Hmm.. sometimes the simplest solution is the best solution. But i have a disbelief, that it can be that easy.

The Task is: If The Supply Voltage is above a certain level AND the Light Level is below a certain level, light up a LED or an other load.

Theory of Operation: If the level at Pin 3 of the LM324 is higher than 0.7V (Diode Voltage) Output at Pin 1 will raise to V+. If the Level at Pin 6 is lower than the Level of Pin 5 the Output at Pin 7 will drop to GND. If those two conditions are true, the Optocoupler will get conductive ant LED1 will light up. The Parts are not the final decision, i just took some from the EAGLE Library ;-)

Here you find the Schematic:

and a board proposal:

What do you think? Will it work as desired?

UPDATE 18.12.14: Yepp it is :-)

With a few small changes it works as desired :) I'm very pleased by this outcome.

I think I'm using it soon in a Project with the ATMEGA Hearth :)

Unfortunately I'm going to use a bread board for the build (due to money and equippement), but with the Layout above it will not be that hard :) (But there is always place to improve :))

Comments are as alway very appreciated ;)

Update 16.02.2015: I've built the circuit on perfboard :) But not sure if i can show it off ;-P