I aligned the mirrors and powered up the draft hardware for the Nuiteblaster, and it didn’t work. This kind of fails to surprise me, as I changed a lot of stuff in the design. I think my speakers are good, but that using MOSFETs was an error. I used MOSFETs because they have a very low resistance when turned on, so they don’t waste energy as heat.
However, the MOSFETs in the design are switching 12V, and are getting a base drive of 5V. This means the voltage from the gate to the grounded source (Vgs) is at most 5V, because that’s what the Arduino can source, voltage-wise. The voltage from the drain to the source (Vds) is 12V. Since Vgs is > Vth (the point at which the MOSFET turns on), but nowhere near 12V, the MOSFETs are probably not getting driven into saturation, even when the outputs are fully on. This is compounded by the fact that since the code runs the PWM of the Arduino very fast, it is unlikely that that gate capacitance gets fully charged, and so the MOSFETs probably see a much lower average voltage at the gate, and so are probably operating in ohmic mode most of the time.
Whatever the cause, the speaker motion was quite small, and the MOSFETs got so hot that when I licked my finger and touched one of them, it sizzled.
I’m contemplating two possible fixes for this. The first is to replace the MOSFETs with the TIP120 power darlingtons that the schematic calls for. The second is to build my own power darlington transistor out of a 2n2222 switching the gate of the MOSFET to the full 12V of the beefy power supply that I’m driving the whole thing with. This would almost certainly drive the MOSFET into saturation, and hopefully get me the gain that my current rig lacks.
The speakers might also not be the best speakers for the job, but they are the ones I have. If I can’t get the system to work by changing the electronics, I’ll try new speakers next.
There is an Instructable up on using speakers as galvanometers for a laser projector. This looks just about optimal for the Nuiteblaster, as it provides readable text without defocusing or otherwise spreading the laser beam.
I’ve started building one, with a couple of modifications. Instead of resistors, I’m using diodes to snub the back-EMF from the speakers. I’m also using MOSFETS instead of transistors to switch the power to the speakers. MOSFETS have lower on-resistance than transistors, and so transfer more power and waste less energy as heat. They also have VERY low gate current (low enough to treat as non-existent for my purposes), so there’s no need for current-limiting resistors on the gates, although a resistor might be good to limit any ringing that might happen from slamming 5V into it. Since I’m driving it directly from a 5V microcontroller, gate drive and switching time hopefully won’t be a concern.
Every year, in early October, there is an arts festival in Toronto, called Nuite Blanche. The real title is “Scotiabank Nuite Blanche”, after a surprisingly large bankcorp that apparently decided that giving an arts event a bunch of money was a good move. Whether this makes for a bunch of overly bland corporate art is a question for people who spend more time thinking about that sort of thing, but I will note that the independent/outsider art pieces that were allowed to participate were in a parking lot in the distant reaches of bumblefuck nowhwere.
Also, at least in 2010, a lot of the pieces could be summed up as “Revenge of the Son of the Guy with a 20k Lumen Projector”. There were a lot of illuminated rectangles of shifting colors or scrolling text messages projected on the flat sides of buildings. That’s not how you do engagement with the urban space. Really, they could have at least done projection mapping on interesting buildings, or responded to the historical and geographical placement of the building. That’s also not how you do engagement with the audience. Any pinhead with $200 to rub together can have “LOL DONGS :-P” in fifteen foot high letters on the side of a building. They don’t really need an an artist with a SMS-to-projector gateway to set it up.
The cumulative effect of both the administrative actions (street closings, businesses staying open later) and the placement of the art seems to say that art is still a one-way channel from the artist to the spectator, but also a transient thing that doesn’t leave a permanent change in the urban environment. Art is something other people make for you to watch at special times, not something for you to do whenever you want.
So I’m restarting my project to build the NUITEBLASTER. It’s a portable projector that people can use to project responses (critical or otherwise) directly onto the art at Nuite Blanche. The heart of the projector will probably end up based on this laser projector, using a 200mW laser shining through a tiny LCD. Laser-based projectors have a couple of unique advantages over white-light projectors, not the least of which is that they are in focus at all distances. By choosing an output lens with minimal enlargement, I can probably throw a readable image quite a long way. The whole device will be lightweight and small.
For the display and input, I may end up using a combination of an LCD from one of my old wearable computer monocular displays and a BOB-II video overlay board that I have sitting around. I may also just skip the video overlay board entirely and have a laptop with S-video or composite out generate the video signal directly. Either way, people will be able to put the generated video stream up on the sides of buildings. I’ll let members of the public put whatever they want up, right next to (or over) the approved artists.