The circuit shown in the figure can be designed with more modern operational amplifiers, as it is from 1994 documentation by National Semiconductor. The power supply must be symmetrical.
We found this circuit in an old American publication. The transistors can be BC558 and diode 1N34....
This circuit will trigger an audible warning signal after the end of the pre-programmed timing. Using...
This is the commercial circuit for the TRF-44 amplifier sold in Japan in the 1960s. The circuit...