Freeze Alarm (CB14454E)

The circuit of a 1968 publication can be implemented with more modern transistors such as BC548 for NPN and BC558 for PNP.

two-tone generator (CB22455E)
T Found in a 1992 Popular Electronics, this sophisticated circuit uses two speakers on a different...
Electret Microphone Preamplifier (CB17046E)
We found this circuit in a 1995 documentation. It is a two-channel preamplifier for supplying a...
timer (CB12115E)
This timer was found in a 1980 publication. With the BC548 for Q1, an 8-ohm speaker with 6 V power...