How do I power an LED from a 2.8V CMOS GPIO pin?
Answer:
I'd use a 2N3904 to drive the LED.
One possible configuration...
2.8V /--------------------
... ... . . . . . . |
.. . . . . . . . . . . . . . . . ---
. . . . . . . . . . . . . . . . . \/ LED
. . . . . . . . . . . . . . . . . --
. . . . . . . . . . . . . . . . . .| c
. . . . . . . . . . . . . . . . ./
.GPIO-- 0-----------| 2N3904
. . . . . . . . . . . . . . b \ e
. . . . . . . . . . . . . . . . . | . . . . . . . . . . .R = 2V / 10ma
. . . . . . . . . . . . . . . . . ------/\/\/\/\/\/--. . . = 0.2 k (or 200 ohms)
. . . . . . . . . . . . . . . . . .. . . . . . . . . . |
. . . . . . . . . . . . . . . . . .. . . . . . . . . . |
. . . . . . . . . . . . . . . . . .. . . . . . . . . . |
. . . . . . . . . . . . . . . . . .. . . . . . . . . Gnd
The point is depending on the type of MCU in general you don't want your MCU supplying the 10 ma.
Put about 68 ohm in series with the LED and remember to wire the LED the right way round, then it should work OK for 2.8V. If it is a complementary ("push-pull") output then you can connect the resistor and LED either from the output to ground, or the output to the supply rail, depending on whether you want the LED to light when the output is high or when it is low. Good luck!
You need to look at several factors.
1. What is the current output specification of the GPIO pin?
2. What is the voltage drop of the LED?
3. What is the current required for the LED?
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