Make the led work only if a certain voltage exceeded
As the earlier comments and answer pointed out, if you want an accurate indication of voltage level, you need an accurate reference.

The more normal method is to use a voltage reference and multiple comparitors or op-amps to compare your input voltages to the reference.  The output of the comparitors or op-amps would then drive the LEDs.

However, there is a simple trick that is inexpensive and very simple.  It works best with positive voltages but can be made to work with negative voltages with some effort.  Because you want to measure only two positive voltages, I'll offer it as a solution for you.

It uses only 3 resistors, a LED, and a TL431 adjustable shunt regulator per voltage rail to be monitored.  Very simple and quite accurate.







simulate this circuit &ndash; Schematic created using CircuitLab

The TL431 is normally intended to be an adjustable shunt regulator - sort of an adjustable zener diode.  It contains an accurate 2.5V reference and is normally configured so that the cathode connects to the voltage to be regulated.

But it will happily run open-loop.  In that case, the TL431 has a high impedance from the cathode to the anode so long as the adjust pin voltage is less than the internal 2.5V reference.  When the adjust pin voltage is more than the internal 2.5V reference, the cathode has a low impedance between itself and the anode.

It is that low impedance that turns the LED ON.

You will need to set the voltage divider ratio on the adjust pin so that the TL431 conducts when the input voltage is above your desired value.

Duplicate the circuit for each voltage rail to be monitored.