What will do if you want a low voltage less than 3 volts as reference voltage for a circuit? Usually we use a low volt Zener to give low voltage to a particular section of the circuit. But Zener below 3.1 V is not available. So do this simple method. The LED can act a Zener to give a reference voltage. Each type of LED has a forward voltage drop that range between 1.7 V to 3.3 V. This forward voltage drop slightly increases when the input voltage increases. This is due to the increase in the luminosity of the LED. So let us exploit the LED to do the job of the Zener.
Nothing complicated. Connect the LED with current limiting resistor and tap the voltage from the junction of resistor and the Anode of LED. You will get the reference voltage. Reference voltage from three typical LEDs are shown in the figure. You can use other LEDs for different reference voltages. Images below show the forward voltage drop of all LEDs now available. The current limiting resistor 470 Ohms limits the LED current to around 10 mA.
Note – The reference voltage measured here is using a 5 volt regulated power supply. If the input voltage is 9 or 12 V, the forward voltage drop will increase 0.1 volt to 0.3 volts. For example, here the Red LED gives 1.7 V at 5 volt input. It will be 1.8 V if the input voltage is 9 or 12 V.
Images showing the forward voltage drop of different LEDs at 5 volt regulated power supply