Simple method to pulse Blue LEDs from sub-Vf voltage
If you have a microcontroller, then from the top of my head:







simulate this circuit &ndash; Schematic created using CircuitLab

To light the LED, set the "DRV" pin to output a "0" then pulse the "CP" pin with a square wave. This makes a crude voltage doubler / charge pump.

The LED current limit comes from the fact that the cap will transfer a certain amount of charge per pulse, and charge multiplied by frequency (ie charge per second) is current. Also your micro's IO pins will have 20-50 ohm output resistance. So you don't need a resistor... you can add one if you want.

That's gotta be the cheapest way to do it. Efficiency won't be that good, but it will light your LED. If you only want to light it once in a while as an indicator, not keep it lit for a long time to make a flashlight, it'll be fine. You can also dim it by adjusting the frequency.

Not bad for 2 parts ;)