# Detecting numbered cards in a stack?

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:
(source)

• 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 Sep 2 '15 at 21:15
• 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. Sep 2 '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. Sep 2 '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. Sep 2 '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. Sep 2 '15 at 21:49