The ever innovative and prolific Mr. Woodward has offered “PWM-programmed LM317 constant current source ... Rs and Rc could form the basis for such a circuit if it were driven from a suitable control ...

In his video [Afroman] describes how the LM317 can be used to regulate current rather than voltage to provide a constant current ... While this circuit may not be as efficient as an LED driver ...

How easy is that? An LED tester can be as easy as a constant-current power supply, and in fact that’s what [Dave]’s first LED tester was, in essence. Set an LM317 circuit up to output 10mA ...