It depends on what the LED is connected to. Without a current-limiting resistor, you can blow either the LED or the thing that's driving it. If you're connecting it to a pin - say for example, to show the state of an LFO - then you should probably use a transistor to drive it otherwise it will affect the circuit driving it.
You don't really need to do much arithmetic to work out the series resistor. For 9V, somewhere around 1k is probably about right. For 5V, somewhere around 470 ohms is about right. Try different values until you get a brightness level you like. The LED will have some data which tells you the rated current and forward voltage, from which you can calculate the resistance you need:
Vf = 2.5V, If = 20mA
So for a 9V supply you want to drop 6.5V with a current of 20mA. Plug that into your magic triangle, giving:
R = V / I = 6.5 / 0.02 = 325 ohms, for a fairly high-power LED. Obviously you can't get 325 ohm resistors, but 330 is probably close enough.
Most modern LEDs are "low current" ones, so they might take typically as little as 2mA. So, once again:
R = 1.7 / 0.002 = 850 ohms so somewhere between 820 ohms and 1k will be just fine.
Bear in mind that with multiplexed LEDs they are only on for a short period of time, so they can be run at significantly higher current.
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Topic: Adding LEDs (Read 5543 times)