This project shows you how a Capacitor's charging and discharging can be used to dim a light (an LED in this case).
When you finish the wiring, set the Switch to the ON (up) position and the LED segments will slowly light up, displaying an "L". In a few seconds the LED will reach its brightest point and will continue to give off a steady light.
Now move the Switch to the OFF (down) position, and the "L" will gradually fade and go dark.
Take a look at the Schematic and see if you can figure out why this happens. Then look at the answer below.
With the Switch ON, current flows from the Battery to charge the 100 uF Capacitor. As the Capacitor approaches full charge, more and more electricity flows to the base of the Transistor, gradually turning it ON (and therefore the LED segments).
When the Capacitor is completely charged, the current continues to flow to the Transistor base, and the LED stays ON.
When, the Switch is turned OFF, the Battery is taken out of the circuit and the Capacitor begins to discharge through the LED. The "L" gradually dims and goes dark when the 100uF is fully discharged.
If you want a really "slow" dimmer circuit, replace the 100uF with the 470uF. You'll have to be patient, but the LED will come ON, eventually.