I have a bunch of eBay-sourced DC-DC converters that I use for a bunch of purposes around the house. Most are ordinary “LM2596” (in scare quotes, as most seem to be clones: they’re marked as LM2596 and generally work well, but have different switching frequencies. Supposedly this is an issue with such things.) buck converters configured as adjustable, constant voltage power supplies where the output voltage is set by a multi-turn potentiometer. Very handy.
Others can be used in either constant voltage mode or constant current mode. For the latter, a serpentine strip of PCB trace acts as a low-value sense resistor. An LM358 dual op-amp integrates the difference between the voltage across the trace and a voltage set by a potentiometer, with the output connected to the regulator’s feedback pin via an LED so you can tell when the regulator is in constant current mode. Another potentiometer sets when the “charging” LED lights up; this is purely cosmetic, and the LED turns off when the current through the regulator drops below the setpoint set by the potentiometer.
Caleb Engineering has an excellent teardown of such a regulator here.