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.