Magnetic Sensors

After researching on the Internet, checking that both commercial and DIY projects boards use reed switches as magnetic sensors, I decide to use them in my project, too.

Selecting the reed switches

There is plenty of information on the Internet about these switches. These are two of the sites which have helped me to understand their specs, to learn how they operate, etc:

  • Reed Switch Info: www.reed-switch-info.com

  • Reed Relays and Electronics India Limited: www.reed-sensor.com

My choice:

VISHAY RI-23B

Dimensions:

Alternative:

  • Slightly smaller and more fragile bulb, but it works well: MKA-14103

Selecting the magnets

There are several factors to take into account when selecting magnets to mount into chess pieces: shape, material, size, strength, etc.

  • Shape and size: Due to the common shape of the bottom of the pieces, the circular disc magnet is the most suitable. It will be as thin as possible (since it must be embedded in the piece), and will have a diameter somewhat lower than the bottom of the pawn.

  • Material: To achieve a good pull force with a thin magnet, we have to discard materials such as ferrite. Neodymium is ideal.

  • Strength: We have to find a balanced value of strength, since excessive strenght may result in premature malfunction of the switches (by deformation of the reeds), in addition to not only activate the switch of its square but the others around. On the other hand, a magnet with very little strength, could not activate the switch if the piece is not located right in the center of the square. In any case, this factor depends on the distance between the magnet and the reed switch (which is determined by our design: wood thickness, distance at which we place the switch, ...) and the sensitivity of the reed switch chosen.

  • Other factors: There are other factors to be considered, such as the type of magnetization. In a circular disc magnet it's usually axial, ie, the N pole is on one circular face and the S pole is on the opposite face. This affects when positioning the reed switches with respect to the board squares.

My choice: MAGNET Expert F309

Circular Disc Neodymium Magnets (15 mm dia. x 1 mm thick)

Note:

  • I have tested other far cheaper neodymium magnets, purchased on eBay, and they have worked well. They are more fragile and break easily, but that does not matter when they are already assembled.

Magnet-Switch relationship

It's important to experience with reed switches and magnets before designing a board like this, in order to not to get a nasty surprise when finished.

For example: If the switches are placed (horizontally) in the center of the squares, a disc magnet with axial magnetization (each pole on one side) will not activate the switch by staying over it, ie, on the center of the square.

In the following graphic you can see such behavior:

Testing

I experimented with several thicknesses of wood, until I found the right reed switch position:

I stick a reed switch to a sheet of paper, and I stick this sheet of paper to the bottom of wooden remnants of different thicknesses. Then I connect a continuity detector to its pins.

At the top of the wood I put another sheet of paper, on which I draw the reed switch position and the active zone: the area inside which the switch is triggered by the magnet.

Reed switch position, relative to the square, which finally I choose for my design. (For 45mm squares)

Addendum

Thereafter I have done more tests with different reed switches and varying its position, the size of the square and the magnet, etc.

Here are some pictures that may help to understand the process. It's important to consider the "actual" distance from the magnet to the sensor, so you have to keep in mind if the chess piece will take any material on the bottom, such as cloth or velvet.

50mm squares and 19x2mm magnet. I add 1mm adhesive tape to the magnet, in order to simulate the actual test distance (6mm).

Reed switch fixed on the bottom of the board, and two wires soldered to their ends.

4 spacers are installed to the bottom of the board, to work on the other side in comfort.

Multimeter in "continuity check". In that square the magnet doesn't affect the sensor.

By placing the magnet in the appropriate square, the multimeter beeps.

The pencil drawing delimits the area where the magnet activates the magnetic sensor. The 19x2mm magnet is too big/strong for this combination.

I make the following tests with a 14x1mm magnet...

And finally with the magnet fixed to a pawn, and using a LED to not bother (with the constant beeping of the multimeter) to the other living creatures with which I live. ;-)