PAGE 108 OF 150 · MICROCONTROLLERS & LEDS
Sink vs Source Current
A pin can supply current or draw it.
LED SYMBOLTriangle points from anode to cathode; the arrows show light leaving the part.
Two directions
A pin can supply current out, called sourcing, or accept current in, called sinking.
- Sourcing: the pin goes high, current flows through the resistor and LED to ground.
- Sinking: the LED goes from supply to the pin, and the pin goes low to light it.
Why it matters
Some chips can sink more current than they source, and common-anode displays need sinking. The resistor calculation is the same either way. A sinking LED lights when the pin is low, so the logic looks inverted.
REMEMBER
A sinking LED turns on when the pin is low.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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