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We are attempting to electronically determine which of 10 cards, numbered 1-10, is currently in front of a stack. Initially card 1 is in front. Eventually card 1 will be flipped around to the back of the stack, and card 2 will then be in the front. Then card 2 will be flipped around to the back of the stack and card 3 will be in front. This process repeats for all cards labeled 1-10.

These cards will be viewed by people so we are looking for a solution which won't require a device positioned in front of the cards (such as a barcode scanner). These cards will also be outdoors making any type of IR sensor not possible.

Does anyone have any ideas on how we can detect which card is in front?

See an example image:
enter image description here (source)

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  • \$\begingroup\$ Will they always be flipped in that order (i.e. not flipping several at once)? Perhaps have something sense when a card passes by it and increment a counter? In this case a beam sensor could work, or you could even use a hall effect sensor with miniature magnets connected to each card. Lots of possibilities, need some more specifications from your end \$\endgroup\$
    – DerStrom8
    Sep 2 '15 at 21:15
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    \$\begingroup\$ Just as an input for thought - is it possible to use the backside of the last card? If the order is always the same, the backside of the last card will tell you which card is in front currently. That might open up some options. \$\endgroup\$
    – Arsenal
    Sep 2 '15 at 21:31
  • \$\begingroup\$ Assuming that the cards are bound so as to always be in order (as shown in your photo), you could have bar codes on the back of the cards and a scanner looking at the back-most card. For example, if you have cards 1-10, the bar code of #1 would be on the back of #10, the code for 2 would be on the back of #1, the code for #3 on the back of #2, etc. - Edit: @Arsenal beat me to it. \$\endgroup\$
    – DoxyLover
    Sep 2 '15 at 21:33
  • \$\begingroup\$ @Arsenal. I know these scoreboards as I play tennis too. They have numbers on both sides so they can be viewed from both ends of the court. \$\endgroup\$
    – Alexxx
    Sep 2 '15 at 21:45
  • \$\begingroup\$ It's not very simple though. For example the card that has a 6 on it (see picture) has number 7 on the reverse side. Therefore if you flip it to the other side of the net the next card with number 7 will be visible on this side while 7 will also be on the other side of the net. \$\endgroup\$
    – Alexxx
    Sep 2 '15 at 21:49
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These days, machines can read just as well as humans can. Take a Raspberry Pi, a video camera module and a suitable lens that can be zoomed in on the cards from the edge of the court. OpenCV software (or other OCR or machine vision software) will be able to tell you what the digits are fairly easily.

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  • \$\begingroup\$ Thanks Dave. That's one idea I didn't consider. A bit more complicated of a solution than I originally hoped, but definitely something to consider since no other solutions seem practical. \$\endgroup\$
    – John Teel
    Sep 8 '15 at 17:33
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How about mounting these small neodymium magnets at the bottom of each card but at different positions along the bottom edge. Then add 20 hall-effect sensors in a single line at the bottom of each bracket (10 on each side) so they can sense which cards are on each side.

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    \$\begingroup\$ Alexx thanks for the suggestion. I did already test out this idea. The problem is that even if a card isn't at the front, the hanging magnet will be close enough to it's designated sensor to detect it. So it detects the front several cards all at once. \$\endgroup\$
    – John Teel
    Sep 8 '15 at 17:35

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