I recently bought a new camera, an Electronic Viewfinder, Interchangeable Lens (EVIL) or if you prefer the official acronym, Mirrorless interchangeable-lens camera (MILC). I went for the Nikon 1 V1 for several reasons, not least the small body and the ability to film at 1200 frames per second.
One downside of this model is that it does not have a standard shoe mount for flash units but instead a smaller proprietary shoe specifically made for this camera series, neither does the camera have a PC sync jack for older flash units.
I needed a way to sync an external flash to the light from the flash unit provided with the camera, you would think that one could simply buy a slave flash controller and attach it to the external flash, while this would work for most electronic xenon flash units, it turns out it does not work for old disposable bulbs, probably because the voltage is too low and the current too high for the slave controller to give enough of a pulse. I could have just built additional electronics for a slave controller to make it work, but i happened to have a tube of VO2223A, an opto coupler triac that can handle 600V and several amps pulsed. Since it's an opto coupler it will be triggered by light, and since it's often used for dimming it has to have rather fast response. I simply cut it in half between the primary and secondary side, soldered wires from it to an external flash shoe adapter, attached my old flash unit to the shoe and the sensor to my camera.
This solution works great, and the triac should handle both high voltage and high current flash units with no problems, and since it only costs a dollar, breaking it isn't a great concern to me. The only real downsides are that it needs a lot of light so it needs to be mounted very near to the master flash, also it only works as a slave for flash units that emit a single flash so no TTL or modern compact cameras that use metering flash or red eye reduction. I could of course add a micro controller to the lot and program it to wait for the appropriate number of flashes before firing, but then it wouldn't be a simple 5 minute build anymore.
The old gray flash unit is a National PB-3S from the 60's or 70's, and the bulb is a #5B, the reason i use this flash is partially it's fun to photograph using old disposable bulbs, but they also give better color and a huge lot more light, the pictured bulb outputs over 10 times more light than the electronic flash unit that came with the camera.
On the left is a picture taken with F/4 and the built in electronic flash, on the right with F/16 and a #5b flashbulb. Both with ISO100. Note the distinct color difference.
Here is an example of a whole building being photographed at night using a #5b bulb, F/3.5, ISO200.