Hi, Theoertically, this circuit should work, because when water reaches the probe sensor (Level n), it will have a finite conductance and the input to the 7407 hex buffer will register as logic 1. This will lead to its output as logic 1 also, and since 7407 uses positive logic convenetion, this will mean H voltage, and will light up the LED. But I test it out on breadboard, and it doesn't work. I have checked and checked the connections again, but they are all correct. Is there anything wrong with the design?