LEDs (Light Emitting Diode) part are interesting things. They only operate at VDC 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 against polarity reversal and can be connected to a AC power supply of 5 to 20 VAC.
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