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.
The indicator instrument of this transistor tester, from an Argentine publication of the 1970s,...
The circuit below is suggested by National Semiconductor in its 1994 Linear Applications Handbook. The...
This well-designed bistable circuit was found in a 1970s documentation. Transistors can be current...