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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)

Nick Alexeev
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John Teel
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  • 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 – DerStrom8 Sep 02 '15 at 21:15
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    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. – Arsenal Sep 02 '15 at 21:31
  • 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. – DoxyLover Sep 02 '15 at 21:33
  • @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. – Alexxx Sep 02 '15 at 21:45
  • 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. – Alexxx Sep 02 '15 at 21:49
  • @John Teel Can you make these cards from two thiner cards and insert an RFID sticker between then? – Alexxx Sep 02 '15 at 22:05
  • @Alexxx: RFID won't help -- all of the cards are in close proximity to each other, and they'll all respond. You won't be able to tell which one is on the front. – Dave Tweed Sep 02 '15 at 22:27
  • Some standards like the 13.66 MHz ISO 14443A/B and 15693 include anti-collision features but not all reader manufacturers actually implement it. The protocol lets a reader talk to an individual tag by its serial number. – Alexxx Sep 02 '15 at 23:46
  • Thanks everyone for all the feedback. Yeah, RFID won't work since they are to close together. Infrared sensors won't work because they will be in direct sunlight. Barcode scanners would require something in front of the cards which might block them from being viewed. Also not sure how well a barcode scanner will work outside in direct sunlight. The cards should be flipped one at a time, so perhaps an incremental counter might work. – John Teel Sep 08 '15 at 17:30
  • It seems like a simple thing to do, but I'm finding it's not so trivial. I also testing out using a capacitive sensor but they require the cards be perfectly flush against the sensor to detect it. I was thinking I could have tab hanging down from each card (in a different location) and could use a cap sensor, but like I said it requires the tab to be flush which isn't practical. – John Teel Sep 08 '15 at 17:32
  • See my answers on . – Transistor Dec 06 '15 at 14:11

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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.

Dave Tweed
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  • 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. – John Teel Sep 08 '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.

Alexxx
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    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. – John Teel Sep 08 '15 at 17:35