We found this circuit in a 1976 documentation. The FET can be BF245 or MPF102 and the power supply need not be symmetrical. The FET in the feedback acts as an automatic gain control.
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...