DIY Flash Trigger Safe Sync

Most modern cameras allow a max flash trigger voltage of 6 volts (Canon) or 8 volts (Nikon). The trigger voltage runs between the 2 main pins on the hotshoe of a flash or between the pins on the trigger connector of a studio strobe, shorting these pins together fires the flash. Before digital cameras this was done using a mechanical switch attached to the shutter but now this is done electronically (hence the max voltages). Old strobes use a trigger voltage of anywhere between 12 to 350 volts and this can be checked using a multimeter set on volts DC - using anything above the 6/8v will fry or seriously shorten the life of the trigger circuitry in your digital camera. In order to reduce the sync voltage you need a 'safe sync' there are manufacturers in the US that make these but they are pretty easy (and more importantly, really cheap!) to build one yourself. The simplest one I found was http://www.repairfaq.org/sam/zpaofu1.pdf. The main advantages (compared to ones using an optocoupler) are the parts are easily available and no external battery is needed, sync voltages are reduced to ~4v using the circuit. Pretty much any triac will work, I only had 1M resistors instead or a 10M or 2 x 4.7M so my circuit is bigger than it needs to be. I hooked mine circuit up in the middle of a sync extension cable to make it really easy to use. Obviously I suggest you test the circuit just using your strobe before plugging it into your D-SLR!