Build an Incremental Encoder Quick Tester

How to build a portable incremental encoder tester.

Every once in while it’s nice to have a hand-held device that can be used to see quickly if encoder signals are present.  I designed and built a quick-tester that allows for fast interface to an encoder without having to drag out a power supply and oscilloscope.

The Quick-tester is an optical encoder tester that is powered by a 9V battery. An internal 5V regulator drops the voltage to 5V, as two of the connections are power connections for the optical encoder.

Current limiting resistors are wired in to red LEDs.   The LED’s illuminate when the optical encoder signals are High.  By rotating the base by hand we can see if a channel is dead or even improperly phased.

Here is the schematic for the Optical Encoder Quick-Tester

Things the Quick Tester can tell you:

Which channels are working.

The LED’s should rapidly blink on and off while the optical encoder hub is slowly rotated. Keep in mind that turning the hub too fast will make the LED’s switch on and off quickly and cause them to appear to be continuously illuminated.

Proper phasing

When channel A is illuminated, channel A- should be dark and vice versa. If both lights are on or off at the same time, there is a problem with the encoder.

If A leads B, or B leads A

Rotate the encoder shaft slowly until both A and B are low (off), then look for the next high channel, that channel is the leading channel for that direction of rotation.

Location of the index pulse

It can be easy to fly right past an index pulse, particularly on higher line count encoders, but the Quick tester can tell you precisely when the index pulse occurs. This can be handy for mechanically timing the optical encoder to a real world position.

Other things to note:

If you have an optical encoder with  open collector outputs, the Quick-tester’s LED’s will not illuminate unless you use pull up resistors between each channel and the positive supply of the Quick Tester.

If all lights are on, you have probably lost the signal ground connection to the Quick tester.

The Quick tester is set up to look at six channels at a time, so it is likely you will be looking at only incremental channel or only commutation channels.

For more information on encoders, go to

Jim can be reached at