First I wanted to show that the concept I thought of works. To do that, I hooked up the laser diode and photoresistor from my sensor kit to an Arduino Nano on a breadboard.

Testing components

I detected the changing resistance when the laser hit the photoresistor and when it didn't. I added a RGB LED and an active buzzer as alarm signals and wrote some code for my Arduino. Now the LED turned red when the laser didn't hit the sensor and green when it did. Changes were accompanied by some buzzing sounds.

To make things clearer, I changed the buzzing sound to a high-pitched beep when the laser didn't hit the sensor anymore (water needs refill!). When it hit the sensor again (water is refilled), the LED blinks blue three times with electronic buzzing to indicate that everything is OK now.

Testing application

Next up, I tested this setup on the water tank of my coffeemaker. Luckily, it worked great! When I position sensor and laser in such a way that the light hits the sensor beneath water surface, it will cease to do so above the water surface. I refined the "ON" and "OFF" thresholds for the photoresistor so that ambient light and dimming of the laser by water and plastic would be compensated.