I found a couple of old disposable cameras in storage that I played around with 15 years ago, shorting the caps to make a loud bang, wiring up the flash trigger to a remote-controlled relay kit I had assembled. I thought I’d do something useful with them.
I decided to turn them into optical slave flashes, since on-camera flashes are not very flexible. I was thinking of a way to detect the camera flash so that the slave could be fired, maybe using an LDR with the ADC to detect an increase in light intensity? It turns out there’s an even easier way to do this – with an infrared sensor. Apparently when flash tubes are fired, they give off infrared which can be detected more reliably than light intensity changes. When I read about this, I tested it out with a simple Arduino sketch and it works as advertised.
Disposable cameras usually have metal contacts that are placed near the shutter mechanism. When the shutter opens, the contacts are closed and if the flash was charged it would fire. To control the flash firing, I replaced the contacts with an SCR.