Sweep a Servo and Control It with a Knob on Arduino Uno

A hobby servo is a small motor that turns to an angle you ask for and then holds it. It is how robot arms bend, how camera gimbals tilt and how a model plane moves its flaps. In this post you will do two things on an Arduino Uno: sweep a servo back and forth on its own, then put a potentiometer in charge so you can steer the servo by turning a knob.

Both parts are short, and the second one teaches a useful skill: reading a sensor, converting the number into something else, and using it to drive an output.

What you need

• An Arduino Uno
• A servo (Mokxi draws the common nine-gram SG90 style)
• A potentiometer, the three-pin knob from most kits
• Jumper wires and a breadboard

A servo has three wires: ground, power and signal. The signal wire is not a plain on and off. It carries a pulse repeated about fifty times a second, and the width of the pulse picks the angle. The Arduino Servo library makes those pulses for you. In Mokxi's servo part, a pulse of 544 microseconds puts the horn at 0 degrees, and 2400 microseconds puts it at 180.

Wire it up

Part pin	Uno pin	Notes
Servo GND	GND	Ground
Servo VCC	5V	Power
Servo PWM	9	The signal wire
Potentiometer pin 1	5V	One end of the track
Potentiometer pin 2 (wiper)	A0	The wiper gives a voltage
Potentiometer pin 3	GND	The other end of the track

The potentiometer is a voltage divider. The ends go to 5 V and ground, and the middle pin, the wiper, slides between them as you turn the shaft. analogRead() turns that voltage into a number from 0 to 1023.

A note about power

In the simulator, the Uno's 5V pin powers the servo without trouble. On a real bench, a servo is a motor, and motors can pull far more current than a USB-powered Uno should hand out, especially when the horn pushes against something. For anything bigger than a tiny servo, give it its own 5 V supply and connect that supply's ground to the Uno's GND so both share a reference. Check your servo's datasheet for its current.

Part one: the sweep

Start with the simplest sketch. It moves the horn from 0 to 180 degrees and back.

{{SKETCH:p2a.ino}}

#include <Servo.h> brings in the library. Servo arm; makes a servo object. arm.attach(9) tells it which pin carries the signal. Each arm.write(angle) sets a new target, and delay(15) gives the horn time to get there before the next step. The two loops count up and then down, forever.

Press Run and watch the horn in the simulator. If you want it slower, raise the delay. If you want a smaller sweep, change the loop limits, for example 45 to 135.

Part two: control it with a knob

Now replace the sweep. This sketch reads the potentiometer and moves the servo to match.

{{SKETCH:p2b.ino}}

Walkthrough, line by line

Line 1 includes the Servo library, the same as before.

Lines 3 and 4 name the two pins. A0 is the analog input where the wiper is connected, and 9 is the signal pin for the servo.

Line 6 creates the servo object. Line 7 remembers the last angle we sent. It starts at -1, which no real angle can match, so the very first reading always counts as a change.

Lines 9 to 12, setup(), start the serial monitor and attach the servo to pin 9.

Line 15 reads the knob. You get a whole number from 0 (wiper at ground) to 1023 (wiper at 5 V).

Line 16 uses map() to rescale that range. It says: when the reading is 0, give me 0; when it is 1023, give me 180; scale everything in between. The result is an angle a servo understands.

Line 18 is the jitter fix, and it deserves a closer look. An analog reading wobbles by a count or two even when your hand is still, because of electrical noise. Without a check, that wobble would send a new angle every loop and the horn would twitch. The abs(angle - lastAngle) >= 2 test says: only move when the knob has changed by at least two degrees. A small gap like this is called a dead band.

Lines 19 to 22 move the servo, remember the new angle, and print it to the serial monitor.

Line 24, delay(20), lets the loop run about fifty times a second. That matches how often a servo expects a new pulse, and it keeps the serial monitor readable.

What you should see

When I ran this in the Mokxi simulator with the knob at its starting position, the serial monitor printed angle 89 and the horn sat near the middle. Drag the knob on the canvas toward one end and the horn follows, with a new angle line each time you move it by two degrees or more.

If it jitters

Jitter means the horn twitches or buzzes when it should be still. Try these in order.

1. Widen the dead band. Change the 2 on line 18 to 3 or 4. Your horn will move in slightly bigger steps, but it will sit still.
2. Average a few readings. Read the knob four times, add the results, and divide by four before you map. Noise tends to cancel out.
3. Check your wiring on real hardware. A loose ground wire is the classic cause. So is a servo sharing a weak supply with the board.
4. Add a capacitor on the servo supply. A real servo draws sudden bursts of current, and a capacitor across the power and ground wires smooths them.

What to try next

• Limit the travel. Use map(reading, 0, 1023, 30, 150) so the horn never hits its end stops.
• Make it smooth. Instead of jumping to the new angle, move one degree toward it each loop.
• Add a second knob and a second servo for a pan and tilt pair. Give each servo its own pin and its own Servo object.
• Add a button that centers the horn at 90 degrees. Read the button debounce guide first.

For a longer walk through servo basics, read the Arduino servo motor guide. For a ready-made circuit, open the servo sweep kit project.

Try it in your browser

Open the servo part page and the potentiometer part page to see their pins and properties, then build this circuit on an Uno in the editor.

Ready to build it yourself? Create your free Mokxi account and open the editor in your browser. No installs, no parts to buy.