This circuit is from an American 80's documentation. The circuit indicates when the voltage of a 9 V battery drops below 7 V. The transistors can be BC548.
This circuit was obtained in a 90s documentation. However, it can be developed with modern...
This circuit uses a 455 kHz crystal or for close frequencies. We found it in documentation from...
This circuit turns off after a time that depends on C1. The circuit is from an Electronics Now...