Servo Tester

A servo tester is a small box with a knob that makes a servo move, without a microcontroller or a radio. It costs a few euros, and it is the quickest way to check that a servo works, find its centre and see how far it can travel. For an animatronic build it is a must-have: you centre every servo with it before you fit the arms.

In this guide we cover:

• What a servo tester does and the modes it has
• What to look for when you buy one
• How to centre an MG996R, step by step
• Powering it safely
• What a tester can’t do
• Alternatives

What Is a Servo Tester?

A servo tester generates the same control signal that a radio receiver or a microcontroller would send: a pulse of about 1 to 2 ms every 20 ms. The knob changes the pulse width, so you can turn the servo by hand. Most also test ESCs (the speed controller of a brushless motor), because those use the same signal.

Almost every tester has these modes:

Disclosure: links marked * are affiliate links. If you buy through them I may earn a small commission at no extra cost to you. It doesn’t change what I recommend.

The one I use: servo tester*.

Typical Specifications

These are typical values for the cheap testers. Check the listing of the one you buy.

What to Look For

• A centre mode or a mark on the knob. Without one, you can’t set the exact centre.
• An input range of 4.8 to 6 V. The MG996R accepts up to 7.2 V, but most testers don’t.
• A knob you can turn smoothly. A rough knob makes a servo jitter.
• More than one connector is handy, but not needed. One servo at a time is how you test.
• The power connector. Check how the tester is powered, usually through a servo-style lead from a separate battery or supply, not from the board you are testing.

Centring an MG996R

This is how the Pit Droid sheets do it: centre the servo, then fit the horn or lever so it is at 90 degrees to the servo body and to the linkage.

1. Power the tester from a 5 to 6 V supply, not from a tiny USB charger. The MG996R can draw a lot of current when it starts to move.
2. Plug the servo into the tester. The signal wire (orange) goes on the signal pin, the red wire on the middle pin, the brown wire on the ground pin.
3. Switch to centre mode, or turn the knob to its middle mark. The servo moves to its centre and holds.
4. Without removing the power, fit the horn or lever at 90 degrees to the servo body. Press it fully onto the spline, and tighten the screw.
5. Turn the knob and check the travel. Stop if the servo buzzes at the end, the arm hits a stop or a linkage binds.
6. Leave the servo in the centre before you fit the arm, so it starts at the right angle when the controller takes over.

What a Tester Can’t Do

• It is not exact. The centre of a cheap tester can be off by a few tens of microseconds, so after final assembly set the centre in your own code too.
• It sets no limits. It happily drives a servo into its mechanical stop. Move slowly near the ends.
• It cannot test a 360 degree (continuous rotation) servo for position. Such a servo just spins faster or slower, and a position test is not possible.
• It tells you nothing about the load. Test with the real load as well, because a servo that runs fine on the bench can stall when it lifts an arm.

Alternatives

Conclusion

A servo tester costs next to nothing and saves hours: centre every servo with it before you build it in, and test each one on the bench. Combine it with the MG996R guide and the Pit Droid hardware list.