I need to power a ~180W (so about 15A) heating element when the board is exposed to sunlight and there are two HIGH signals from the MCU. I’ve connected a comparator to a phototransistor to generate a digital signal, which is then fed into the second input of an AND gate. However, I am not sure that the comparator will function properly with such voltage levels.

I’m powering the load directly from +12V and have only connected the GND pins of the XT30 connectors to prevent short circuits in case of rain. There are two connectors so it's clear when the circuit is capable of conducting and when it's not (if the cable is disconnected, there will be no current).

Spoiler: PCB layout and schematic

 

The light detector will not work as shown. It needs a resistor from U4 pin 3 to gnd to form a voltage divider with the photo-transistor.

 

The light detector will not work as shown. It needs a resistor from U4 pin 3 to gnd to form a voltage divider with the photo-transistor.

How many Ohms?

 

The value of the resistor will adjust the light level at which the circuit switches. Perhaps start at 1k. You could put a 10k pot here to be able to adjust the brightness level.