Found in a 90s documentation, this circuit makes use of D1, a switching diode that is uncommon today. D3 is an LED. The circuit monitors 12 V batteries.
When the voltage exceeds the value given by the zener, this circuit from a 1990 documentation...
This power supply may have 5, 6, 9 or 12 V outputs depending on the 78XX and 79XX series...
In the figure we have a simple 5 V power supply obtained from an American documentation from the...