Read a sensor, drive an output

Understand the idea

A digital input answers a threshold question: is the voltage low or high? An analog input uses an analog-to-digital converter (ADC) to represent a voltage as a number. The count depends on the board's ADC resolution and reference voltage, so it is not automatically a reading in volts.

A potentiometer wired as a divider gives a useful first experiment: its middle pin moves between ground and the reference rail. Keep every signal within the board's allowed input range; 5 V-tolerant and 3.3 V-only boards are not interchangeable.

PWM output is also easy to misunderstand. `analogWrite` commonly changes the fraction of time a digital pin is high; it does not create a smooth analog voltage on most Arduino boards. For a motor, use PWM through a suitable driver.

Worked example

On a 10-bit ADC with a 5 V reference, a reading near 512 represents about 512/1023 × 5 V ≈ 2.50 V. Check the specific board: ADC resolution, reference, and PWM-capable pins differ.

Try it at the bench

1. Wire a potentiometer's outer legs to the board's appropriate supply and ground, and its wiper to an analog input.
2. Print the raw ADC count while turning the knob slowly. Note the low and high counts.
3. Map the measured range to a safe LED brightness using a PWM-capable pin and a series resistor.

Check your understanding

Does PWM value 128 necessarily mean the pin is outputting a steady half-voltage?