This schematic diagram shows a water level sensor/detector/monitor circuit. An alarm is also featured in this circuit. Any fluid with a resistance under 900K between the maximum separation distance of the probes will trigger this circuit. A 4050B CMOS hex buffer is used on this circuit that is working at 5 volt supply. The 10M resistors connected between buffer input and ground bias all gates off.
The “common” probe the topmost probe above probe 1 in the schematic diagram above is connected to the positive 5 volt supply. The top gate is activated and the LED 1 will light, when probe 1 is spaced 1 cm away from the common probe and tap water at 25 C is detected between the probes (a resistance of 20k). Similarly if probe 2 at 2 cm distance from the common probe detects water, LED 2 will light and so on. To select which output from the hex buffer, this circuit use Switch 1. It will trigger audible oscillator made from the gates of a CMOS 4011B IC. [Circuit’s schematic diagram source: www.zen22142.zen.co.uk]