✔ 最佳答案
A 22 uF capacitor could only store a very small amount of charge.
Charged stored in capacitor
= 3 x (22 x 10^-6) C = 6.6 x 10^-5 C
Since you did not provide the technicl specification for your LED, assuming the current consumption of your LED is 20 mA, then time to comsume all the charges from the capacitor = (6.6x10^-5)/(20 x 10^-3) s = 3.3 x 10^-3 s, or just about 3 milli-second.
Hence, the LED only light up for 3 ms. This is the reason why you could not observe the LED to glow.
2011-06-06 22:51:36 補充:
To have the LED lit up for longer time, you need to use a large capacitor. For example, to light up for 3 s, a 22 mF capacitor has to be used.