This circuit was found in a 1970s American documentation. It can be applied to almost any regulator. The transistor used admits equivalent, depending on the circuit current.
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...
Found in a Linear Technology 80s integrated circuit manual this circuit has its gain controlled from...
This very low consumption circuit can be powered by a solar cell. The time indication is given by a...