I knew my parents had an old (probably cheap) external flash unit they used in combination with an old compact film camera. They triggered the flash by using its optical sensor that detects when the camera's built-in flash goes off.
|The old flash (triggered here via the DIY adapter).|
Some head-scratching later, I realised that it's because of the fancy TTL flash metering system of today's cameras. Before the actual exposure, the camera pops the flash once to judge the light and then pops the flash a second time for the actual exposure, with the correct power as calculated from the first test shot.
It was this test shot that made optically triggering the external flash impossible. The second, real, pop came while the external flash was still recharging from when it went off at the test shot.
I found one work-around to continue using the optical trigger of the flash: set the camera's flash to fire at the end of the exposure (also called 'second curtain'). With a shutter time that's long enough for the external flash to recharge, the flash will be ready to pop once more, this time during the actual exposure.
This work-around is effective in some cases, but it's pretty ugly and you cannot always afford such a long exposure time for the shot (and thus also delay until the actual flash).
Luckily enough my flash can also be triggered with a 2.5mm mono jack plug. (Note: just as the with remote trigger, I'm actually using a 3.5mm mono jack with a 3.5mm to 2.5mm stereo adapter. It's no big deal that the adapter is stereo.) The obvious thing to do was to hook it up to my camera's hotshoe!
|The hotshoe of my camera, a Canon EOS 500D.|
A quick check on Google confirmed my findings. It also mentioned an important property of the Canon hotshoe: it is only rated to switch 6 Volts or less!
Knowing that there is a lot of high-voltage action going on inside strobes, I grabbed my voltmeter to measure the voltage between the connectors of the 2.5mm jack of my flash. It turned out to be about 10V. Not a few hundreds of Volts (which same strobes will put out!), but still almost double than what the Canon hotshoe is rated for.
Luckily for me, the 10V I get from the strobe is pretty high impedance. In fact, just the output resistance of putting a finger between the contacts was enough to drop the voltage down by a Volt or so.
The easy solution was to wire a 100kOhm resistor over the contacts. That is high enough to not cause the flash to go off, and low enough to get the voltage down to the 6V range. I'm pretty sure the internal impedance of the camera hotshoe would have been enough by itself, but better safe than sorry.
|100kOhm resistor inside a 3.5mm mono jack salvaged from an old microphone.|
Note: If you find yourself having a high voltage strobe (say, 20V and up), the above trick probably won't work (you'll have to load the input so hard that the flash will think it is being triggered). You could alternatively try hooking it up via an optocoupler system to keep things safe and separated.
The only problem left for me was making a connection to the hotshoe. Some PVC plate I had laying around came to the rescue! It was just a matter of sawing and filing it down to fit the camera.
|Top view of hotshoe connector. The upper left corner is the front side.|
To unmount the connector, simply release the screw by hand and slide off. To attach the connector, slide on and fasten screw by hand. It does the job.
The only thing left is making the connection to ground, the metal bracket. This one was more easy. Simply drill a hole, put the shielding wire of the chord through, make a small 'gutter' in the plastic and glue the wire in place. When the glue has set, sand of the top of the wires to expose the bare copper on the extruding points.
|Bottom view of hotshoe connector. On top: the wire that makes contact with the metal bracket. In the center: the nut and screw that touch the center pin.|
|Sliding on the connector.|
|The connector attached and the screw fastened.|
Below is a test shot in a mirror. I rolled up the wire a bit. I might shorten the wire in the future so that it's just right when mounted on my camera. I can still use a good old 3.5mm jack extension chord when I need to cover some distance.
|Test shot in mirror with old flash mounted to camera.|
To conclude: I put an old an unused flash to good use for basically no money at all. There are no fancy automatic TTL goodies and I have no strobe light meter, but who cares when you shoot digital?
To close I give you a small look-ahead: I'm working on a ring flash for macro work that will use this old strobe and connector. I'll post it here when it's finished.