Servos

Servos are one of the simplest and least expensive ways to add motion to a holiday display. They are commonly used to animate figures, moving props, singing faces, opening doors, waving arms, and many other projects requiring controlled movement.

Overview

A servo is a self-contained motor with built-in position control electronics. Most hobby servos have a limited range of motion, typically 90° or 180°, although continuous-rotation versions are also available.

Servos are widely used in radio-controlled cars, airplanes, boats, and robots because they provide accurate positioning without requiring complex control circuitry.

How Servos Work

Unlike a standard DC motor, a servo contains:

• A DC motor
• A gear train
• A position sensor (typically a potentiometer)
• Internal control electronics

The controller compares the desired shaft position with the actual shaft position and automatically drives the motor until both match.

This closed-loop feedback system allows the servo to accurately hold its position.

Servo Control

Most inexpensive hobby servos use Pulse Width Modulation (PWM) to determine the desired shaft position.

Instead of changing the voltage supplied to the motor, the controller sends a short control pulse approximately every 20 milliseconds (about 50 times per second).

Changing the pulse width changes the desired shaft position.

If the pulses stop, the servo no longer receives position updates and external forces may move the shaft. While valid control pulses are present, the servo continuously corrects its position and resists outside forces until it reaches its torque limit.

⚠️ Warning:

Holding a stalled servo against a heavy load for long periods can overheat and permanently damage the motor or gears.

Typical Pulse Widths

The exact pulse widths vary slightly between manufacturers, but most hobby servos use values similar to the following:

Pulse Width	Typical Position
1.0 ms	0°
1.5 ms	Center (90°)
2.0 ms	180°

The following illustration, originally adapted from an Arduino forum discussion, shows the relationship between pulse width and shaft position.

Typical hobby servo pulse widths and corresponding shaft positions.

Servo Types

Standard Position Servos

These are the most common servos used in holiday displays. The shaft moves to a commanded position and holds that position until a new command is received.

Continuous Rotation Servos

Continuous rotation servos do not control position. Instead, the control signal determines the speed and direction of rotation.

These are useful for spinning props, rotating signs, and other applications requiring continuous movement.

Power Requirements

Most hobby servos operate from 4.8 to 6 volts, although larger digital servos may support higher voltages.

💡 Note:

Servos can draw several times their normal operating current when starting or when stalled. Multiple servos should normally be powered from a dedicated power supply rather than directly from a microcontroller.

Common Christmas Display Applications

Servos are frequently used for:

• Waving figures
• Singing faces
• Moving arms and heads
• Opening doors or windows
• Animated presents
• Rotating signs
• Small mechanical props

Choosing a Servo

When selecting a servo, consider:

• Required torque
• Range of motion
• Operating voltage
• Gear material (plastic or metal)
• Environmental protection if used outdoors

Metal gear servos generally provide greater durability for larger or heavier props.

Lessons Learned

• Do not force a servo beyond its mechanical limits.
• Allow adequate power capacity for startup and stall current.
• Test the mechanism before attaching decorations.
• Protect outdoor servos from moisture whenever possible.
• Continuous-rotation servos control speed, not position.

See Also

• How to add Motion to your Display
• Stepper Motors
• Wiper Motors
• Cheap ULN2803A Motor Driver