My primary goal this week was to figure out what I need to build one magnet detection square. In order to accomplish this, I first had to revisit physics 2 to relearn how magnetic fields work and are measured. After doing this, I took into consideration the magnitude of fields that our hall effect sensors will be able to detect, and came up with what seemed like a reasonable board thickness (since our sensors will be under the board, and the pieces will be above the board). These numbers were a range of -45 to 45 mT and a thickness of 1/8 inches. With these in hand, I settled on a set of N42 magnets with a disc diameter of 1 inch and a thickness of 1/16 inches. Their measured surface field was 819 Gauss, and one Tesla is equivalent to 10,000 Gauss. As distance r increases, magnetic field decreases with respect to 1/r. Since the surface field is assuming a point in the center of the magnet, the surface field is at a distance of 1/32 inches, and our board will be 4 times as thick, so it will be ¼ the strength on the other side. As a result, this magnet should be detectable at 20 mT by the detector if it is flush against the board. I plan on using thin spacers to elevate magnets higher to change the field for different pieces. I placed an order for the appropriate magnets and sensors, and hope to settle on a spacing system next week and also determine any changes that must be made based on differences between my readings/calculations and how the sensors and magnets actually end up working.
In addition to this, I started my basic circuit design. I have already designed a testing circuit I can use with my arduino, since the actual raspberry pi and ADC will be arriving later than the sensor and magnets. Conveniently, the input voltage can range pretty significantly, so for testing purposes a single sensor can be powered straight from the arduino. I also spent time researching analog multiplexers to use for each row. Our plan is to have an 8-input ADC that receives input from 8 8-to-1 muxes, which will receive input from the 8 squares on each row. I found one that looks like it should suit our purposes. The CD74HC4051-EP is an 8-to-1 digitally controlled analog multiplexer, so it will be easy to send lines from one pin on the raspberry pi to all the multiplexers and then sweep the 8 inputs to the ADC. I am behind right now, but am bottlenecked by not having parts to run tests with yet. I hope to drop a lot of hours into testing after the presentations next week to try and catch myself back up in time to have a form of functioning square before leaving for spring break.