Electronics > Projects, Designs, and Technical Stuff
RFID chessboard - "switchboard" similar to relay multiplexer and other questions
Rora:
My idea is to use a single RFID reader connected to an 8x8 crosshatch of antennas. One lead of each antenna would be joined with all others in its row, the other lead with all others in its column. Each row and column would have a switching mechanism then join together to run to the terminals of the reader. This would allow antennas to be connected to the reader individually by activating one row and column at a time.
I need a way to toggle the rows and columns. I know a relay multiplexer could do this, but it's a bit oversized, most use physical contacts, and possibly a little slow for such purposes. I was wondering if there's a board or IC that can act as a set of programmable SPST switches.
I'm planning to use 125Khz RFID for very short range/to eliminate the chance of scanning an adjacent piece. I don't suspect the additional lead length (roughly 24" max) shared by "dead" antennas would affect the reactance so drastically that it can't read, but wanted to run this by those of you who know more about RF.
Since I only need one reader, I can afford one with high performance features. Does anyone have any suggestions? Being able to adjust the power level and tune antenna characteristics would be nice, ideally being able to take a reading with custom antenna values tweaked for each squares characteristics.
Any input or other ideas are much appreciated!
Domagoj T:
Is there a reason you want to use RFID and not something simpler, like a reed switch or hall sensor?
Chess starting positions are known, and moves are done one by one, so a microcontroller can easily keep track.
Thinking more about it, hall sensors are better, since you can polarize magnets on the pieces so that white has north and black has south. That makes keeping track of pieces being captured a bit easier.
Rora:
It's true you can do almost everything required with deductive logic, however there are several reasons that I'm doing it this way such as automatic piece promotion and illegal move detection.
The primary reason, though, is because this is actually for a robotic arm that plays against you. Per-piece identification ensures a consistent experience by validating whether pieces are where the programming expects them. The player can't set up the board wrong and the goal is that pieces cannot otherwise end up where they shouldn't be (illegal move detection), or for example the player promoting the computer's pieces with the wrong type (a queen is not always desired) and the arm grabbing said piece in the wrong place, etc.
It does complicate things a bit, but it also simplifies the code for getting the board state and detecting changes. For instance, as you say a means to detect black or white is still required with deductive logic, in the case where a piece moves/disappears off of a square and could have taken one of two different enemy pieces. Computer vision is also a solution people have used, although I think this is slightly more reliable. There's trade-offs with each solution, this one just offers the ability to do some fancier features down the road and this is a project I plan to be working on for many years.
mikeselectricstuff:
For close-up reading like this, 125KHz RFID reading is trivially simple - you can do it with an opamp and simple data slicer, and decode in software. Antenna tuning is very noncritical.
The main issue you need to deal with is that the signal is bipolar, so you can't easily use diodes to isolate the matrix nodes as you can with buttons or LEDs, and that you need to avoid activating tags on adjacent squares.
If I was doing it I wouldn't bother with a matrix - I'd use a reader topology that grounds one end of the RFID coil ( so you only need to switch one end), and use a 1:64 analogue mux, with a bipolar +/- 5V supply. 5V should be enough to read button-sized 125KHz tags at a range this close, but you could up this to whatever your mux can handle - 4000 series could do +/-7.5v, I'm sure there are plenty that will do more.
The circuitry would be something likei
One leg of RFID coil to common ground, with a parallel damping resistor. Other leg to the multiplexer.
Reader would be a 125khz TTL-level squarewave, fed via a tuning cap and series resistor, maybe back-to-back zener clamps, to the mux common. This node is also connected to a diode detector, then to a limiting amplifier and adaptive data slicer - both functions could be done with a single LM358.
An issue you may have is the read time of a 125khz tag, which will probably take several seconds to read the whole board. Maybe split it into four 16-square sections that you can read simultaneously using either four small MCUs, or something a bit more powerful to handle all channels simultaneously.
mikeselectricstuff:
Just found a schematic of a reader I did many years ago - this has additional zeners as it was intrinsically safe, and also had some other functions, which can be ignored . This used an antenna about 50mm dia, to read credit-card sized card from about 75mm away.
Ignore U2 - the output of U2A is a 125KHz squarewave from a PWM or compare peripheral.
D6 provides limiting to avoid saturation issues with strong signals.
Your board would just be 64 coils, each connected to a mux input, and the mux output connected to the C1/D1 junction. It may be possible to avoid the need for a bipolar mux supply if you split the supply and connect the coil ground to +2.5V
If you can, use the larger ~20mm dia disc tags.
If you want to use PCB coils, you'll need to use less than the 1mH inductance, and increase the tuning cap to match, but tuning isn't too critical and you don't want a sharp resonance due to drift issues.
Note that without clamping, the voltage across the coil can reach many tens of volts, so you may want a parallel damping R from the mux input to ground.
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