Voltage is greater than 12V
current flows through the Z -Diode 11V and the series resistor.
T1 is open and T 2 blocked, the LED lights not up.
If the rated voltage below,
then the Z-Diode current rapidly and the voltage drops.
T 1 is locked and T 2 opens,
about 20 mA flow through the LED diode so that it lights up.
