Tim's Servo Tester

There are plenty of cheap servo testers on the market — and for basic tasks, they work well. Most simply rotate a servo by turning a knob, which was perfectly fine back in the days of analogue servos, when “does it move?” was all you needed to know.

But modern servos — especially digital ones — are far more capable, and far more varied. Most budget testers still operate within the traditional servo control range: 1ms to 2ms pulse width at 50Hz frequency. That’s no longer enough.

Today’s servos often contain dedicated microcontrollers. Manufacturers have extended the usable pulse width range to 0.5ms to 2.5ms, and many servos now support higher PWM frequencies — up to 300Hz. If your tester can’t reach those extremes, you’re not seeing the full behaviour of your servo.

One thing remains consistent: 1.5ms pulse width is the midpoint for nearly all servos. But the amount of travel on either side of that midpoint varies widely. Using only 1ms to 2ms, you’ll often get just 90° total rotation — 45° each side. To achieve a full 180° sweep, you need to go beyond.

🔧 Why I Built My Own Tester

My tester allows:

• Pulse width adjustment from 0.4ms to 2.6ms
• Fine control in 0.01ms steps, and 1µs precision when holding a button — perfect for measuring deadband
• Selectable frequencies: 50Hz, 100Hz, 150Hz, 200Hz, 250Hz, 300Hz
• Live square wave display, showing Pulse Width, Period, and expected Angle

🤖 Why It Matters

Servos aren’t just for RC planes anymore. They’re everywhere in robotics, where precise control, repeatability, and matching behaviour between units really matters.

Microcontrollers give us the ability to position servos with sub-degree accuracy — but only if we understand the servo’s true limits.

This tester lets you:

• Explore a servo’s full range
• Measure its deadband
• Match pairs for synchronized movement
• See how it responds to different frequencies

📘More Details

👉 Full build guide, schematics, and programming steps are available in my Instructable: Tim's Servo Tester

PCBWay is where I get all my PCBs made.

You can get the PCB from my Shared Projects: Tim's Servo Tester.

🧭 Using the Servo Tester

Main Control: Rotary Encoder Switch

• Pressing the encoder toggles between Pulse Width and Period modes.
• A cursor on the display indicates the current mode.
• Rotating the encoder adjusts the value for the selected mode.

🧪 Deadband Testing To test servo deadband:

• Hold down the “1µs” button while rotating the encoder in Pulse Width mode.
• This adjusts the pulse width in 1µs steps, allowing you to find the smallest change that causes the servo horn to move.

📶 Frequency Control

• Modern servos often include dedicated microcontrollers — that’s why they’re called digital servos.
• In Period mode, rotating the encoder changes the signal frequency sent to the servo.
• Available frequencies: 50Hz, 100Hz, 150Hz, 200Hz, 250Hz, 300Hz
• Frequency is capped at 300Hz to allow for extended pulse widths (0.5ms to 2.5ms).
• Some racing servos support higher frequencies (e.g. 500Hz), but only with standard pulse widths.
• You can’t have both extended pulse width and high frequency — the longer pulse won’t fit in a shorter cycle.

🎯 Pre-Set Positions

• 1.5ms – Standard centre position for most servos.
• 1ms – Typically -90°, but may vary with extended pulse width servos.
• 2ms – Typically +90°, also varies with extended pulse width servos.

📐 Angle Display

• Based on classic analogue servos: assumes 180° movement between 1ms and 2ms.
• Modern digital servos may move less or more — the displayed angle may not match actual movement.
• Some servos only move 90° total, even if the display shows 180°.

⚙️ Pulse Width Notes

• The only consistent standard is that 1.5ms means centre position.
• Beyond that, pulse width ranges vary by manufacturer:
• Some support extended ranges (e.g. 0.5ms to 2.5ms).
• Others are fixed at 1ms to 2ms.

📡 Transmitters and Receivers Notes

Digital Systems

• Built around microcontrollers.
• Support:
• Custom pulse widths.
• Servo type selection.
• Endpoint adjustment.
• Sub-trim and travel.
• Ideal for extended rotation or non-standard setups.

Analog Systems

• Use fixed circuitry (e.g. 555 timers).
• Pulse width locked to 1ms–2ms.
• No software configurability — changes require hardware mods.
• May limit servo movement and prevent full use of extended-range servos.