TL430 and TL431 integrated circuits manufactured by Texas Instruments, are active zener diode, with 2.5 V internal reference, and comparator output stage. Maximum supply voltage is 30V, 100 mA. This electronic device makes the practical realization of a voltage monitor for variable power supply.Resistance R1 has the value (Uin-4.5) / 10, where the factor of 10 indicates a current of 10 mA through the LED. Depending on the setting of P1, LED lights when the input voltage is too high and off when it gets too small.
Using two identical assemblies with different colored LEDs, you get a real voltage monitor.