Actuators and Controlling Servo Motors

1From sensing to doing

A sensor on its own only reports. To open a gate, sound an alarm, pump water or move a robot arm, the Arduino must drive an actuator. Some actuators, such as a small LED, can run directly from a pin. Most cannot: a motor or a pump needs far more current than a pin's 20 mA, and must be switched through a transistor, a motor driver or a relay. This lesson covers the actuators you will use most, how to control how much they do, and how to position a servo motor precisely.

2Common actuators

3PWM: in-between values from a digital pin

A pin can only be at 0 V or 5 V. PWM switches it about 490 times a second (about 980 times on pins 5 and 6), and the LED or motor responds to the average:

$$V_{\text{avg}}=5\text{V}\times \frac{\text{value}}{255}V\_\{\backslash text\{avg\}\}\; =\; 5\backslash ,\backslash text\{V\}\; \backslash times\; \backslash frac\{\backslash text\{value\}\}\{255\}$$

An LED switching 490 times a second looks steadily lit, just dimmer. This sketch fades an LED on PWM pin 9 and beeps a passive buzzer on pin 7 at full brightness:

// Fade an LED on PWM pin 9 up and down; beep a passive buzzer on pin 7 at full brightness
const int LED_PIN = 9;      // must be a ~ (PWM) pin
const int BUZZER_PIN = 7;

void setup() {
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  for (int level = 0; level <= 255; level += 5) {   // brighter
    analogWrite(LED_PIN, level);
    delay(20);
  }
  tone(BUZZER_PIN, 880, 200);                        // 880 Hz for 200 ms
  for (int level = 255; level >= 0; level -= 5) {   // dimmer
    analogWrite(LED_PIN, level);
    delay(20);
  }
}

4Servo motors

A servo is controlled by a pulse sent about 50 times a second (every 20 ms). The width of the pulse sets the angle: roughly 1 ms for one end of the range and 2 ms for the other, with exact values that vary between models. The Arduino Servo library generates these pulses for you; by default it uses 544 µs for 0° and 2400 µs for 180°. You only call attach(pin) once and then write(angle).

An SG90 has three wires: brown to GND, red to 5 V and orange to the signal pin. (Other servos use black, red and white or yellow.)

// Turn a potentiometer on A0 to set the angle of a servo on pin 9
#include <Servo.h>

const int POT_PIN = A0;
const int SERVO_PIN = 9;
Servo arm;

void setup() {
  arm.attach(SERVO_PIN);           // the library makes the control pulses
  Serial.begin(9600);
}

void loop() {
  int reading = analogRead(POT_PIN);             // 0 to 1023
  int angle = map(reading, 0, 1023, 0, 180);     // 0 to 180 degrees
  arm.write(angle);
  Serial.print("Angle: ");
  Serial.println(angle);
  delay(15);                                     // give the servo time to move
}

Turning the knob changes the voltage at A0 from 0 to 5 V; map turns the reading 0 to 1023 into an angle 0 to 180, and the servo follows.

5Switching bigger loads: the relay

A relay module contains a relay, a transistor to drive its coil, and usually an indicator LED. Its IN pin takes the Arduino's signal; its COM, NO (normally open) and NC (normally closed) screw terminals switch the separate circuit, for example a 12 V pump with its own battery. This sketch, which would water a plant pot when a soil sensor on A2 reads dry, shows the pattern:

// Switch a small 12 V pump through a relay module on pin 6 when the soil is dry
const int SOIL_PIN = A2;     // analog soil-moisture sensor: higher reading = drier
const int RELAY_PIN = 6;     // relay module IN pin
const int DRY_LEVEL = 600;   // threshold found by testing
const int RELAY_ON = HIGH;   // many modules switch on with LOW: if so, write LOW here
const int RELAY_OFF = (RELAY_ON == HIGH) ? LOW : HIGH;

void setup() {
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, RELAY_OFF);   // pump off at start
  Serial.begin(9600);
}

void loop() {
  int soil = analogRead(SOIL_PIN);
  bool dry = soil > DRY_LEVEL;
  digitalWrite(RELAY_PIN, dry ? RELAY_ON : RELAY_OFF);
  Serial.print("Soil: ");
  Serial.print(soil);
  Serial.println(dry ? "  pump ON" : "  pump off");
  delay(1000);
}

Many relay modules are active low: the relay switches on when IN is LOW. The constant RELAY_ON lets you change that in one place. All the sketches in this lesson were compiled for the Uno with arduino-cli without errors.

6Hands-on: a car-park barrier

1. In the simulator, connect an SG90 servo to pin 9, a potentiometer to A0 and upload knob_servo. Check that the angle printed matches the servo.
2. Replace the potentiometer with the HC-SR04 from the last lesson (Trig on 2, Echo on 3, so pins 9 and 10 stay with the servo).
3. Write a sketch: when a car is closer than 15 cm, the barrier turns to 90° (open) and a green LED on pin 5 lights; 3 seconds after the car leaves, it returns to 0° and a red LED on pin 6 lights.
4. Add a short beep from a passive buzzer each time the barrier moves.

Check: the barrier never moves while a car is under it, closes 3 seconds after the car leaves, and the LEDs always match its position.