So, for people who are seasoned in photography, you'll probably know that when using Speedlite flashes on consumer-grade cameras, the camera can only synchronize the flash up to a speed around 1/160th of a second. This is a problem if you want to do high speed photography, using an external controller and sensory input (such as a microphone or light sensor) so the solution is to add external flash control, to allow the controller to trigger the flash itself, instead of using the camera. This mod also allows you to connect your flash off-camera, using a modified hot shoe connector. It also doesn't compromise the on-camera functionality of the flash.
The modification is quite simple, requiring only a few tools and parts.
Step 1: Tools and Materials
Step 2: Disassemble Flash
For this modification, I used a Yongnuo YN460ii flash, which are about $55 on eBay. They are full manual, so we don't have to worry about complications from TTL flashes. To disassemble a YN460ii flash, start by removing the four screws that hold the hot shoe connector onto the base of the flash. Once it is free, you will need to use a knife to cut through the stickers in the battery compartment that show battery orientation. Now the two halves of the flash can separate.
Identify a location inside the flash that will allow enough free space for the jack to go in. Its a good idea to leave a good safe amount of extra room near the jack so it won't collide with anything inside the flash.
Step 3: Drill the Hole
Once you've figured out where to put the jack, mark the approximate location on the outside of the flash. Use the small 3/32" drill bit to drill a small pilot hole into the side, and then test the positioning before drilling the full size hole.
Step 4: Solder Wires and Mount the Jack
Next step is to solder wires to the 3.5mm jack. Its easier to solder wires to it before gluing the jack in place. I've soldered the Fire pin to the tip of the jack and the Negative pin to the base of the jack. Shrink some small heatshrink over the pins so the don't short out anything inside the flash housing.
Using a hot glue gun, glue the jack into place inside the flash. Route the wires around the circuit board and out the hole in the bottom of the casing where the hot shoe is, and re-attach the two halves of the flash body.
Step 5: Connecting the Jack
With the wires protruding from the bottom of the flash, connect the wire from the tip of the jack to the Fire pin, and the wire from the base of the jack to the Negative pin, inside the hot shoe.
On a manual flash like the YN460ii, these are the only two connected pins inside the hot shoe connector. All you have to do is short them out to trigger the flash! This is what the controller or whatever device you connect the flash to will have to do to trigger it. You can use an opto-coupler or a relay to trigger from any kind of device.
Re-attach the hot shoe to the flash and you're finished!
Step 6: Complete!
You now have an easily triggerable flash which you can connect to your favourite microcontroller, wire a handheld trigger button, or connect to a modified hot shoe or camera for off-camera flash triggering without expensive radio transmitters or hot shoe tethers. Also, by using 3.5mm jacks, you can buy cheap headphone extension cables (I got a bunch of 12ft cables for $2 from the local dollar store) to set up your lighting arrangement for a fraction of the cost of a purchased system.
I'm using this flash with the Camera Axe Project I purchased a while back, along with some custom designed sensors to detect light and sound. Since cameras can't sync with fast shutter speeds, this is the only way to do high speed photography for cheap.
Thanks for reading everyone. Feel free to ask me any questions.