PAGE 109 OF 150 · MICROCONTROLLERS & LEDS
Driving LEDs with a Transistor from a Pin
A small pin controls a larger load safely.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Boosting a pin
When an LED needs more current than a pin can give, let the pin drive a transistor and let the transistor carry the load.
Example
A 5 V pin through a 2.2 kilohm base resistor gives a base current of (5 − 0.7) ÷ 2,200 ≈ 2 mA. That saturates an NPN transistor carrying about 20 mA, with the LED and its resistor in the collector.
For strips and power LEDs use a logic-level MOSFET, with a 10 kilohm resistor from gate to ground to keep it off at start-up.
REMEMBER
The pin signals; the transistor supplies the power.Try it yourself
Sketch the circuit on paper first, calculate the values, then build it on a solderless breadboard using a low-voltage supply. Measure with a multimeter before you make anything permanent.
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