This is the LED pilot light (or LED indicator) circuit. LEDs (Light Emitting Diode) part are interesting things. They only operate at DC supply within specific tolerances, and generally connected with a current limiting resistor towards the power source. Rather than a resistor, it is possible to use a FET (Field Effect Transistor) like the ECG31 and, NTE312 (you may try other types).
When the gate and the source are connected together, it acts as a current regulator.
In the circuit above, the current is constant between 6 and 8 mA at 5 to 30Vdc. If the diode is added (the 1N4148 is optional), this circuit is secured agains polarity reversal and can be connected to a AC power supply of 5 to 20 VAC.
LED Pilot Light using FET, circuit designed by Tony Van Roon