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Hacking the Vivitar Zoom Thyristor 3500 flash

Manual power hack

How to hack the Vivitar 3500 flash

I've been looking for a cheap flash to mess around with for one of the projects (don't wanna fry my 580ex ii) So having seen a manual power mod in one of the strobist posts a while ago, I decided to have a go myself so I bought the Vivitar Zoom Thyristor 3500 on ebay, the plan was to hardwire it into a microcontroller but as it happens, it works ok with my dslr too which is handy.

One particularly interesting thing about this flash is the control module which is connected using snap-on lock and communicates via 6 pins. It would be interesting to figure out what each one does (hook it up to an Arduino,  see where I'm going?) Please email me if you've got any info. ilya at
UPD: got the rough pinout

Manual power hack

Now the great thing about this hack is that it'll probably work with other automatic strobes too. Just this one seems to have enough space inside for the potentiometer knob and the removable plastic cap which makes it very easy to do.

It is a great way to get a manual flash on the cheap, and it can also be modded to use a different power source for faster recycling. I've triggered it with my 40d and they both still work, but don't take my word for it:

Follow the instructions below at your own risk.

You will need:

If you use a smaller value potentiometer, you will only be able to control the weaker side of the power range.

You will also need a fine phillips screwdriver, soldering iron and some solder.

Vivitar 3500

Disconnect the control module from the flash by pressing the button on the side.

Vivitar 3500

Remove the 4 small screws and pull gently, there is a ribbon cable holding the pieces together.

Vivitar 3500 remove screws

Vivitar 3500 remove screws

On the front side of the unit there is a phototransistor that needs removing. (This will mean flash won't be able to work automatically. If you need manual and automatic you can drill a new hole for the knob and use a switch in parallel with the phototransistor)

Vivitar 3500 remove sensor

If there is any glue, cut through it with a stanley and press the bits of plastic together. The sensor should just pop out.

Vivitar 3500 mount potentiometer

Now you can desolder the wires and prepare to mount the potentiometer. The potentiometer has a pin on the front to stop it from moving. Drill a small hole for it (or burn one with a soldering iron.)

Vivitar 3500 mount potentiometer

Remove the nut and the washer from the potentiometer and solder it to the wires from the light sensor. Then mount.

Vivitar 3500 mount potentiometer

Now carefully reposition the top, so that the ribbon cable is not pressed between the top and the potentiometer.

Vivitar 3500 put it back together

Make sure that the switch fits into the plastic runner, press the top and reposition the nut.

Well done, now test.

Vivitar 3500 test

Woo-hoo manual control flash for under £10


If your flash fires full power with the control module attached, you've most probably got a connection problem. One of the units I've got had a bit of glue between the pcb an the spring of the pin which was easy to fix by cleaning the contacts (just use a pencil eraser)

If your flash doesn't fire and the pilot light doesn't come on but you can hear the whining sound  — try new batteries. If not then most likely one of the capacitors inside the flash needs replacing but make sure you know what you are doing.

If the flash doesn't power on and doesn't whine — try to clean contacts with sand paper including the battery door. Old batteries can leak if left inside the flash which'll most likely prevent it from charging.