This circuit indicates when the voltage of a 12 V battery is low. With changes, for example, the zener, the circuit can be used to monitor other voltages.
This simple expandable circuit was obtained from a Popular Electronics from the 80's. The supply...
The frequency of the LC resonant circuit must be the same as that of the crystal and the JFET...
This circuit was obtained in a Popular Electronics of 1991. The magazine no longer exists, but the...