A microcontroller reads inputs and controls outputs

A microcontroller is a small computer on a chip, with a processor, memory and connections to the outside world. Its program can read a switch, decide what that input means, and turn an output on or off. The Arduino Uno is a board that supplies the chip with power connections, a USB interface and accessible pins.

This topic uses the classic Uno R3 with an ATmega328P, operating with 5 V logic. Other Arduino boards, including newer Uno models, can have different converters and electrical limits.

The drawing groups functions; it is not a wiring pinout. Check the labels on the actual board. Start with Digital and Analog Signals, then Analog-to-Digital Conversion and Pulse-Width Modulation.

A sketch runs setup once, then repeats loop

An Arduino program is called a sketch. It uses C/C++ syntax: rules for names, parentheses, braces and statements. It is a programming language, not ordinary English. Capitalization matters: digitalRead and digitalread are different names.

setup() runs once after startup or reset. loop() repeats while the program runs. This original example turns the built-in LED on for half a second and off for half a second.

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
}
void loop() {
  digitalWrite(LED_BUILTIN, HIGH);
  delay(500);
  digitalWrite(LED_BUILTIN, LOW);
  delay(500);
}

delay uses milliseconds and pauses normal progress through the sketch. The braces group the statements belonging to each function.

Choose the pin mode before using the pin

pinMode(pin, mode) configures the pin. digitalRead(pin) returns HIGH or LOW. On an OUTPUT pin, digitalWrite(pin, HIGH) drives it toward the logic supply; LOW drives it toward ground. These are limited-current signals, not ideal power supplies. On an INPUT pin, digitalWrite instead controls the pull-up.

A button to ground reads LOW when pressed

With INPUT_PULLUP, an open switch reads HIGH. Closing the switch to ground reads LOW. This is called active-low: the action happens when the signal is low.

void setup() {
  pinMode(2, INPUT_PULLUP);
  pinMode(LED_BUILTIN, OUTPUT);
}
void loop() {
  bool pressed = (digitalRead(2) == LOW);
  digitalWrite(LED_BUILTIN, pressed ? HIGH : LOW);
}

The pull-up supplies a small current through the closed switch to ground. The switch sinks that current. A physical switch can bounce between states briefly; counting presses reliably needs debouncing beyond this simple state display.

Supporting detail: Addendum 1 — Pull-up resistance and current.

Digital uses two states; analog uses a range

A digital input interprets two states: LOW/HIGH, or 0/1. The wire still carries a physical voltage; the receiving circuit decides which logic state it represents. A voltage in the undefined region between valid LOW and HIGH levels is not a useful third state.

An analog signal represents a quantity by a continuous range of values. A potentiometer wired between ground and 5 V can provide a changing voltage at its middle terminal. On a classic Uno R3, the usual analog measurement range is nominally 0–5 V with the default reference. That is not a universal sensor range.

Classify the sensor's output circuit, not just its name

A bare resistive sensor does not generate a usable voltage by itself: its surrounding circuit matters. A module may offer both analog and thresholded digital outputs.

Supporting detail: Addendum 2 — Phototransistor circuits can measure light or switch at a threshold.

Supporting detail: Addendum 3 — Why an ultrasonic measurement can be digital.

The ADC reads voltage; PWM controls time

Analog-to-Digital Conversion turns an analog input into a number the program can use. Pulse-Width Modulation switches a digital output on and off for a controlled fraction of each cycle. Neither operation changes the fact that the processor stores and works with digital data.

Practice identifying sensor signals

Ten bits give 1024 codes, numbered 0 through 1023

An analog-to-digital converter (ADC) samples a voltage and reports a number. The classic Uno R3 has a 10-bit ADC: 210 = 1024 possible codes. analogRead(A0) reads analog input A0 and returns an integer from 0 to 1023.

The reference voltage sets the scale. With an assumed exact 5.000 V reference, one nominal step is 5/1024 = 0.0048828125 V = 4.883 mV, or about 4.9 mV. Resolution describes the step size; it does not guarantee that a real voltage measurement is accurate to that amount.

Supporting detail: Addendum 4 — Arduino's resolution and timing figures.

Use 1024 for the nominal voltage step

For these exercises, use a clearly defined ideal bin model: divide the 0–5 V span into 1024 equal bins, round down to the bin number, and limit the top code to 1023.

code = min(1023, floor(Vin × 1024 / Vref)). Here floor means round down. At 3.000 V with a 5.000 V reference: floor(614.4) = 614.

To report the nominal voltage represented by a code, use its lower bin edge: V = code × Vref/1024. Code 800 represents 800 × 5/1024 = 3.90625 V. In this model its bin extends up to, but not including, 3.9111328125 V.

Supporting detail: Addendum 5 — Why dividing by 1023 answers a different question.

Explore the steps and check the units

The two-way ADC controls in the PWM and ADC explorer let you enter a voltage or a code. Try nearby voltages within one bin, then cross its boundary. In reverse, code 1023 gives 4.9951171875 V as its lower edge, not proof that the input equals that value.

The same explorer also separates when a voltage is sampled from how its code is calculated. It shows instantaneous raw PWM or settled RC values, the finite mean of the selected point samples, and the mean of their reconstructed lower-edge voltages. Those are different from the waveform's complete-cycle time mean. A held code is the latest point result, not a window average.

Supporting detail: Addendum 6 — Quantize first or average first?.

Practice converting a reading to volts

Practice sampling phase

Duty cycle is the fraction of each cycle spent HIGH

Pulse-width modulation (PWM) repeats a rectangular waveform. The period T is the time for one cycle; frequency f is cycles per second. Duty cycle D is the fraction of the period spent HIGH.

T = 1/f, ton = D × T, and for ideal switching between 0 V and a supply V_s, Vaverage = D × Vs. Convert a percentage to a fraction: 25% means D = 0.25.

At 500 Hz, T = 0.002 s = 2 ms = 2000 µs. At 25% duty, the on-time is 500 µs and the average of a 0–5 V output is 1.25 V. The pin itself still switches between 0 and 5 V.

The stock Uno R3 uses six PWM pins

Use digital pins 3, 5, 6, 9, 10 and 11. Default PWM is approximately 490 Hz on 3, 9, 10 and 11, and 980 Hz on 5 and 6. These correspond to periods of about 2.04 ms and 1.02 ms.

analogWrite(pin, value) requests duty using values from 0 (always off) to 255 (always on). A useful nominal mapping is value = round(255 × D): 25% gives 64. Hardware timer modes introduce small differences in the exact duty, particularly near the ends of the range.

Supporting detail: Addendum 7 — Official frequency table and timer limitations.

An RC filter smooths PWM but leaves ripple

Put a resistor in series with the PWM signal and a capacitor from the output node to ground; measure the voltage across the capacitor. This RC low-pass filter slows voltage changes. The capacitor charges while PWM is HIGH and discharges while it is LOW.

The time constant is τ = R × C. A longer time constant relative to the PWM period reduces ripple but makes the output respond more slowly when duty changes. The explorer shows the settled waveform with τ = 10 ms, assuming an ideal source and no load. A load changes the result; the filter is not a motor power supply.

Microchip's AVR131 PWM filtering note explains the tradeoff: the low-pass filter must reject PWM base-frequency ripple without removing the slower signal of interest, and a low-impedance load can discharge the capacitor unless buffered. This explorer deliberately assumes no load.

Supporting detail: Addendum 9 — What the smoothed trace calculates.

PWM changes LED brightness and motor drive

Rapid switching changes an LED's average light output while a resistor or driver limits its on-state current. Perceived brightness is not perfectly proportional to duty. For a DC motor, PWM commands a suitable power driver; the board pin does not supply the motor current. The driver must handle the motor and its inductive current.

Increasing duty usually increases the available drive, but motor speed also depends on load. Use DC Motor Torque, Speed, and Power to connect speed, torque and current. See Electrical Safety before wiring.

Practice PWM calculations