Stepper Motor Zero Flag
Bertho Boman


For some mechanical projects there is a need to know a "home" position or to obtained a pulse once per revolution for a stepper motor. For commercial applications stepper motors can be purchased with an extended rear shaft but they are not commonly available for the hobbyist market.

Here are three ways of adding a pulse output, once per turn, to regular stepper motors, using different assembly methods.

In all cases no dimensions are given since they will vary based on the stepper motor and the available opto-interrupter.

Opto-Interrupter Information
First, a comment about opto-interrupters. Basically there is an LED in one "leg" illuminating a photo transistor in the other "leg". An output is obtained from the transistor when the light is blocked by the "flag".

The resolution (how well-defined the state of change is) depends on the size of the light beam reaching the transistor since it will be gradually blocked by the approaching flag. The smaller the beam, the better the resolution but the weaker the light to drive the transistor.

Since almost all opto-interrupters are used with a flag approaching from the side, many opto-interrupters have vertical slots to get better resolution (sharpness of change).

In other words, it is important for best performance that the flag is approaching in a direction that will quickly cut off the light beam going through the slots. That is also why it is important that the flag edge is really aligned parallel with the slot.

For information on a high accuracy "Home" switch see:

The First Design

First a picture of a stepper motor's recessed rear shaft:

Stepper Rear

Adding a Flag:
A shaft extension can be added by "super-gluing" a short shaft to the end of the rear shaft. That shaft is also shaped to be the active flag for the photo interrupter. The base of the shaft should match the motor shaft and the top section slightly smaller diameter than the opto-interrupter's slot used. See the third item in the picture.

Stepper Parts

To make it easier to accurately glue the shaft extension to the recessed shaft, I made a simple little guide that just fits into the hole in the back of the stepper and has a center opening slightly larger than the base of the shaft extension. It is used to align the shaft extension when it is glued to the shaft. See item number 1 (left) in the picture.

Clean the end of the stepper shaft and the shaft extension with a cotton swab (Q-Tip) dipped in acetone or other cleaner. Add a very small drop of super-glue to the shaft extension and slide it through the guide and then press it down for about 20 seconds. Do not disturb it for a minute and then remove the guide.

Test run the stepper to verify that the shaft extension is glued properly.

Congratulation! You now have a stepper with a shaft mounted flag.

Adding the Base:
The base is the second item in the picture above. It also has a small protrusion on the bottom that is made to just fit in the hole in the stepper motor to align it with the shaft. Of course, with the limitations of the accuracy of the stepper motor housing itself.

Several drops of super-glue is placed on the back and it simply glued to the back of the stepper and is let to cure for a minute or so.

Adding the Opto-Interrupter:
It is tempting to just lay it down to keep a low profile. Unfortunately the slots are vertical so to get best resolution it needs to be mounted vertically, actually up-side down. It is held in place by the little clamp, the item just below the opto-interrupter in the picture.

Wire the opto-interrupter as normally done with a resistor driving the LED and a pull-up resistor on the transistor collector. The values depend on the opto-interrupter brand and the circuit interfaced to. Typical values would be 220 Ohm for the LED resistor and 4.7 kOhm for the transistor assuming a 5 Volt supply.

Aligning the Opto-Interrupter:
The desired way is to use an oscilloscope to see the switching waveform when the motor is run. Do not tighten down the opto-interrupter yet! If misaligned the rotating flag can break it or break off the shaft. The opto-interrupter needs to be mounted slightly off center. Monitor the scope and slightly move the opto-interrupter for the best waveform and then gently tighten it down.

It can also be done using a volt meter and turning the stepper very slowly. Even just an LED could be used to see the status.

Here is a CAD model of the assembly:

Opto Asm

Adding a Cover:
When tested and the screws tightened, a cover should be added to keep out dirt and stray light.

Stepper Cover

The Second Design

If no machining equipment is available or there is a rush, a much simplified version can be made. Please read the above section first to better understand this simplified part.

Adding a Flag:
The flag for this version is a flat head screw that has been hand filed to have a section removed. See:

Stepper Flag

It is super-glued to the shaft without a guide. Run the stepper to see if it was mounted successfully. If not, start over.

Adding the Opto-Interrupter:
In this case the opto-interrupter is placed sideways and a good position found using the same procedure as described above. After that, it is simply glued in place.

Stepper Sideway
I am sorry for the poor picture quality!

Adding a Cover:
When tested and glued, a cover should be added to keep out dirt and stray light. The easy way is to use a bottle cap and add a slight notch for the wires to exit. It too was super-glued.

Stepper Cap

The Third Design

A third option, in this line of glued on shaft extensions, would be to glue on a straight short shaft that would stick out about 8mm (depends on the opto-interrupter) and at the end glue on a horizontal flag to extend out about 15mm.

The opto-interrupter was then mounted with the legs pointing toward the shaft and adjusted so that the flag's edge was parallel with the opto-interrupter slot. The opto-interrupter was glued with one leg to the back of the stepper motor. This style gives very good resolution since the flag movement is much larger and therefore goves a sharper transition.

Stepper Type3-0.png

The flag was cut from an old credit card. It is a suitable 0.8mm thick. The shape is not important except the edges should be pointing to the center of the shaft to match the opto-interrupter's slot.

The two resistors were glued down to make it self-contained. A cover was also added afterwards.

Stepper Type-3-1.jpg

Comparison of the Three Types

The blue top trace is from a stepper using the first type and the yellow bottom trace is from the second one. As expected, the first one has much "cleaner" transitions, faster fall and rise times which in turn translates to better definition of the reference point.

Stepper Good-Simple

The display below is from testing the third type. As predicted, this third version has even faster rise and fall times. It is also the easiest one to make although physically bigger.

Stepper Type3-2

Depending on the application, either type might be acceptable but for best resolution the flag ought to be lined up with the slot in the opto-interrupter.

Bertho Boman
Vinland Corporation
11600 NW 20th Street
Fort Lauderdale
FL 33323
(954) 475-9093
Email: Boman03 at Vinland dot com