PCB design for pattern matrix 2

This is the pattern matrix 2 tangible sensor schematic, which is fairly simple – just 4 hall effect sensors and a capacitor to smooth out any noise on the power supply.

sensor-schematic

We need to make hundreds of these for the Penelope Project, and we can save some costs by using the built in pull up resistors in the MCP32017 to get a decent signal from the sensors. The difficulty with this PCB is arranging the sensors so they align with the magnets in the tangible programming block in the optimum manner. From tests with the prototype Lego rig, this isn’t actually too critical – but it’s set up so the lead length can be tweaked a bit at soldering time.

sensor-render

This took me about 20 variations to finally get right, but the circuit is just about simple enough that it can be made single sided – this is good because the top side will be partly exposed, while the lower side with all the copper traces can be protected. It’s good practice to have large areas of copper left connected to ground, partly as it’s a common connection needed all over the board, partly for stability but also it reduces the amount of chemicals required to etch the circuit – as only the parts around the traces need to be removed.

sensor-render2

The i2c expander board is a little more complicated. The design is made to be modular so we can stack up any number of these connected to the Raspberry Pi for different arrangements of sensors. Each board can deal with 8 sensor locations (each comprising 4 individual hall effect sensors). Their job is to convert the digital signals from each sensor into serial data (using the i2c protocol) so the Raspberry Pi can read them all just using 2 wires, plus 2 for power.

i2c-schematic

Each board can be configured to a separate i2c device address to tell it apart from the others using jumper connectors. This one had to be 2 sided, but I managed it without any ‘vias’ (holes to pass traces from one side of the board to another). I also added a power indicator LED as a last minute addition.

i2c

I’ve been learning the open source Kicad software to design these, which is now used by CERN for building the LHC, so it’s pretty fully featured! The idea is that you draw the schematic first, link each component with a physical ‘footprint’, then switch to the PCB design stage. Other software I’ve used in the past tries to route everything in one go for you (and can come up with some pretty strange and messy results). Kicad works in a semi-automatic manner – you need to draw each trace by hand, but it routes them around components and other traces, and suggests the shortest path for you. This is quite a lot better than a fully automatic approach as you have more control over the end result, and easily end up with a decent placement of all the parts.

i2c-render

This project is of course open hardware, and can be found on github here.

Sonic Kayaks Hacklab

Part one of our two events for British Science Week was the Sonic Kayak open Hacklab with Kaffe Matthews and Dr. Kirsty Kemp. Amber has reported our findings here, this was the first time we successfully trialled the technology and ideas behind the Sonic Kayak, in future we will be refining them into instruments for experiencing the marine world. More on that soon!

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Pattern matrix – putting it together

Here is a member of staff at Miners Court trying some tangible weave coding in the midst of our crafts area – at the moment it’s simply displaying the weave structure on the simulated warp weighed loom with a single colour each for warp and weft threads, the next thing is to get ‘colour & weave’ patterns working.

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The pattern matrix is the second generation of tangible programming device from the weavecoding project. It’s been built as an open hardware project in collaboration with Falmouth University’s Makernow fablab, who have designed and built the chassis using many 3D printed parts and assembled the electronics using surface mount components (far beyond my stripboard skills).

Here you can see the aluminium framework supporting the AVR based row controller boards with the Raspberry Pi in the corner. The hall effect sensors detect magnetic fields – this picture was taken before any of the wiring was started.

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The row controllers are designed to read the sensor data and dispatch it to the Raspberry Pi using i2c serial communication running on their atmega328 processors. This design was arrived at after the experience of building flotsam which centralised all of the logic in the Raspberry Pi, resulting in lots of wiring required to collect the 128 bits of information and pass it to the GPIO port on the Pi. Using i2c has the advantage that you only need two wires to communicate everything, processing can be distributed and it can be far more modular and extendible in future. In fact we plan to try different sensors and configurations – so this is a great platform for experimenting with tangible programming.

This video shows the current operation of the sensors and row controllers, I’ve programmed the board with test code that displays the state of the magnetic field with the status LED, making sure that it can tell the orientation of the programming block:

The row controllers have a set of multiplexers that allow you to choose between 20 sensor inputs all routed to an analogue pin on the AVR. We’re just using digital here, but it means we can try totally different combinations of sensors without changing the rest of the hardware.

After getting the first couple of rows working and testing it with elderly people at our Miners Court residency there were a couple of issues. Firstly the magnets were really strong, and I worried about leaving it unattended with the programming blocks snapping together so violently (as we plan to use it in museum settings as well as at Miners Court). The other problem was that even with strong magnets, the placement of the blocks needed to be very precise. This is probably to do with the shape of the magnets, and the fact that the fields bend around them and reverse quite short distances from their edges.

To fix these bugs it was a fairly simple matter to take the blocks apart, remove 2 of the 3 magnets and add some rings to guide placement over the sensors properly:

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