This is a mod that allows you to effectively use an electronic flash with cameras built before the 1970s. However, before I get into why this mod works, I should explain briefly why electronic flashes typically do not work with old cameras.
Before the 1970s, cameras that operated with a flash used flash bulbs. Unlike electronic flashes which fire seemingly instantaneously, flash bulbs had a slight delay between being triggered and flashing. The reason for this delay was that the filament within the flash bulb needed a few milliseconds to heat up before illuminating. To compensate for this, the camera manufacturers built in a (typically) mechanical shutter delay that allowed the shutter and flash to both occur simultaneously. The length of this delay is dependent on the type of flash bulb used, since all of the bulbs have slightly different response times. For instance, the m-series flash bulbs used by a number of classic cameras (such as the Polaroid Land Camera) had a .026 second (26 millisecond) delay. This common delay is referred to as m-sync because it means the camera was synchronized to work with m-series flash bulbs.
If you have any technical aptitude, it should now be very apparent why an electronic flash won't work with older camera. Basically, when you press the shutter, it will trigger the electronic flash, which will immediately illuminate, and then a few milliseconds later, the shutter will open. By the time the shutter has opened, the flash has already happened and is either finished or greatly decayed. This results in not enough light reaching the film. In more advanced cameras that have light sensors to detect the flash (like the Polaroid Land Camera), this will result in the exposure always being off, as the flash will always happen before the sensor has the chance to register it.
The fix for this timing problem is relatively simple. Since the shutter built into the camera can not be sped up, the electronic flash needs to be slowed down. Fortunately, the circuit for doing this is both pretty simple and pretty small. This is good because it means that it can be easily squished into a cumbersome old electronic flash unit.
I am calling this an m-sync hack because I am using it to work with an m-sync camera (26 millisecond delay). However, I have built the circuit with a trim pot such that you can set a delay between 0 and 51 milliseconds. This should allow you to use it with just about any camera that operates on a flash bulb.
Step 1: Go Get Stuff
Step 2: Open It Up
Remove the batteries if you have not done so already.
Open up the flash by removing all of the screws and carefully prying the case apart.
Note that some of the screws may be hidden under stickers.
Once you get the case open, be careful not to touch any of the circuitry. The flash needs to be properly discharged before you can do that.
Step 3: Discharge
Press the 'flash test' button to trigger the flash and get rid of the bulk of the charge.
Keep in mind that even though the flash went off, there can still be a pretty nasty charge stored up in the capacitor.
To get rid of the remaining excess charge in the capacitor, bridge both capacitor pins with a long screwdriver that has an insulated handle.
Step 4: Remove the Jack
Desolder the 3/32" jack from the circuit board.
Solder a red and black wire to each of the terminal pins.
Notice that my jack had an extra thin black wire going to the board. This black wire was from an internal switch that was closed to complete the flash circuit when the plug was inserted. When the plug was not inserted, the switch was open and it defaulted to the hot shoe attachment.
I had no plans to use the hot shoe, so I disconnected those wires from the board and connected the thin black wire to the large black wire terminal on the underside of the board. This means that when I connect the red and black wires together (the ones I attached), the flash will trigger. If I had not connected the thin black wire in this manner, the flash would not have triggered when I pressed the wires together because the (now nonexistent) switch would have still been open.
Step 5: Trim the Board
Trim the perf board to be small enough to fit inside the flash casing and just large enough to hold the entirety of the circuitry.
I used a guillotine paper cutter to do this, but barring one of those, you can get the same results with a good pair of scissors.
Step 6: Program the Chip
Program the chip using this code:
Step 7: Build the Circuit
Build the circuit as specified in the schematic.
Do not go solely by these pictures because I made some of the critical bus (power/ground) connections on the underside of the board.
If you have a 6V flash, you will want to add the 3.3v regulator and the 10uF capacitor.
Even should you not need to use the regulator, you may want to consider adding a 10uF capacitor between power and ground.
Step 8: Jack It
Install the new 3/32" jack in the spot where the previous one used to be.
Widen the mounting hole with a drill if necessary.
Step 9: Shutter Switch
Connect one of the terminals on the jack to pin 6 on the Arduino.
Connect the other terminal to +3.3V on the board (or whatever other voltage you are using to power the chip).
Step 10: Wire the Flash
Connect the red wire that you soldered to the flash terminal to pin 6.
Connect the black wire to pin 4.
Step 11: Power
Connect the ground terminal from the battery connector to the ground on the circuit board.
Find the power switch for the flash.
Connect the switch's far terminal (the one that receives power when the switch is toggled on) to the V-in pin on the voltage regulator. If you are not using a voltage regulator, simply wire it to the power bus on the circuit board.
Step 12: Drill
Drill a 1/4" hole in the case directly over where the trim pot will be located. This will allow you to fine tune the timing with a screwdriver without having to open the case.
Step 13: Reassemble
Put the case back together.
Make certain the hole lines up with the trimpot before you screw it shut. If it does not line up, install a little bit of foam to move the circuit board around and position the trimpot into place.
When all looks good, reinstall the screws and put all of the labeling back on.
Finally, reinstall the batteries.
You should now be good to go.
Step 14: Fine Tuning
M-Sync can be found roughly a little past twelve o'clock on the trimpot's dial.
Adjust to suit your needs and tastes.