Keith's Electronics Blog

This NewScientist article from last spring shows a very cool system of motion sensors for occupancy tracking. The sensors are installed more densely than in typical buildings, so they can detect occupants’ positions fairly precisely within hallways.

The most interesting part of the system for me is the video showing playback of motion through a building — you can clearly see where people were walking, where they ended up, etc.

This seems like a natural fit for a smart house — instead of just watching where there’s motion now, pay attention to where the motion has gone and remember that there must still be someone in that room (even if they’re not moving much now).

$10 Parallax motion sensors + blinders to narrow the cone of coverage + write open-source software for the capture and playback, anyone?

A long time back, I was over at Lawrence’s house and Gail had a number puzzle up on the whiteboard that she was using with her homeschool consortium gradeschool math class.

For the students, I think Gail drew the circles with pebbles in them and the younglings had to count the pebbles and write the sums of the nodes into the edges.

For adults, it’s a lot more interesting to start with the numbered edges and fill in the vertices with numbers that make the edge sums work. It’s not hard; it’s just a little bit of a twisty way of thinking.

And you have to keep your vertices balanced enough that you don’t “overflow” an edge. For example, if you started with one pebble in the upper left, then you’d have to have four in the upper right, and then zero in the lower right, and let’s just say that zero pebbles isn’t permitted. Any solvable configuration will have multiple correct answers, but they’re constrained by overflow and underflow of all the edges of the graph.

Puzzle Ball

All of which made me think that this concept could make for an interesting LED puzzle ball. Make a dodecahedron (or a smaller regular polyhedron, or even a larger geodesic shape) with LEDs along the edges and LEDs and buttons (up/down?) at the vertices.

Start it up, the LEDs flash and wink all over the place as the controller randomizes the configuration. Use the buttons to adjust the numbers at the vertices (maybe blue LEDs) so that adjacent vertices add up to the number of LEDs on that edge. As each edge is “solved,” its LEDs change from red to green. Once you have all the edges green, you’re done.

Aesthetics

Seems like it’d be coolest as a “wireframe” polyhedron rather than a solid (or solid surface).

I think I’d prefer SMT LEDs over through-hole. 3mm through-hole if absolutely necessary.

I think I’d like the edge LEDs to be strips of nine that light up fairly symmetrically from the middle out, i.e. something like

----o----              ----o----
---o-o---              ---oo----
--o-o-o--              ---ooo---
-o-o-o-o-          or  --oooo---
o-o-o-o-o         the  --ooooo--
-ooo-ooo-      boring  -oooooo--
-ooooooo-              -ooooooo-
oooo-oooo              oooooooo-
ooooooooo              ooooooooo

I’d like the whole thing to be constructed from PCBs, no separate case. There’ll need to be room for microcontrollers and/or LED drivers. Maybe instead of having the PCBs on the face of the polyhedron, have them edgewise so the PCBs’ outer edges are the edges of the polyhedron. Internally, PCBs could be held together by jumper wires soldered together at the board edges and carrying the signaling bus from one board to another.

Gameplay

I still haven’t decided whether solving the puzzle ball would be fun or merely tedious.

I remember a particular game mode in the old Merlin where you’d press buttons and all the adjacent squares would invert state, with the goal being to get all the lights on or off. I always enjoyed that game, and the way local changes impacted neighboring cells, which impacted strategy for dealing with those neighbors. I would hope that the puzzle ball would be fun in much the same way.

If each edge has nine LEDs (1 – 9) and the game is played with non-modular addition, then the average number of lit LEDs per edge will be 5.5, making the average number of lit LEDs per vertex only 2.75 — that is, on average, only a very few numbers that need to be tried at each vertex. That doesn’t sound like a very interesting game. I could increase the number of LEDs per edge to 20ish, making the average edge sum 10ish, making the average vertex 5ish, which seems okay.

Or should there be ten LEDs per edge and operate in modular arithmetic? Would that make the puzzle too easy, having more correct solutions?

Jeremy and I have been playing with my “new” Akai Headrush E2 looper pedal, which allows you to record an instrument (or whatever) into it; then it plays your mini-recording back for you over and over and over in a loop, and you can play along. You can also overdub, recording more stuff on top of what’s already there and building up a more complex mix in the loop — this is how KT Tunstall does solo performances of “Black Horse and the Cherry Tree”, which are just amazing to watch.

However, Jeremy and I both have some issues with the feature set and the user interface of the Akai, and we’re thinking about what it would take to build a better looper — or at least, one more suited to our desires.

Feature Wishlist

Here’s what we want:

• At least 60 seconds of loop time, which would be enough to play 12-bar blues at a fairly slow tempo. The Akai’s 17.8 seconds in low-fi mode is enough for KT to do two measures of rhythm and backing vocals, but not enough to record a full chord sequence.
• Tap a Record footswitch to begin recording, tap again to end the recording and start looping. It doesn’t absolutely have to be the same switch; but some loopers out there use a potentiometer to set loop duration. We want duration to be determined by the duration of the phrase we played, as it is with the Akai.
• Independently-addressable tracks or channels. The Akai lets you discard all of your mix except your very first pass and rebuild from there. We want to have (say) eight tracks/channels in the loop mix, each with a footswitch and red/green LED. Tap Record and the next unused channel winks green and records. Tap any channel’s footswitch to put it in (green LED) or out (red LED) of the mix. Not-yet-used channels are dark.
• Independently rerecordable channels. To rerecord a channel, tap it out of the mix and immediately tap the Rerecord footswitch (which is separate from Record and a little further back or off to the side). That channel winks green and rerecords. Various loopers have varying levels of undo or redub, but none (that I’ve found) let you do this.
• Ideally have stereo recording so I can loop with the full stereo chorus effect of my analog synths.

There’s room to negotiate on the exact UI, but that should convey the feature set we’re after.

How to Build It?

John suggested using an AudioPint and Pd, and I think I may prototype this with an Arduino to run the control panel and Pd on a laptop to do the audio. But ultimately I’d like to build at least two of these in nice stage-ready stompable enclosures, and I really don’t feel like using a general-purpose OS, for a number of reasons.

It seems to me that this might be a good project for a DSP or an FPGA, but I don’t know enough to make a choice. Any suggestions? (I know I don’t need the full power of a DSP.)

What I’d love to find in one IC would be:

• Dual 16-bit ADC and DAC capable of 44.1kHz
• Address and data buses capable of addressing enough external RAM for the audio (say about 1G for stereo * 16 bits * 44100 samples/second * 60 seconds * 8 channels) without my having to worry about DRAM refresh circuitry
• A processor that I can program for the very simple, core task of summing/averaging samples from the active channels to feed to the DAC
• ~32 I/O pins to manage the control panel, and/or I²C or SPI to use offboard I/O chips
• Ideally a USB interface for uploading samples to a computer, but this would be an extremely low priority

Does such a thing exist? Is that necessarily an FPGA? Can I get a development environment for < $500 and a chip for < $30?

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