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Project Help and Ideas » Servo control

March 03, 2012
by avidflyer
avidflyer's Avatar

Hey fellow hobbyists.

I'm interested in building a USB-powered railroad semaphore signal. The semaphore signal has an arm that moves to 3 positions: straight out, 45-degrees down, and full down. As the semaphore flag moves, lenses mounted on the other side of the arm pivot move colored lenses in front of a lamp in order to give a lighted signal indication in hours of darkness.

So I'm trying to figure out the best way to control a servo in order to set the arm through the three positions, and have it stop at the proper points. I was thinking at first that I could mount a small disk to the pivot point, and in that disk drill small holes corresponding to the correct positions of the arm, and use a small lamp and detector to indicate to the controller when to stop moving the servo. If I go that route, I haven't figured out how to detect which one of the three position the arm is at without using another detector.

Any input on how to approach this challenge?


March 04, 2012
by pcbolt
pcbolt's Avatar


If you put the lamp on the moving arm and photo diodes at each of the stop points on the mounted disk, only one of the diodes should be outputting a signal. Shouldn't that tell you the position of the arm? I like the small disk idea.

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