out As-Is to help some aspiring hacker to perhaps carry on from where I left off.  Or I may pick it up again in the future.