First output: an LED and a resistor

A GPIO pin (general purpose input/output) is a pin your program can set high or low. High is 3.3 V on most modern boards and 5 V on an Arduino Uno; low is 0 V. Every light, relay and motor driver you control works this way: a pin goes high or low, and a circuit turns that into a physical action.

The formula you need is Ohm's law: V = I x R, with voltage in volts, current in amps and resistance in ohms. Rearranged, it gives I = V / R. If 3.3 V is across a 470 ohm resistor, the current is 3.3 / 470 = 0.007 A, which is 7 mA.

An LED does not behave like a resistor. It drops a roughly fixed voltage, called the forward voltage: about 2.0 V for red and about 3.0 V for blue and white. The rest of the supply voltage falls across the series resistor, and that resistor sets the current. For a red LED on a 3.3 V pin at 10 mA: (3.3 - 2.0) / 0.010 = 130 ohms. Without the resistor, the LED draws far too much current, and the LED or the pin can be damaged.

A pin can supply only a small current, typically 8 to 20 mA. This is enough for an LED, but not for a motor, a buzzer or a relay coil. Those loads need a transistor or a driver chip.

A pin can be configured in 2 output modes. Push-pull is the normal mode: the pin actively drives both high and low. Open-drain drives only low. When it is set to 1, it releases the line, and another component, usually a pull-up resistor, has to bring the line high. I2C uses open-drain, and a later lesson covers it.