Wednesday, September 22, 2010

Don't throw away that old flash yet!

To continue with the DIY-photography-on-a-budget theme from last post, I present you a way to use that old 2.5mm jack triggered flash you might have laying around!

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).
That ought to work on my Canon EOS 500D too, so I thought. After some pretty dark test shots, I figured something was wrong. I could see the external flash firing, but when I shot into a mirror, only the small pop-up flash was active in the photo.

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 test with a multimeter showed that during an exposure, the center pin shorts to ground (the metal bracket). One note, you need to watch out with polarity. The center pin should be positive, the casing negative, otherwise stuff won't work (there's an internal blocking diode from the switching transistor).

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.
Making the connection to the center pin was tricky. I was thinking about some spring system (possibly salvaged from an old ballpoint pen) that slides over and then makes contact. But I thought screw it (no bad pun intended) and  just drilled a hole, jammed in two nuts and use a screw to make the connection to the center pin. The positive connection to the jack connector (in my case the tip) was soldered to the top nut, as can be seen in the photo above.

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.
That's it! Now it's just a matter of sliding it on and trying it out!

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.
I've already used the old flash with this adapter a few times and it does the job for me :-). Here are some examples when I was playing around with 'freezing' water drops:

Full resolution available here and here.

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.


  1. I wouldn't do this. There have been arguments about how using old flashes can fry your modern camera electronics.

    What i would rather do is just to buy really cheap 20 dollar wireless radio triggers. Mine came with the connection cable and i'm using a trio of over 20 years old flashes with my modern dslr.

  2. Congrats on the Lifehacker reference! But yes, Demens is right. Many )if not most) old flashes have "sync voltages" in the range of hundreds of volts. That is the voltage across the sync terminals that your new digital camera is completing when the flash is electrically connected to your hot shoe in this manner.

    Modern DSLRs are generally expecting single-digit sync voltages. The result can literally be your camera going bye-bye in a puff of smoke.

    Here is a great table to see if your new camera and your old flash are physically compatible on a sync voltage basis:

    David Hobby

  3. Hello,
    you can also use this schematic... It works very well!


  4. Hi! I find your project very interesting. Do you think this would work with remote triggers such as these


  5. For $16 you can get an adapter from midwest photo exchange to let you attach via cable:,12111.html

    I know I wouldn't trust myself to build the adapter in this article, but I've been using this and am happy with the results.

  6. You could also look into setting your popup flash on manual (non TTL), that way it doesn't do the preflashes to measure correct exposure. Since the old big flash is manual, it shouldn't matter too much ... I don't know about Canons, but on Nikons I have, this is possible.

  7. Holy cow, I did not expect this much attention!

    Thanks for the Lifehacker reference!

    Once again my view on the safety: This project is something you should *only* do when you have the tools to measure the sync voltage of your flash! I could, and I managed to get it down to a safe rated voltage. Everything in my set-up is according to the specifications, so I'm pretty sure it won't fry my camera.

    Also, I have finished my macro ringflash. Will post it next week-end when I have some more time (college has started again).

  8. Nicely done! I just remember finding an old box of flashbulbs at a garage sale as a kid, and wiring them all up inside of the mailbox so they'd go off when the mail lady opened it. Thank god she had a sense of humor! Thanks for a great post & jogging a funny memory...

  9. I believe the 300D was the last of that range to rate at 6v on the flash sync, thereafter (350D and on) I believe they're rated at 250v, so you probably had more overhead than you thought on that 500D. Same for the 10D, From the 20D on I believe that range is also 250v, as is my 5D. I had an 80v sync flash (agfatronic 201B) on it firing away happily for several days before I found your article, got worried, and did some research.