What is required to trigger an optical slave?

Question

I previously asked a question about how to trigger a flash unit with a smartphone such as an iPhone here: Can I trigger my Canon flash with an iPhone?. The top answer suggested that I simply put the flash into optical mode, which it does not have(see: Do Canon 430EXII speedlite flash units have an optical slave function?).

Much to my surprise, when I did get a flash that has optical triggering capabilities(Yongnuo YN 560 III for example) I found that multiple different smartphone LED's did not trigger the optical sensor on my flash.

As noted in this answer, smartphone apps exist to work the opposite way - but why exactly does a smartphone LED not trigger the optical sensor in my Yongnuo flash? What is needed to trigger one such as on my Yongnuo flash if it differs by brand.

Answer 1

It could be that the LED flash doesn't ramp up fast enough to register the flash, or put out enough light when it does flash.

Usually, optical triggers detect the rate of change of the light level to detect when a flash has occurred (as opposed to an increase in the ambient light level).

I assume that you have the light sensor on the flash reasonably well facing the phone?

You could always try building your own optical trigger, as that would give you the scope to swap out components to alter the sensitivity of the trigger. I've successfully used this circuit before (both optically and sound-triggered): DIY trigger.