We found this circuit in a documentation for radio amateurs from the 70s. The transistors can be the BC548. With it, small speakers of 4 or 8 ohms can be used as microphones.
This circuit interrupts the power supply when grounding is missing. The circuit is from an old...
We found this circuit in an old American documentation. The circuit applies to modern equivalent...
The trigger shown in the figure, built around a 4093 can generate pulses that reach 1 hour with a...