Introduction: How to Modify Canon 155a Flash to Make It Fully Manual

I have an old Canon 155a flash that is fully functional but it's an automatic flash that calculates the output power based on the available light. It works great with AE-1 film camera that it came with, but it is practically useless with the newer DSLRs. So, after I saw how other people modded their flashes (lot's of online examples of the Vivitar 283 mod), I decided to investigate if it was possible for my Canon unit. And indeed it was.

How does it work? In automatic flashes there is usually a photo cell that detects light levels. All a photo cell is, is a light sensitive resistor. Remove the cell and replace it with appropriate resistors and voila - you have a full manual control. For the large electrical resistances the flash is more intense and for the small resistances it's weaker.
I suppose it should work with similar flashes - 199A, 188A, 177A, 166A, 133A, 577G and 533G which have the photo cell. In fact, the principle is the same for all the flashes that use the photo cell, of course the resistors would differ.

Step 1: Tools and Materials

- a set of small "jeweler's" screwdrivers
- tweezers
- soldering iron and solder
- resistors (10k, 5k, 2k, 1k, 450, 250, 150, 50 [ohms])
- a rotary 12 position 2-pole switch (and a knob)
- some wire and shrink wrap
- a multimeter
- a "third hand" thing
- casing of some sort - I used a film canister
- epoxy or hot glue
- drill

Step 2: Disassembling the Unit

DISCLAIMER: if you disassemble the unit, you do this at your own risk!


The unit uses large capacitors for storing the electrical energy. Capacitors can stay charged for a long time after you turned of the device and removed the batteries. If it discharges on your fingers it can be very painful(as I experienced) and dangerous! So, be very careful not to tuch the bare wires on the opened unit. Read this: as it applies to my intractable as well.

To disassemble the unit you need to remove four small screws at the base of the hot shoe, and one of the two screws in the battery compartment. You can leave the other one inside because the screws are actually holding each other via small metal rod (You'll understand when you open it), and it is easier to reassemble the unit this way. Now you can pop it open using some force. Be careful not to break the wire connections as these are now the only thing holding the two parts of the flash together.

Step 3: Replacing the Photo Cell With Wires

Carefully unsolder the photo cell contacts from the small circuit board. The cell is marked red in the photo below. Use the tweezers to gently pull on the cell as you apply heat to the soldered contact. Be careful not to unsolder other components as some are soldered to the same spot as the cell. Try to be quick so that you don't overheat the board and surrounding components.
Once you removed the cell, solder two about 10cm long wires to the spot where the cell's contacts were. Use shrink wrap to protect the contact.

Step 4: Reassembling the Unit

Using a screw driver or similar tool punch out a little plastic window that protected the cell. Pull the wires you soldered in the previous step through the hole you've just made. Reassemble the unit. You may have some space issues, but organize the wires so that they don't get in the way.

Step 5: Assembling the Control Module

I did some experimentation with variable resistors to get the values of resistance for different output powers. I came to this results:
- full power - 10kohm or greater resistor. (to be sure you can leave open circuit)
- 1/2 power - 5 kohm
- 1/4 power - 2 kohm
- 1/8 power - 1 kohm
- 1/16 power - 450 ohm
- 1/32 power - 250 ohm
- 1/64 power - 150 ohm
- 1/128 power - 50 ohm (you can even short circuit this because it's very close to minimum power)

You should take this results with a grain of salt because they are not that accurate. According to this you would get 7 stops of control, which sounds nice but more experimentation is needed to confirm this.

To assemble the module, solder one end of each of the resistors to the numbered contacts of the rotary switch and join the other ends of the resistors together. If you want open circuit just leave the contact empty and for short circuit put a piece of wire instead of the resistor. Isolate parts that could short out with shrink wrap or electrical tape. Later you'll connect the wires you soldered in step 3 - one to the central contact of the switch and other to the place where all the resistors meet.
You can connect the photo cell you removed from the unit in step 3 to one of the contacts (as I planned, but dropped the idea), to keep the original "auto" functionality of the flash.

You can even drop the rotary switch design and replace it with a variable resistors to get the full precise control of the output power.

Step 6: Fitting the Module to a Casing

For the casing I used a black film canister. I've cut the canister to size then drilled the hole in the bottom for the operating knob. The module fitted in the canister quite snugly. Now is a good time to trim the operating pole to size and fit the knob.
I've cut an opening in the cap for the little bulge around the ex-photo-cell hole, then I glued the cap of the canister upside down to the flash body so I could later just pop on the module.

Step 7: Joining the Control Module and the Flash Unit

This step is very simple. Just solder one wire protruding from the flash body to a central contact of the rotary switch and the other wire to the place where all the resistors ends meet. Pack the wires in, and pop the module to a cap. Glue it in place with a few dabs of hot glue so you can easily reopen it if you wish. finally put a label on the front of the module. I've included a label image here, it is also in psd format so you can modify it to your liking.

That's it, you have a fully functional manual flash unit!

This concludes my first instructable. Hope you liked it.

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