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This is the circuit I plan to implement to control a LED by a logic bit

schematic

simulate this circuit – Schematic created using CircuitLab

The datasheet of the transistor can be found here

I used FET transistor as amplifier, so I know about the saturation zone that let you get a voltage gain. However, I never used fet as switch, therefore correct if I am wrong.

In that zone, the MOSFET behave as a variable resistance. So if I work in 3.3 logic, when I put 1 into the gate, The mosfet will behave as a 2-4 ohms resistance (from the datasheet), where the channel will let pass current up to 50 mA? When I put a 0 into the gate ( Vgs < Vth) , the mosfet will not conduct.

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That looks good.

A few things to consider.

This FET looks good but when using other types be careful about the extremes of gate threshold voltage, many common types such as 2N7000 have a worst case threshold of >3V and so are not suitable for use with 3.3V logic.

The voltage required for the LED varies with the color - red is lowest at ~1.7V green is next at ~2.1 but blue and white may need more than 3V and so can't be driven from a 3.3V power rail without some more complexity (a charge pump for instance).

These days you can get enough light out of an LED for most uses with only 1-2mA.

Since the current requirement is so low you can drive the LED straight from a logic signal (with a resistor) and you don't really need a discrete driver.

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  • \$\begingroup\$ I will only use red & green LED, the world purpose is to show a "is charging" and "is complete" signal. \$\endgroup\$ – MathieuL Aug 7 '15 at 19:43

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