What is the idea behind all those diodes on the inputs and outputs of the opto isolators? Those are just waste of parts. If your proximity sensors are fed with 24V then calculate the opto isolator R-value for that and that's it. No need for any other diodes and resistors.

That is, unless they are LEDs, in which case I would only have them on the outputs (inputs of UC300). I would use very high intensity LEDs which require very low current ( ~1-2mA or less ) for normal conditions. I am a big fan of 100,000 mcd green or blue LEDs and use them even for purposes where high intensity light is not needed because they require such low current for normal conditions that it is almost unbelievable.



Here you can see what I mean. The green LED is driven through a 51k resistor and the blue one via a 6.8k from 12V. Note that the room is very well lit.