How a servo works: SG90 pulses, angles and Arduino code explained

#What it is

A servo is a motor that knows where it is. Ask it for an angle from 0° to 180°, and it turns there and holds. The one in our kits is the SG90 micro servo: a 9 g plastic box with a white arm on top (the horn) and a three-wire lead — brown GND, red 5 V, orange signal.

Servos steer model cars, move robot arms, open little doors and point needles on gauges. See it in 3D.

#How it works inside

Open one up and you find four things:

1. A tiny DC motor, spinning fast.
2. A row of gears, slowing it down and making it strong.
3. A potentiometer (a knob) fixed to the output shaft — it reports the current angle.
4. A small control chip.

The chip compares where the arm is (from the potentiometer) with where you asked it to be (from the signal). Too far left? Drive the motor right. Too far right? Drive it left. Just right? Stop. That loop runs all the time, which is why a servo pushes back when you try to turn its arm by hand.

#The signal: pulses every 20 ms

The orange wire gets one pulse every 20 ms (50 times a second). The width of the pulse is the angle. Our 3D lab uses the same numbers as the Arduino Servo library:

Every extra 10.3 µs is about one more degree: (2400 − 544) ÷ 180 ≈ 10.3. You never have to make these pulses yourself — servo.write(angle) does it in the background.

#Pins

#How to wire it to an Arduino

No breadboard needed — three jumper wires go into the servo's plug:

1. Brown → GND
2. Red → 5V
3. Orange → pin 9

Play in 3DRun the code · press the parts · turn it around

#Example code

The sketch from the servo project:

#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> brings in the Servo library, which makes the 20 ms pulses for you.
• Servo arm; creates one servo and gives it a name.
• arm.attach(9); — start sending pulses on pin 9.
• arm.write(angle); — go to this angle, 0–180.
• delay(15) gives the arm time to move 2° before the next step: 91 steps × 15 ms ≈ 1.4 s for a sweep. Make it smaller to sweep faster.

#Common mistakes

• The servo jitters or the board resets. A servo can draw a few hundred milliamps when it starts or is blocked. One SG90 on USB is fine; for several, use a separate 5 V supply with its GND joined to the Arduino's GND.
• analogWrite() stopped working on pin 10. On the UNO the Servo library takes over the timer behind pins 9 and 10, so PWM on those pins stops.
• It won't go to exactly 0° or 180°. Real servos differ a little. Use arm.attach(9, min, max) with your own pulse widths, or stay between 5° and 175°.
• Nothing moves. Check the signal wire is on the pin named in attach().

#Check yourself

#Questions people ask

#Can a servo turn all the way round?

A normal SG90 stops at 0° and 180°. A continuous rotation servo spins freely instead: write(90) stops it, values below and above set the direction and speed.

#How many servos can an Arduino UNO control?

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

#Does a servo need a resistor or transistor?

No. Its own chip drives the motor; the Arduino only sends a small signal. Just mind the power when you use more than one.

#Where it is used

The servo sweeps in the servo project and becomes a needle in the strength tester in the 3D lab. Need exact turns past 360°? Meet the 28BYJ-48 stepper.

Last updated 2026-10-29