This circuit will emit an intermittent beep (or will flash a LED) when the water contained into a recipient has reached the desired level. It should be mounted on top of the recipient (e.g. a plastic tank) by means of two crocodile clips, acting also as probes. If a deeper sensing level is needed, the clips can be extended by means of two pieces of stiff wire.
IC1, a 555 CMos timer chip, is wired as an astable multivibrator whose operating frequency is set by C1, R1 and R2, plus the resistance presented by water across the probes. If the resistance across the probes is zero (i.e. probes shorted), the output fre