Door buzzer or alarm (CB20492E)

This solar powered circuit was found in a 1991 documentation. The transistor is a general purpose NPN.

Ultra Sensitive Photo Trigger (CB17159E)
This fast-acting circuit uses any phototransistor as a sensor. The circuit drives a 30 V relay, but...
Digital Volume Control Treble and Bass Balance (CB4576E)
This circuit is from 1995 documentation. Integrated circuits are used which can still be found...
Stair signal generator (CB9144E)
This circuit was found in a Popular Electronics from the 90s. The circuit uses common components...