Here is a good tutorial on using transistors and LEDs. Transistors can be used as switches, or they can be used as amplifiers. Of course LEDs have a polarity, and only work in one direction. It is important to match the correct resistor to the LED as well. You don’t want to connect it straight to a battery, you need to limit the current going through the LED or you may fry it. I found this article through my friend over at zedomax
To make an LED work, you need a voltage supply and a resistor. If you try to use an LED without a resistor, you will probably burn out the LED. The LED has very little resistance so large amounts of current will try to flow through it unless you limit the current with a resistor. If you try to use an LED without a power supply, you will be highly disappointed.
You can find this tutorial here
If you need help finding the right resistor, here is a caculator program.
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