I'm experimenting with circuits to trigger my camera shutter and camera flash. My plan is to build a 2-stage trigger for high speed photography that triggers the camera a fraction of a second before an event like a balloon popping, and then a second trigger for the flash using sound, vibration, breaking an "electric eye" light beam, etc.
Both cameras and flashes use a 2-wire trigger that applies a DC voltage. When the 2 wires are connected with a near-short, it triggers the flash to fire/camera to take a picture.
Different cameras and flashes use widely varying trigger voltages. Older flashes sometimes use trigger voltages as high as 300 volts, although those are uncommon in modern flashes. It looks like the phototransistor is rated at 250VDC max, so it would probably be destroyed with a flash who's trigger voltage exceeds that level, but that's fine. Better to sacrifice the optocoupler than burn out my controller or my camera.
So far I'm using analog electronics for my trigger circuits, but I plan on using CMOS logic circuits (run at 5VDs) built around a micro controller. The microcontroller will allow me to add things like variable delay, enabling/disabling the whole setup, sequencing the triggering, quenching LED room lights just before opening the shutter, etc, etc.
The circuits I've built so far use a low-power PNP switching transistor like a C1740 to do the final triggering of the flash or camera. (I just connect the flash/camera trigger directly to the collector an emitter of the PNP transistor.) That works fine, but I wanted to provide some protection to both my trigger circuit and to my cameras/flashes. I've been using a 5-6 volt supply for my test analog circuits in anticipation of using regulated 5 volt supplies for my digital circuits.
I use Nikon cameras and flashes, which use very low voltage triggering (According to my meter, my hotshoe flash puts out about 3.6 volts. I haven't measured the trigger voltage from the camera, but I think it's also <= 5VDC.)
I bought some PS2501A-1-A Optocouplers, and am having trouble understanding the data sheet. Here's a link:
Is the "diode" mentioned in the data sheet the LED on the trigger side of the photo coupler?
What is the minimum current needed at 5VDC to trigger the optotransistor "wide open"? It looks to me from the data sheet as if the LED needs 30 mA, but that it can tolerate .5A forward current. Is that right?
If I drive the LED at 30 mA and 5 volts, can I push 250V through the phototransistor? (I don't know how much forward current one of the older high trigger voltage flashes needs to trigger. It's my understanding that it is a transient voltage though. I believe that for these older flashes, the flash feeds it's flash tube voltage through the trigger line to a step-up transformer that generates something like 10kV, which ionizes the xenon gas in the flash tube, triggering the flash. If that's the case the current would be quite high for a tiny fraction of a second.)
If my trigger circuit can't trigger these old flashes that would be ok, although not ideal. At least the optocoupler would prevent damage to the other components in the system.