Led Pilot Light

Led Pilot Light by Tony van Roon, CET

LED's are funny things. They only work at Vdc within specific tolerances, and normally connected with a current limiting resistor to the powersource. Instead of a resistor, you can use a FET (Field Effect Transistor) such as the ECG312, NTE312, or others.
When the gate and the source are connected together, it behaves as a current regulator. In the circuit above the current is constant between 6 and 8 mA at 5 to 30Vdc.
If a diode is added (the 1N4148 is optional) this circuit is protected agains polarity reversal and can be connected to a AC source of 5 to 20 Vac.