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Low-power X25.xx - X29.xx stepper motors

No doubt, one of the best choices for replicating most of aircraft panel instruments is using small bipolar stepper motors X27.168 (or X25, VID29 and other similar ones) that are used in car and motorcycles dashboards nowdays. You can get these steppers for a price of $3..5 a piece on Ebay, AliExpress, etc.

Instruments with limited rotation

X25.xx - X27.xx stepper motors have an internal mechanical stop (see the picture below) that limits shaft rotation to 315 degrees. For most instruments with an arrow rotation angle less than 320 degrees you can use these stepper motors without any modification - no need to make zero-position sensor (though you can).

In this case, when SimVimCockpit is started or reconnected, the firmware program moves the motor shaft until it stops against the limiter and resets its position counter.

Instruments with a full-circle rotation

To make an instrument with continuous rotation (like compass card, altitude indicator, directional needle) you need a motor without internal limiter and thus, it should have a position sensor to define the initial needle location. It can be a thin electric contact wire or IR opto sensor.

You can easily remove the stop for the Switec X27 motor, just carefully open the pair of plastic half shells of the stepper's body (they snap together without glue), and cut off the plastic stop located on the gearwheel using a sharp blade (see the picture).

In this case, when SimVimCockpit is started or reconnected, the firmware program moves the motor shaft until the needle/card cross the position sensor and resets its position counter.


IR Positioning Sensor

A good way to make a position sensor for X27 stepper is to use an optical (infra-red) sensor - a pair of infrared LED and IR photodiode, as shown in the picture, using the white instrument needle as the "mirror" that deflects the light from the LED onto the sensor.

The optical pair is placed behind the faceplate at a distance of about 3..5 mm and the faceplate has a small vertical gap in front of it (about 2 mm width) - this will exclude external light interference.

For instruments that have no needle but have a rotating card just use a black matte surface on the back side of this card with a thin white line or strip of foil aligned with "zero" on the face side.



Stepper Control

To control these very low-power motors (which draw about 20 mA only), you need to use the driver chips which are specially designed for such motors:

VID6606, STI6606Z, AX1201728SG or STI6608 chips

One such controller chip includes 4 low-current drivers and provide very precise positioning - 12 micro-steps per degree for 4 stepper motors (the STI6608 controls 2 motors).


These controllers can be found online for $1 ... $2 for the chip (check the links above). Also, along with driver chips, I recommend you to buy very useful small adapter boards (for $0.1 for a piece) that will allow you to make an excellent breakout board without need to order or make PCB, and save your time and money.

Using a small soldering iron you will make a good "breakout board" for 4 stepper motors just in a few minutes and spending $2 (chip + board + header strip). If you want, you can make PCB (below is the sample of a possible one-side PCB design), but this is absolutely not necessary with technique desctibed above.

All 4 Switec stepper motors should be connected directly to the controller outputs, without using additional diodes or resistors, as shown in the connection diagram for VID6606 below. Pay attention to the steppers pin numbering.



X27.xx Stepper Control using A3769 driver





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