Servo motor with Arduino — sweep an SG90 from 0° to 180°, explained

#What you will build

A servo whose arm (the horn) sweeps slowly from 0° to 180°, then back to 0°, over and over — like a windscreen wiper. The Serial Monitor prints 180 and 0 each time it reaches an end.

Servos are everywhere small things must move to an exact position: the steering of RC cars, the flaps of model planes, robot arms and legs, camera gimbals, automatic pet feeders. Once you can sweep one, you can point one anywhere.

#How it works

#What is inside a servo

An SG90 micro servo weighs only 9 g, but it holds four things:

1. a small DC motor,
2. a gearbox that makes it slower and much stronger,
3. a potentiometer on the output shaft, which tells the servo where its arm is now,
4. a tiny control chip that compares where the arm is with where it should be, and turns the motor until they match.

That loop — measure, compare, correct — is why a servo holds its angle even when you push on it. More in how a servo works.

#Talking to a servo: pulses

The servo's signal wire listens for a pulse every 20 ms (50 times a second). The length of each pulse is the angle:

You do not have to make these pulses yourself. The Servo library does it in the background: you just say write(90).

#The wiring

No breadboard needed: the servo's three wires go straight to the UNO.

1. The orange (signal) wire to pin 9.
2. The red (V+) wire to 5V.
3. The brown (GND) wire to GND.

#The code, line by line

#include <Servo.h>
Servo arm;

void setup() {
  arm.attach(9);
  Serial.begin(9600);
}

void loop() {
  for (int angle = 0; angle <= 180; angle += 2) {
    arm.write(angle);
    delay(15);
  }
  Serial.println("180");
  for (int angle = 180; angle >= 0; angle -= 2) {
    arm.write(angle);
    delay(15);
  }
  Serial.println("0");
}

• #include <Servo.h> adds the Servo library — ready-made code that comes with the Arduino IDE. You do not need to install anything.
• Servo arm; makes a servo object called arm. You could call it anything; with two servos you would make two objects.
• arm.attach(9) tells the library which pin the signal wire is on. From now on it sends pulses on pin 9 by itself.
• for (int angle = 0; angle <= 180; angle += 2) counts 0, 2, 4 … 180 — 91 steps.
• arm.write(angle) sets the angle. The library changes the pulse length; the servo does the rest.
• delay(15) gives the arm time to move 2°. 91 steps × 15 ms ≈ 1.4 seconds for each sweep.
• The second for loop counts down from 180 to 0 with angle -= 2, so the arm comes back.

#Common mistakes

• The servo jitters or the board resets. The servo draws too much current from USB. Use a separate 5 V supply for it (GND joined), or add a big capacitor (470 µF) across its power.
• It buzzes at 0° or 180°. Some servos cannot reach the full range and push against their stop. Use 5°–175° instead.
• Nothing moves. The signal wire is not on the pin given in attach(), or the servo has no 5 V. Check the colours: orange signal, red 5 V, brown GND.
• analogWrite() on pin 10 stops working. On the UNO the Servo library takes over the timer for pins 9 and 10, so PWM on them does not work while a servo is attached.

#Try these challenges

1. Make it stop at 45°, 90° and 135°, one second at each.
2. Sweep faster (delay(5)) or slower (delay(40)). Which is smoother?
3. Add a potentiometer on A0 and turn the arm with the knob: arm.write(map(analogRead(A0), 0, 1023, 0, 180));.
4. In the 3D lab, change the step from 2 to 10 and watch how jerky the sweep gets; then turn on labels to read the angle.

#Check yourself

#Questions people ask

#What is the difference between a servo and a DC motor?

A DC motor spins freely as long as it has power; you control its speed. A servo has a sensor inside and goes to an angle you choose, then holds it. Use a servo to point at something, a DC motor to drive wheels. See the DC motor project.

#Can a servo turn 360 degrees?

A normal SG90 turns only 0° to 180°. "Continuous rotation" servos turn all the way round, but then write() sets the speed and direction instead of an angle.

#How many servos can one Arduino UNO control?

The Servo library can drive up to 12 servos on an UNO, on any pins. The limit in practice is power: give them their own 5 V supply.

#Do I need a resistor or transistor for a servo?

No. The servo has its own driver inside, and the signal wire needs only a tiny current. Just power, ground and a signal pin.

#What's next

A servo knows its angle; a stepper motor counts it. Next, spin a prize wheel with a stepper motor that stops on an exact segment every time.

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Last updated 2026-10-29