Today a couple of guys literally walked up to our place and rang the doorbell, looking for help. They are medical types who know basically nothing about electronics.
They want to plop a still image on a PC screen and simultaneously trigger some external gear. They would like those to be coincident to within maybe a millisecond. This will be a Windows machine, hardly a realtime OS. I don't think I could load a screen and simultaneously bang a serial port pin, or some USB device, accurate to a millisecond. Well, probably not.
I'm helping them for free, so I don't want to launch some big coding or hardware project.
One idea is to tape a photodiode in the corner of their screen. Their image could always include a bright spot there, and we could trigger off that.
Another is to use a laptop that has a video port, like you'd connect to a projector. Then somehow look at the (VGA?) signal and use that as the trigger.
In both cases, they'd have to manually arm the system before a shot so normal PC screens wouldn't trigger the external gear. Make screen black, throw a switch, begin the show. Maybe a serial port pin could be the on/off switch?
A serial or USB port pulse wouldn't have the false alarm problem, but time sync might be tricky. I have no idea how close in time the video and the electrical outputs could be aligned. I guess I could experiment.
Just possibly the VGA image/signal could contain some distinct trigger pattern, like a 7 MHz or some such waveform that I could bandpass filter and trigger on. Maybe RGB at different frequencies? Sounds like work.
These guys have a small chance of doing a lot of good in the world, so I'd like to give them some help, without it becoming a giant project.