Simple Nerf Gun Shot Counter Using a Keychain Photo Viewer

I am in college, and zombie hunting with Nerf guns is a common pastime.  In the course of blasting away at the imaginary undead, it is easy to loose track of just how many darts are left in that clip.  Never fear!  With a very simple hack and a cheap digital photo keychain viewer, you can easily track how many shots you've fired.  And it looks really professional.
The counter works by loading a set of images into the photo viewer.  The images can be made as fancy as you want, but essentially each image displays a number - in my case, 1 through 6 for my six-shot Maverick, as well as a "reload" or 0 image.  A small switch mounted in the handle of the Nerf gun is pressed every time the trigger is pressed, and this switch is wired across the "up" button of the photo viewer, so that each shot changes the image on the viewer to display the number of shots left in the gun.  My viewer is cyclic, so pressing the trigger while reloading changes it back to "6".

Step 1: Find and Paint a Photo Keychain

First, get your hands on a digital photo keychain viewer.  Mine was from a company called "The Sharper Image" and came to me at a Christmas grab bag.  I painted it to conceal the ugly purple color and make it match my Nerf gun.  The best part was that the screen was covered by a screen protector, so all I did was remove the electronics from the case and spray both sides of the case.  When the paint dried, I removed the screen protector.  Protip: make sure you cover the back of the screen cover with tape - I neglected to do this and had to scrape of bits of paint from the inside of the screen.

Step 2: Wire a Switch to the Trigger of the Nerf Gun

In order to make the photo viewer change its display when the Nerf gun is fired, I mounted a switch inside the handle of the Nerf gun. The switch is a small "leaf" or "micro" switch of the kind that can be found at Radioshack or ordered from electronics companies like Sparkfun.  It is mounted in the handle of this Maverick so that when the trigger is pulled all the way back, the switch is switched (some slight plastic modification was involved to make sure the switch didn't catch on the trigger).  The three metal points on the switch are common, normally open, and normally closed.  I soldered two wires to the switch, one to the common and one to the normally open, so that the switch closes when the trigger of the gun is pressed.  This mimics how the buttons on the photo keychain work.  The wires are routed out through a small hole in the handle.

Step 3: Wire Switch to Photo Viewer

When I opened up my photo viewer, I immediately noticed that the buttons were mounted on a piece of circuit board that was mounted upright beside the main circuit board, and that there were four good solder connections between it and the main board.  One of these connections was separated from the others, so I guessed that this one was common and the others actuated the various buttons.  While attempting to test this theory with a multimeter, the board kept turning on (there was no way to unplug the battery) and I figured out that crossing the common and the button pads would make the viewer behave as if a key was pressed.  This method is probably specific to this model, though, so be aware your mileage may vary.  I suspect that on most viewers you could simply pop the top contact off the button and solder the wires from the gun to the contacts below.  I drilled a small hole in the back of the viewer to run the wires through.  I wish I had put in some kind of connector, but the ones I had all seemed too bulky.

Step 4: Mount Viewer and Load Images

I mounted the photo viewer on the back of the reloading slide of the Maverick by cutting off the little clip thing and gluing the viewer onto the flat surface that was left.  The wires running outside the gun worry me, but I couldn't think of a better mount point.  It is easily viewed there and does not stick out (read: easily broken off).  You can see an example of one of the images I uploaded to the viewer.  It is large enough to be easily seen at night while hunting zoms.  The viewer is connected to the computer by mini USB and includes software to load the images.  
Thanks for reading, I have always been amazed by these little photo viewers and I believe there are lots of uses for them.  I would like to see other projects of a similar nature.  I stumbled across a website that describes how to modify the firmware of certain viewers to show videos streamed from a computer.  http://spritesmods.com/?art=picframe&page=1  Mini display for Raspberry Pi, anyone?