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Massive dot-matrix LED sign, need help picking a technology

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ajb:
The conditions in which it needs to be visible will really determine the feasibility here.  Maximum solar illuminance can be up to 120,000 lux, and since five acres is about 20,000 m^2, that means you have about 2.4 gigalumens to compete with over the entire area.  Making the background dark will certainly help provide contrast, but even with a 1% reflective material, that only takes you down to 24 megalumens.  You can reduce that further by only targeting a limited viewing angle, and using lenses and/or reflectors to send your light in that direction, but remember that lenses and reflectors work both ways, so you'll need to take care to make sure that sunlight isn't focused down into an LED-death ray inside your contraption.

Obviously, something that only needs to be visible in the dark is going to be much easier, and at that point it's mainly a question of what sort of visible size each pixel needs.  Maybe you only need a couple of PAR lamps every meter or so, or maybe you need something that illuminates the entire ~square meter area.  You probably will need to do some tests on a small scale to see what sort of visual size and brightness you want before you really go any further on the project.

IDEngineer:
The single largest question on this project is "Daytime or nighttime?"

If daytime, then keep it simple which will make it easier and cheaper to build. First thought is an automated version of Nusa's suggestion: Rotating flat panels, each of which represents a pixel, that have contrasting colors on either side and can be flipped remotely. This is exactly the technology used by several roadside sign designs prior to the advent of high brightness, high efficiency LED's. Each pixel was a brightly colored object that could be revealed or obscured. It's like CMOS... nearly zero static power consumption. Just scale up the size for your project. The active element could be anything... a solenoid could work, and require power only during switching with the panel axle having a slight detent to retain its position with power off.

As for electronic architecture, I'd keep all modules identical and run a serial data line where each module reads its bit. That way servicing becomes easy, just swap in any module and use a dipswitch to set its X and Y "address". You don't have to keep track of which wiring harness goes where, etc.

This whole thing could run off a car battery.

Cyberdragon:
Flip dots are WAY more expensive (like 10x) than LEDs. A single pixel can cost up to several dollars whereas LEDs are cents. If you are willing to spend the money, you can even get lit flip-dot displays. Otherwise standard modular matrix displays are yout best option. The controller can range anything from a dumb repeating ROM computer that just sets and repeats to a full computer with video. But either way, way cheaper than flip-dot.

richard.cs:
Agreed that conventional flip dots are more expensive, but the power requirements for a daylight-visible emissive display are enormous, and providing/distributing that power does not come cheap.

I'm wondering if there's some existing thing which can be adapted to make large reflective pixels. Window blinds would seem promising, spray one face black, the other white (or maybe one side in some sort of dirt colour to blend in with the background as your pixels will not be butted close together), and then it's just pulling strings which has a modest power requirement.

ebastler:
Unless the "marriage proposal on a private plane" market is much larger than I think, I doubt that this will be commercially viable... To approach this from the other side, do you have a budget you are willing to commit to this?

I think we are looking at a large, mains-powered system, which you will need to build in a way that keeps it safe from wildlife, weather, and plant growth. More likely than not, you will also end up spending more time than you like on maintenance -- replacing failed components, fixing water ingress, mowing the lawn or meadow. (With the installation getting in the way of mowing...)

If you are considering a stationary (non-programmable) solution, do you need to be able to switch it on and off at all? Couldn't you simply ask a local farmer to mow or seed a field in the right pattern?

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