The original idea comes from Laurier's Handy Dandy Little Circuits.
Step 1: Step 1: Get the Components!
1 Resistor (try different values, in this tutorial I used a 510 Ohm resistor)
1 LED (Pretty much any single color, normal 20mA LED.)
1 Capacitor (Yet again, try different values, must widthstand atleast 16V. In this tutorial I used a 470µF one.)
1 2N2222 transistor (I have not tested the 2N2222A type, so not guaranteed to work, but feel free to experiment with different types.)
And a variable powersupply! (So you can vary the voltage you power the circuit with.)
Step 2: Step 2: Put It Together!
From positive on the powersupply, connect the resistor, other end of the resistor goes to the anode on the LED, cathode goes to positive pin on the capacitor, and the negative capacitor pin goes to ground.
The 2N2222 are then connected as follows: Collector to *ground* and emitter to *positive* pin on the capacitor. Leave the base unconnected, bent upwards (just for the look). :)