I wondered if the subcircuit consisting of the LM317 and resistors Rs and Rc could form the basis for such a circuit if it were driven from a suitable control current. In Figure 1, the first step to ...

While this circuit may not be as efficient as an LED driver module or a switching solution the LM317 is cheap and readily available. [Afroman] also describes how the circuit works in detail ...

Set an LM317 circuit up to output 10mA, say, and you can safely test out about any LED. Read off the operating voltage, subtract that from the supply voltage, and then divide by your desired ...