I would like to monitor a digital sign to make sure it stays 'online' – the setup is a Raspberry Pi connected via HDMI to an LED TV. I have seen some general linux discussion about polling HDMI-attached devices but have not not yet been able to get this working. Since the GPIO pins exist can I just use a "physical" solution and check to see if the TV is on and producing output by taping a small LDR to the corner of the screen?
One caveat is that the slideshow has a black background, and so I'm wondering if there will be enough "light" from this to definitively register the difference between "on" and "off" with the LDR.
I just ordered some cheap LDR's and am planning to taking some readings when they arrive, but am I headed in the right direction? If not, what are some options I could use instead of an LDR?
Monitoring something does not mean there is a mechanism to "make sure it stays 'online' " - that needs some form of interaction with a control but, assuming you mean you just want to detect if it goes off-line then, why not monitor the AC current it takes - if it goes off-line, almost for sure the current taken down its power cable will reduce significantly and this, in the case of an AC power feed, is easy to do with a current transformer on either the live or neutral wire. It could also be done with a hall effect sensor and this would work if the power source is DC.