Sensor to detect rotation

Discussion in 'General Electronics Chat' started by 128ITSH, Dec 9, 2017.

  1. 128ITSH

    Thread Starter Member

    Jul 20, 2017
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    Hello everybody!
    I am looking for a sensor that will be able to tell when a full rotation of a pointer which rotates inside a circle was completed.
    The circle diameter is about 1 cm, and the sensor should have a distance of 0 to 1 cm from it. The pointer is red and also both the circle and pointer are protected by glass about 0.1 - 0.5 cm above them.
    upload_2017-12-9_11-23-15.png
    I understand I need a sort of color sensor, but which one? any help will be appreciated.
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
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    I would think almost any reflective opto sensor should work. The problem may be that there is as much reflection from the background as the pointer. You may have to paint the background matt black. You will find it dificult to find a sensor that detects a particular colour. Most sensors of this type use infra red light. This is an example of one type of sensor that would work.

    Les.
     
  3. 128ITSH

    Thread Starter Member

    Jul 20, 2017
    98
    16
    Thanks for your help!
    I have no way to paint the pointer or background since they are covered in glass. Maybe an RGB sensor could do the job?
    The sensor don't have to output an exact pulse when a rotation has completed, but show a difference in the data it outputs to an MCU when it sees the pointer so the MCU can handle it and convert it to a rotation.
     
    Last edited: Dec 9, 2017
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Is there a height difference between the backgound and the pointer?
    If so, a distance sensor might work.

    Bertus
     
  5. 128ITSH

    Thread Starter Member

    Jul 20, 2017
    98
    16
    The pointer is almost touching the background and about 0.1cm higher than it. Can a relatively cheap sensor see this difference?
     
  6. Danko

    Member

    Nov 22, 2017
    469
    169
    Take Hall sensor, for example DN6851, from dead computer's fan and place it on circle, close to pointer's end. On end of pointers glue small magnet.
    https://forum.allaboutcircuits.com/attachments/hall_effect-pdf.139961/
    Edit: Sensor can be placed on back side of the circle (depends on thickness of circle and material).
     
    Last edited: Dec 9, 2017
  7. 128ITSH

    Thread Starter Member

    Jul 20, 2017
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    16
    Thanks for the advice, but let me make it clear: I don't have any access to the pointer or to the background because they are covered with glass, therefore I cant install the magnet :(
     
  8. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    What is the pointer and the background made of?
    If the pointer is metal and the background isn't then you may be able to detect it with an inductive proximity sensor behind the background.
     
  9. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    Second thoughts: Thinking this may be a utilities meter so you have no access to the inside.
    Do you want to detect from the front through the glass or the back?
     
  10. Danko

    Member

    Nov 22, 2017
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    Glue in parallel two plastic tubes 5 mm inner diameter, about half of inch long (two pieces of pen), it will like binoculars. On one end of this monster tube put to one hole blue LED, in other hole photodiod or phototransistor. Energize LED and point other end of monster tube to pointer. You will have changes of signal from photodiode when pointer travel under monster tube.
     
  11. 128ITSH

    Thread Starter Member

    Jul 20, 2017
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    The pointer and background are both made from plastic. I want to detect this rotation from the front of the glass
    This solution sounds interesting but I don't fully understand it. Can you draw a simple schematic of it?
     
  12. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    A standard reflective opto sensor should work from the front of the glass.
     
  13. 128ITSH

    Thread Starter Member

    Jul 20, 2017
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    Does such sensor can tell the difference between red and white, or the difference of 1 mm height? Can you link me to a good par for this?
     
  14. Danko

    Member

    Nov 22, 2017
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    Pointer.png
     
  15. Danko

    Member

    Nov 22, 2017
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    Under blue light red pointer will seems like black.
    Edit: It will big difference for photodiode between background and pointer.
     
  16. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    I expect there would be a good difference in the amount of light being reflected in each case. You may need to shield this kind of sensor from ambient light to maximise detection.
    For instance: http://www.mouser.com/ds/2/427/tcrt1000-280041.pdf which is useable over a range of a few millimeters.
    upload_2017-12-9_12-53-49.png
     
  17. Danko

    Member

    Nov 22, 2017
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    You may not shield it from ambient light. Simple feed LED from 10 kHz pulse current source. Signal from sensor put on input of amplifier through capacitor.
     
  18. 128ITSH

    Thread Starter Member

    Jul 20, 2017
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    Thank you both for this help! Now I know the direction I should go for when designing this project.
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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  20. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    How far is it between the front of the glass and the pointer?
    The QRE1113 isn't so good at larger distances.
    upload_2017-12-9_16-20-35.png
     
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