This circuit was found in a 1966 General Electric documentation. It makes use of direct contact water sensors and therefore requires an isolation or low voltage relay drive transformer.
This circuit is from an old documentation. However, it can be designed based on modern transistors...
I found this circuit in a Popular Electronics from 1992. The circuit shows how we can modulate a lamp...
This circuit needs only 0.7 V at 40 uA to drive the relay. We found it in the November 1993...