For example, I have a board (A plain board or a board with sensors, this needs to be decided depending on sensors ). On this board, I will place small objects. These small objects are having 2 sides (like a coin). I just want to know how many objects are placed on this board.

What kind of sensors can I fit on the board?
What sensors can I use on these small objects? (might be some sort of magnetic strip attached to these small objects and the sensor on the board can detect these object)

The objects can be placed randomly(not one at a time) on the board. Each object can be of different weight.

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    \$\begingroup\$ Mount a camera above (or below) the board? \$\endgroup\$ – geometrikal Dec 26 '13 at 13:50
  • \$\begingroup\$ Individual RFID tags? You'd have to check whether they can detect more than one tag at once. \$\endgroup\$ – pjc50 Dec 26 '13 at 23:13

Your specification asks "how many objects are placed" and if they all weigh the same, use a sensitive strain gauge and amplifier feeding a small MCU with a display.

If after reading this answer you realize that the objects don't all weigh the same then you'll have learnt a lesson in writing specifications accurately.

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  • \$\begingroup\$ Thanks for your reply. The thing is some times I might place coins, some time I might place marble. I am free to add some kind of small tap (magnetic, which helps to detecting when placed on the board) to these objects. Not getting which sensor to use. \$\endgroup\$ – NJMR Dec 26 '13 at 12:05
  • \$\begingroup\$ If the objects are placed, one at a time onto the board, an incremental change in detected weight tells you how many objects have been placed. Providing the strain gauges fitted are sensitive enough AND the weighing mechanism has very little stiction this is still possible. Look for load cells or strain gauges. Imagine you had a very sensitive bathroom (or kitchen) scale that can measure fractions of a gramme - can you envisage how this would work. If after reading this you decide that two objects may be placed at once then again, writing the spec of what you need is your weakness. \$\endgroup\$ – Andy aka Dec 26 '13 at 12:19

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