I hope I'm doing this right, but I'm looking to identify the light frequency of the LED on the front of my C5.
I'm considering a photo transistor to identify the state of the hub during a power outage. If it's the red frequency I want to use that to drive a small relay or transistor that fully cuts power to the hub when the mains power returns to automatically power cycle the device for reboot.
To me this feels like one where it is important to understand the outcome being targeted...
Identify C5 Indicator lights
For me I would start by asking the question whether this is the best approach... Not to say it is wrong per se.... just that there may be a simpler or more consistent option available.
It also does not discount the benefit to achieving the ability of detecting light colour accurately...
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Interesting approach. If I'm reading this correctly the OP may have a method to issue a hub shutdown command in the event of a power outage, but wants to ensure that it is still shutdown (or has completed the shutdown) before virtually unplugging it and then restoring power to it.
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You are very much on track, and I'll expand my explanation. I have a battery back-up that will allow it to shut down gently. However after shut down, and once the main power returns I need a way to power cycle the device.
One way to do this may be using a photo transistor that reacts to only that light frequency. I siphon a bit of current from my 5Vdc power supply from the mains to that transistor, and if that red light is lit it will allow power to pass to a relay or similar circuit that will cut off ALL power to the hub for just long enough to allow it to reboot.
Now if the outage has been long enough that even standby power has drained the battery, then power cycle for reboot will happen naturally.
Other options being explored are current sensing, or even other time delay relay circuits feeding into each other.
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