Which type of sensor ?

[MichaelNielsen]

Im trying to figure out which type of sensor that suits my need.

I have 12 different objects, the 12 objects are placed randomly on a 8x8 board (64 fields), only the board can be connected to a power source,

I want to detect on which field the items are positioned, I'm thinking on inserting different strength magnets in each item and then using 64 hall sensors, but this
solution seems to be rather expensive - are there a cheaper way to accomplish this.

Edit : changed from 30 to 64 fields.

 

[djsfantasi]

Do you have to distinguish between EACH of the 12 objects or just know which of the 30 locations are occupied?

In the former case, what distinguishes the 12 objects to you? Size, shape, weight or something else? How about photocells on each field of the board in the latter case?

 

[MichaelNielsen]

I need to distinguish between the 12 different objects, the 12 objects are approximatly of same shape, size and weight, but i have some freedom when it comes to the material.

I cannot use photocells, the sides of the board cannot be changed, so im looking for some kind of solution where i can insert the sensors into the board, the height of the board can be up to 4 cm.

Another idea i was playing with, is to use a transparent board, marking the bottom of the objects by some kind of pattern, and then maybe use some kind of camera.

 

[FvM]

You can use RFID tags and multiple reader coíls in a 8x8 row/column arrangement.

 

[franciscorc]

Hi all

Could you use a bar code ? RFID Is a great idea but I do know about costs.

Best

 

[Abeer_h]

RFID tag price around 1$ i think
but if he used RFID tag, does he need to buy a reader?
i think one reader will be enough.

 

[Nick C.]

How about each object containing a resistor with a unique value. Each square would have a matching contact on it - maybe you can get away with some short machine pins on the bottom of the object, sitting on an exposed pad on the 'square'. A lowly 8-bit ADC should be enough for the job of differentiating between the multiple objects.

A similar idea was used on late 80's Chevy Corvettes (and probably others) as a security measure.

To read 64 channels, you could go with something like 8 ADC0838's, but I'm sure there is a better way to limit the parts count (using some CD4051's, a uC with lots of ADC channels, etc).

 

[FvM]

but if he used RFID tag, does he need to buy a reader?
i think one reader will be enough.

Yes, but a special multiplexer for the reader coil matrix must be designed.

 

[akshay_d_2006]

Do you just want to know where the object is placed or also which object is placed.
For just to know where a particular object is placed you can extend the concept of telephone dialing system (DTMF)
Imagine 3x3 grid.
x:A1 A2 A3
y:B1 B2 B3
z:C1 C2 C3
L m n

when you press B2; the x and m freq. mix together and give you a unique o/p. Same applies for all. Same applies to larger grids. Its easy to use. If you decide on this google DTMF or telephone (landline) dialing system. This is just an option. I am sure there will be better options.

For determing particular objects. RFID fits the bill

 

[Sink0]

Are they placed on a random position and orientation or they going to be fitted on a specific way on the board?

 

[djsfantasi]

:idea:If the objects can be oriented on each square, have each square contain 4 magnetic sensors - Hall effect sensors or reed switches. Each object can then be binary encoded by placing magnets in corresponding positions on its base in binary.

E.g.

none, none, none, none is object 0
none, none, none, magnet is object 1
none, none, magnet, none is object 2
etc...​

Then you can scan/address each square by its row and column and determine by the 4-bit value, which object is placed there...