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I was being a little too cheap & opted for a motor w/o an encoder to roll a screen up & down.  I've noticed that depending on the wind, the screen unravels at different speeds.  This causes a huge issue for me because I have no idea how far it unravels.  Although I don't care as much about the unraveling, I do care about rolling the screen back up.  I can't roll it up above a certain height or it will basically tear itself apart.  I've decided the best course of action (besides breaking down & purchasing the same motor w/ the encoder) is to add a contact sensor.  I was thinking about using 2 small/weak magnets (w/ plastic coats) to make sure the contacts line up well.  I was going to wrap the max height sensor w/ the + & - leads separated with a small gap.  Then I was going to wrap the screen sensor with lots of wires going in every direction.  That way no matter how the magnets touch, they will bridge the gap.  Any thoughts?  Anyone have a better example?  Or even something I can purchase online?  Thanks.
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Quote
I was thinking about using 2 small/weak magnets (w/ plastic coats) to make sure the contacts line up well
What kind of magnet has  contacts?
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Well the magnet wouldn't have contacts.  I'd have to make the contacts by wrapping the - & + wires around the 1st magnet w/ a gap.  The 2nd magnet would be covered in wires or a metal plate.  When the magnets touch, then - & + wires will be bridged sending a signal to my arduino.  I don't necessary have to use magnets.  I also thought about just having a metal plate at the bottom of the screen.  Then having just 2 wires sticking out (one from 5v from arduino, the other into a digital input).  When the metal plate touches the 2 wires, the connection will trigger me to shut off the motor.  I just liked the magnet idea a little better because the wires would be more stable & even if I was a few cm off, the magnets would attract each other & create a strong connection between the 2 wires.
« Last Edit: August 06, 2012, 10:52:50 am by nevafuse » Logged

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Still can't comprehend what you saying.   They call it "reed switch":
https://www.sparkfun.com/search/results?term=reed&what=products
Alternatives: hall effect sensor
https://www.sparkfun.com/products/9312
or IR slotted detector
 http://www.google.ca/imgres?imgurl=http://www.sunrom.com/images/media/3565A.jpg&imgrefurl=http://www.sunrom.com/sensors/distance-obstacle/slotted-optical-switch-moc7811&h=463&w=535&sz=52&tbnid=sI6WvH53EddIkM:&tbnh=96&tbnw=111&zoom=1&usg=__xP-Gi0DwqOrfQeYrOM2XIQXH0Kc=&docid=rvn9fdaOKrmusM&sa=X&ei=aO0fUJO-LIjVyQGwr4GoBA&ved=0CGoQ9QEwAg&dur=65

http://mechatronics.mech.northwestern.edu/design_ref/sensors/reflectors.html
« Last Edit: August 06, 2012, 11:18:43 am by Magician » Logged

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Mmm...those could work too.  The magnets in my example were only there to ensure a tight connection between 2 wires.  But sounds like I could use these magnet sensors to eliminate the need for the pieces to even touch.  Which would actually be better because another concern was trying to figure out was which way to "unravel" seeing how I'd have to temporarily disable the fail safe to see which direction would disconnect the 2 wires.  But I could stop the screen before it reaches the sensor based on increased magnetism.  Then to figure out which direction, I could choose the direction that causes a decrease in magnetic detection.  Thanks Magician.
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