A circuit that offers visual indication of fluid level in a vessel, with a switchable audible alarm. Example uses would be to monitor the level of water in a bath or cold storage tank.
This circuit will trigger with any fluid with a resistance under 900K between the maximum separation distance of the probes. Let me explain further. The circuit uses a 4050B CMOS hex buffer working on a 5 volt supply. All gates are biased off by the 10M resistors connected between ground and buffer input. The “common” probe the topmost probe above probe 1 in the diagram above is connected to the positive 5 volt supply. If probe 1 is spaced 1 cm away from the common probe and tap water at 25 Â° C is detected between the probes (a resistance of 20k) then the top gate is activated and the LED 1 will light. Similarly if probe 2 at 2 cm distance from the common probe detects water, LED 2 will light and so on. Switch 1 is used to select which output from the hex buffer will trigger the audible oscillator made from the gates of a CMOS 4011B IC.
Read More Source: Circuit : Andy Collinson