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I am designing a motorised blind (powered by a Raspberry Pi) and need a reliable way of detecting when the blind is fully "up" or "down".

I don't want to use a stepper motor, and rely on counts, as I want it to handle situations where the Pi crashes / power cycles.

The simplest way seems to be a switch or sensor to detect when the blind is up or down, and I think a beam sensor would be neatest. The sensor needs to work in the dark, as the blind should be able to function at night. (I plan to automate it so the blind closes when there is no-one in the room at night.)

Have looked on Adafruit, but can't see anything suitable. Does such a thing exist?

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  • \$\begingroup\$ Can't you use a microswitch? \$\endgroup\$ – Vladimir Cravero Aug 11 '14 at 9:51
  • \$\begingroup\$ Thought about this, but wasn't sure if I could rely on the blind touching it to activate. I'll need one at the top and one at the bottom, so not sure if it would keep the top one activated when the blind was down. \$\endgroup\$ – Ben Aug 11 '14 at 9:54
  • \$\begingroup\$ I don't know how uswitches are arranged but that's a solved problem, and unless you need a beam sensor for particular reasons, like dust, temperature or whatever, just go for uswitches. \$\endgroup\$ – Vladimir Cravero Aug 11 '14 at 9:56
  • \$\begingroup\$ You could sense the motor current when the blind is fully wound up. \$\endgroup\$ – Andy aka Aug 11 '14 at 10:12
  • \$\begingroup\$ @Ben - Consider using an Arduino instead or as well. Arduino nano or mini clones are dirt cheap, and use very little power, the R-Pi will use about 20x more power. Arduino's have PWM motor speed control, and ADC so that you could detect motor stall. Clone Nano's or mini's are under £2. Even if the RPi was essential for some other reason, you might use an Arduino to build an interface anyway. \$\endgroup\$ – gbulmer Aug 11 '14 at 11:27
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I would use a Proximity sensor of the type.

  1. A reflective IR sensor that detects if there is a object in front of it. It contains a infrared LED and a infrared light detector and the detector detects the reflections from the LED. At DigiKey you can find several hundreds of different versions DigiKey

  2. A Hall transistor (Detects magnetic fields) and attach a magnet to the blind.

Reference to proximity sensors Wikipedia

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  • \$\begingroup\$ At risk of this being a dumb question - will a reflective IR sensor work in the dark? (Like the Hall transistor idea, but not sure if there will be room for the magnet on a roller blind.) \$\endgroup\$ – Ben Aug 11 '14 at 11:52
  • \$\begingroup\$ Yes the reflective IR works in darkness, that's one of the features it produces it's own light that it detects. The magnet could be in the size of 1/4 inch and 1/16 thickness depending on distance between magnet and hall sensor. \$\endgroup\$ – MatsK Aug 11 '14 at 16:21

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