An optical slave usually isn’t looking for “any light.” It typically detects a very fast, sudden change in light level—the kind produced by a photographic flash tube. A smartphone LED may fail because it doesn’t get bright enough, doesn’t change quickly enough, or ramps up in a way the slave sensor doesn’t recognize as a flash event.

So the issue is less about brand and more about the trigger signal: optical slaves are generally designed to respond to a brief, intense flash pulse, not a continuous or slower-ramping LED light. Sensor direction and line of sight also matter, so make sure the slave sensor is aimed toward the triggering light.

If you need reliable triggering from a phone, a standard optical slave may not be the right tool. In practice, they work best with another flash. If you want a custom solution, a DIY optical trigger can be built with different components to change sensitivity and response behavior.