This project is also from a few years back, but I decided to post it here.I made a nice instructable which you can check out.
Older cameras used to mechanically trigger the flash and thus there can be a trigger voltage in the 100s of Volts coming from older flashes. New DSLRs have electronics (usually triacs) inside to trigger the flash and might be damaged by these high voltages.
It’s a little circuit which lets you safely use old flashes on modern DSLR cameras.
It protects your camera using an optocoupler from high trigger voltages in old flashes. It is completely safe and will protect the camera’s circuits.