The sensors of this liquid alarm are formed by two pieces of wire with the ends peeled off. The relay depends on the supply voltage that can be between 3 and 12 V. The transistors can also be the BC548.
This circuit is from an old manual of operational amplifiers. The setting can be adapted to work...
The operational amplifiers used on this Motorola circuit from the 1970s are no longer common, but...
This circuit uses a feedback system that latches the relay when closed. The circuit uses an...