You should probably get yourself a multimeter. I'm guessing the 5V is dropping.
Hey forum, im stuck with a problem that i can't deal by myself. In a couple words, my LCD brightness is darker when my relay is turned off but when i turn it on the screen becomes normal. There's some pictures below that i took and my code
If you are using the NO relay contacts (I can't see your picture clearly) so the lights go on when the relay is energized, it should be the other way around... However, if you are using the NC contacts, the lights will be on when the relay is de-energized... It's (probably) related to current through the relay coil "sucking down the voltage", and unrelated to the 120VAC lights being on or off.
the only different is the add of a water sensor DS18B20.
I'll betcha' if you disconnect the sensor it makes no difference. Unless you've wired the sensor wrong, it shouldn't draw much current and shouldn't affect the power supply.