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.

Amplifier with the 4049 (CB4770E)
This different audio amplifier was found in a 90's Electronic Design. The circuit uses components...
Ice Warning on the Track (CB6046E)
We found this circuit in an 80's Radio Electronics. The circuit uses a PTC sensor and an LED...
Tone Recognizer Link 567 (CB19312E)
The receiver circuit LM567 recognizes the tone set on 555. The connection between the circuits can be made...