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Using servo motors to operate switches


Turnout motors always seemed a bit expensive to me until I learned about servos from a radio control (RC) enthusiast friend. He used servos in his airplanes, which he sold for less than $10 each. The name “servo” is an abbreviation of the term “servo mechanism”. These devices use lightweight canister motors to drive high-ratio gearboxes that operate the model aircraft’s flaps and other control surfaces.

This information started my search for servos, which eventually led me to the Tower Hobbies website: www.towerhobbies.com. Tower sells a single Hitec model. HS-311 Servo Motors $7.99 each. If purchased in a group of three for $23.10, the price drops to $7.70 per person (special stock number LM3129).

Even if I add a pair Omron doesn’t. Each device is equipped with 187709 SPDT micro switches (Jameco) for frog power and signal transmission, and the unit cost per device is still just over $10.

Servo modification

For my use, all I needed was a DC motor for the servo and its matching gearbox. Each servo also includes a printed circuit (PC) board (as part of its RC electronics), a three-wire wire, and a plug (which I removed and discarded).

I started modifying the servo by applying a piece of masking tape to the top front of the servo and partially taping the sides to secure the panel to the case. If you omit this step, removing the four screws from the housing will take the whole thing apart and you’ll have some small gears.

Next, I flipped the servo so that the output shaft was facing down and the three wires and plugs were on top. (A Panavise or similar clamping device with nylon jaws is helpful here as a third hand.) I removed the four long screws holding the servo housing together.

After opening the case, the servo motor is at the top with a tri-color cable next to it. I carefully removed the PC board and RC electronics from the motor terminals. Once the board was free, I cut the wires connected to the PC board and tucked the rest into the housing next to the motor.



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