Water Level Indicator (CB6268E)

This circuit was found in an English publication from the 80's. It triggers a buzzer (with oscillator) when water touches the sensor probes.

Simple Photometer (CB523E)
This simple photometer uses a 0-200 µA galvanometer as indicator. The sensor is a common LDR and...
Function Generator (CB4011E)
This circuit is from the Motorola Linear Databook of 1990 and can be implemented with other...
Lights On Reminder (CB4784E)
This circuit triggers a warning oscillator if you forget the lights on when you leave the car. The...