The blog does a really good job of describing the project, but too long for the projects page.

http://www.billporter.info/rebuilding-freeze-frame/

As you may have seen already, I enjoy volunteering for the local Science Museum. This time I took on rebuilding one of my favorite exhibits, Freeze Frame. The exhibit is simple, guests stand in front of a photo sensitive wall while a flash activates to ingrain their silhouette in the wall for a few minutes. The museum had one that had long since died and the previous volunteers simply ripped out all the parts except the photo sensitive wall and added an LED on a pen to let patrons ‘write with light’. While interesting, it’s not the same awe inspiring effect as Freeze Frame, so I took it on myself to resurrect the original function. This is how I did it.

All that was left of the exhibit was the hole where the flash unit was once installed, and the photo sensitive wall. Everything else had to be built from scratch. Ok, so first the flash. I tested a friend’s photographers flash unit and it worked well, so I acquired one for this task. It has a ‘PC-Sync’ connection that is used to remotely trigger a single flash. Searching the internet gave me clues that this is a simple two wire connection that just needs to be shorted for a brief duration to trigger the flash. I tested this with an optoisolator (just to be safe) and I was able to trigger the flash with the flip of an IO pin.
diagram

Example schematic from glacialwanderer.com

Next is the user interface. I already used Sparkfun’s big red dome button for another project in the museum and knew it worked well for kids and adults. I also wanted to add audio feedback to give patrons the cue it was about to flash so I grabbed an small 8 ohm speaker as well. I figured between the audio tones and the flashing light in the button, guests would know exactly when the flash was coming and to strike a pose. I threw together a quick PCB design and had it made by Dorkbot.

http://www.sparkfun.com/products/9181

http://dorkbotpdx.org/wiki/pcb_order

Schematic (All pics are click to enlarge)

PCB

The circuit is centered around an ATtiny85 which I like to use for small projects like this. When pressed, it runs a short three count of flashes and audio tones before triggering the flash. This gives guest enough time to run back to the wall and strike a pose.

[cut - many pictures, a video, and code]