see copyright notice. Page created 3-Mar-2005 updated 9-Nov-2019. Use the button groups above to navigate quickly around the site.
WARNING: this article describes the author's experimental modifications to a commercial product. Readers attempting similar experiments do so entirely at their own risk, and will invalidate any supplier's warranty. Flash guns generate high voltages and intense light, both of which can be hazardous to the experimenter.
There can't be many products that have been around as long as the Vivitar (TM) 283 flashgun. I bought mine in the 1980s, and they're still in great demand! That's because it's a well-engineered device that simply gets on with its job and wears well. My 283 got a new lease of life when I bought a digital camera with a hotshoe. The camera had its own pop-up flash, but it was rather feeble (guide number 12m at ISO 200, compared with 51m for the 283). Also, it was fixed-angle and close to the lens, so no bounce-flash, and plenty of red-eye. And it took ages to recharge. And the camera shut down while that was happening :(
Once I'd seen the improvement to be had with an external flash there was no turning
back. I was lucky though. Older versions of the same product present several hundred
volts across the hotshoe contacts, enough to kill a digital camera; later they
modified the design. Mine measured just 8v and worked fine - but I soon got annoyed
with the automatic flash. I was using manual camera settings more and more, and what
I really wanted was to complement these with manual adjustment of the flash. The
experimental modification described here provided me with 9 levels of flash duration
(energy), selectable in steps of approximately 1 stop.
My 283 came with a plug-in sensor, bearing the boast-words "auto thyristor" (cf. "turbo intercooler" on motor cars), to measure reflected light and shut off the flash when it's gone the distance. My plan was to replace the measuring element, a light-dependent resistor, with a bunch of fixed resistors. So the first thing to do was to draw up a graph of light output vs. resistance value.
I removed the sensor and connected a 100k log-law potentiometer to the socket in place of the light-dependent resistor (rightmost pins of the socket - a shorting link is also needed across the leftmost pins to make the hotshoe contacts work). I rigged the digital camera and flash in an unlit room and adjusted the camera to produce a good image at the lowest flash level (potentiometer at zero resistance). Then I reduced the camera's aperture one stop at a time, each time adjusting the potentiometer to give the same good image, i.e. with one stop more flash light, and measuring the resistance value. At the highest flash levels I had to abandon the room and do the tests along the length of the garden! The final test was done with the potentiometer disconnected, equivalent to the sensor's "M" setting.
I plotted resistance values against exposure stops on log-lin graph
paper, and traced a nice smooth curve from which I was able to choose standard
resistor values to cover the full range.
I used a 10-way "digital" switch, connected as shown so that position 0
was the lowest possible level, and 8 (and 9) the highest. For convenience, I included
a second switch to disable the flash - much easier than fiddling about with the
menus in the camera!
The circuit board with the resistors and switches is mounted on the side of the flashgun with a piece of double-sided adhesive foam, and the whole thing is further secured and protected by lots of insulating tape. The connections to the sensor socket are made via a short piece of ribbon cable terminating on a square of veroboard from which wire "pins" protrude into the socket. It seems to be rugged enough, and while not waterproof it's survived a few showers without mishap.
As a footnote, I've since acquired a Vivitar VP-1 "variopower" unit, which plugs into the 283 in place of the "auto thyristor" and does what its name suggests. But the numbers on its continuously-variable control are hard to read, and become very compressed towards the low-power end. I much prefer my solution!