Computing stuff tied to the physical world

TwitLEDs robot – action shots

In AVR, Hardware, Software on Jun 25, 2010 at 00:01

As promised in the previous post, some pictures and movies of the TwitLEDs robot in action.

First another view of the robot, with the different pieces:

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We don’t have the proper “arena” yet, i.e. a big fenced-off floor area covered with glow-in-the-dark paint. Instead, these first trials used two pieces of foam board, taped together. It’s not perfect because the robot keeps running off track, and because the hump between the two pieces is quite high. Foam board isn’t really suited for this: it curls up too much from the moisture in the paint. We probably should have waited a bit longer for everything to dry completely…

The inital test code just printed out its name (povGlow) and the compilation date. In the first tests, one of the LEDs wasn’t working (a software bug, not hardware), so the text isn’t quite right:

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You can clearly see the fading of the letters over time. This happens very quickly, but those faded letters then remain visible for quite a while. That’s why you can still see several trial runs “printed out” on the foam board.

So the basic idea of printing with light works, as you can see!

Here is a video (sorry, not inline), showing how the robot veers to the right as I put my hand in front of it to prevent it from running off the track. Note that I’m not touching the robot, I just briefly trigger the distance sensor. The on-board LED lights up in red when the correction takes place. It’s not very smooth, but it works.

Another picture, showing the decay of the printed text brightness:

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Maybe 7 pulsed blue lasers (from DVD writers, perhaps?) could be used to create even more intense blue dots. As it is, with these blue LEDs the writing is very clear – but only in a relatively dark room. With the lights on, the text becomes virtually invisible. Even though the LEDs are bright enough to be painful to look at:

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The quality of this “printer” is actually pretty good, considering how simple its technology is:

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Finally, a run where the robot happened to stay a bit longer on track, allowing it to display its brief message a few times. Again as video – you can see the wobbling foam board, and this thing driving like a drunken duck, leaving its trail of fading messages behind.

All that remains, is to try and get some tweets into it in real time. Stay tuned…

Several days of testing have now drained the 4 alkaline AA to the point where the robot advances noticeably slower. Looks like we’re getting no more than an hour or two out of these batteries. Which is not really surprising: the DC motors must be eating quite a bit of current, and the LEDs probably draw up to 200 milliamps or so. Self-powered autonomous motion is really hard!

  1. Have you tried (or are you) using UV LEDs?

    See Ghostmatrix by Jonathan Foote:

    • Have you tried (or are you) using UV LEDs?

      Hey – I didn’t even know they existed. Thanks for the tip!

  2. I would love to see a laser in this. If you want to try a Violet laser (405nm “Blu-Ray”) heres a link off the top of my head. A PHR-803T would run at 100mA & give 90mW of light output focused to a pin-point. Instead of using 7 lasers you could scan it back & forth quickly. I have used a read/write arm from a broken HDD with a couple mirrors mounted. Some background on building a complete laser–47684.html Feel free to email me with any questions. Also a blue 445nm laser can do 1 watt of light output for ~$45 (prices are dropping on that).

  3. Opps that laserpointerforums guide was taken down.

    • Thx. It’d be great to use a laser, but that would have required a more complex mechanical construction (scanning – a servo is probably way too slow). Also, I was wondering: how do you turn lasers on and off quickly? Can you just pulse their power supply with a MOSFET?

  4. Yep, you can pulse ‘m. Cheap scanning system, mirror, rubber band and electromagnet. PWM the electro magnet to set the mirrors position. Rubber band is used to set the mirror to its “home position” once the PWM is low. Take good care with lasers though, 90mw is already more then enough to make an ever lasting impression/burn retinas. With normal RGB lasers you’ll also have an auto response to turn your eyes away(which still may be to slow at high power), but the more you’ll go towards IR or UV spectrum the lazier the eye gets… Personally I wouldn’t go beyond 5 mw with a daughter interested in science.

    It’s great to see the fun you both have though !

    • Ah, great tips. I’ll have to try that one day. Yes, laser beams scare me. But there is incredible potential. This youtube video is amazing – I think it’s all done with laser positioning and a single global light sensor. Would love to figure out how to do that.

  5. Two rubber bands, mirrors, electro magnets plus sensor ? Just kidding ;-) I’ve read about using harddisk read/write arm just as Mike wrote, those are very fast and accurate unlike the rubber band approach.

    Its a pity it’s been a while since I saw it, I also can´t give you an URL for this one, I’ve seen people using mirrors attached to a PWM-ed speaker. Basically the same principal as the rubber band approach though faster/more accurate. I guess the Hard disk arm option probably is easiest…, most accurate and probably fast enough for an application like this. Detection of reflection versus less reflection probably isn´t that hard as well. Writing the software to create such fast/accurate scanning, eh… I’d probably let someone else do it. Come back in a few years, for now i’m almost wondering whether it’s photo shopped ;-) I believe I can tell by the pixels, LOL.

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