Is there any advantage to biasing a transistor with a zener diode rather than a (resistor-based) voltage divider? - Electrical Engineering Stack Exchange
Voltage Regulator using Transistor
Voltage Regulator Circuits using Transistor and Zener Diode | Homemade Circuit Projects
Confused about battery voltage warning circuit with PNP transistor and Zener diode - Electrical Engineering Stack Exchange
Simple Overvoltage Protection Circuit using Zener Diode