GPIO - Raspberry Pi Hardware Reference - page 127

Figure 10-5. GPIO output low
Figure 10-6 shows the active high configuration's R load circuit element substituted
with an LED and limiting resistor R . Since there is no current limiting provided by the
GPIO port, resistor R must be provided to do this.
Figure 10-6. GPIO driving an LED
Driving LEDs
When an LED is hooked up to the GPIO output port, R load becomes the LED and the
limiting resistor (in series). The math is complicated slightly by the fact that the LED is
a diode. As a diode, it has a voltage drop, which should be subtracted from the supply
voltage. For red LEDs, the voltage drop is usually between 1.63 and 2.03 V. 30