Need to run a few new super bright LEDs, from a low voltage source. This will do it using the old reliable MC34063 switching supply chip. Modern GaN superbright LEDs (White, Blue, and true Green), run at higher voltages than the red and yellow LEDs. Often as high as 4.5V. That gives you little head room, if you want to run them off a 5V supply. Using this circuit you can tie a bunch of them in series, so they’ll all get the same current. Tie as many together, as long as the total voltage drop is less than 40V, the MC34063 limit. The circuit is rigged to regulate current, not voltage. That’s what makes LEDs happy. As shown, that current is 18mA. A safe value. The circuit will regulate to 18mA, and the voltage at the top LED will be whatever it needs to be to get 18mA thru.
