Galvanic Separation / Isolation
When building things like camera or flash triggers, one of the most important things in my design plan is starting with the bit that separates the camera from the actual trigger and working back from there.
A lot of the time, my choice of equipment for this is an opto-isolater, usually the PC817, but sometimes others; and whilst using an opto-isolator on the triggering line is slower by microseconds and centimes more expensive than a 2N2222 or another NPN transistor, I’d rather lose a couple of thousands of a second instead of running the risk of cooking one thing with something else.
So what is Galvanic Separation? Well, in simple terms it’s being able to send a signal between two sides of a circuit without sending stray current. In design, if you think the ground potential of either side of the circuit could be different, separate.
Whilst it may not seem much connecting one circuit that is running at around 5v to another one that is running around 5v (in the case of a lightning trigger connected to one of my Nikons, as an example) having a “gap” between the two means less risk of one side, or the other, or both popping magic smoke.
When it comes to flash triggers, some of the old flashes have a voltage in the order of hundreds of volts at the pin, and the x-sync contact on a modern camera sits in the single digit range, so that’s boom waiting to happen right there if there’s no separation.
When designing a circuit, there are two considerations, the speed of the switching and the voltage being switched; most of the time an opto-isolator is quick enough and depending on the variety can handle the voltage, but there are also reed-switches (like the Coto 9000) or full on relays.
In the long term, learning about how to safely isolate one side of a circuit to another, even though quite a lot of tutorials on line will recommend a simple NPN is time well spent. I know an opto-isolator or reed switch may take a bit more space on a circuit board, may cost a few centimes more, may need a different resistance value depending on how it’s being driven but learn when not to go down the simplest and cheapest route, especially if you are attaching what you are building to something you care about.