Originally, this circuit was used to sense a low-water level in a Christmas tree stand, but the circuit can be use
d as a water-level detector for pump controls, water sensors (for garden and lawn applications), etc. A comparator and probe setup with a Linear Technology LTC1040CN comparator drives a 2N3906, which switches a tone generator. Sampling occurs every 20 seconds, which minimizes current drain. A pair of dry cells will power the circuit for several months.