I packed the vehicle parts into the car (using the freshly made packing crate for the straight parts) and headed to Lux Lounge (666 South Ave.) Around 9:30 or so I got some help getting all the parts into the back yard. In all, I think it took about 30 minutes to get the whole thing assembled in the dark. I think about 12-16 people got a chance to try it out … I took some video and posted it on my other website a few weeks ago … same as here.
John and I took it to a nearby parking lot to see if we could get it to go straight. The traction was so much better than in the dirt that we actually broke one of the spokes on the steering wheels in back. In the end, we found it was pretty easy to get it to go straight backwards (although it's hard to get it to change direction) and it's probably impossible to get it to go straight forward (although it's easy to get it to go in circles.)
I had to take it apart in the dark too and I guess it also took 30 minutes or so. I got back home and was somewhat amused to find that my feet got the dusty clay dirt all over them. It was very reminiscent of the playa, except it didn't burn or anything.
(For a while I was using something called the Coral project to host the video, but it has apparently gone away.)
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The title says it all: I made a wood box to hold the 6 straight bars for the vehicle. It's reasonably compact and keeps the bars from knocking against one another.
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I found a website that described DC-DC converter basics. I took a crack at building a "boost" style step-up DC-DC converter (where an inductor is placed in series with a power source and the output side is switched to ground.) I managed to step 5 volts to 24 volts across a 1.2K load (using low-power components) for a current of 20mA or almost 0.5 watts.
I found that transformers work particularly well in the circuit — plus, the secondaries offer useful voltages as well. By switching to better transistors (i.e. 2N2222 instead of 2N4123) I achieved 38V out into 1.2K: 32mA or 1.2W. This looks very promising … now if only I could get it to work from 1 volt.
I measured across 51 ohms and got up to 9.3 volts out. The input current is about 0.46A at 5.7V, so that's 2.62 watts and the output into 51 ohms is 0.18A or 1.70 watts out, so it's about 65% efficient. Using a smaller torroidal inductor, I got 8.62V into 51 ohms or 1.47W with 5.82V at 0.40A in or 2.34 watts for 62% efficiency.
I started building one to work off 1.5 volts or so. At first I didn't get it to work. I rebuilt the whole circuit and got exactly the same bizarre result: a short-cycle square wave that seems to ring down. I couldn't get the thing to work. The capacitor on the NPN transistor seems to be running into negative voltage territory somehow … it actually oscillates, but the final output is a stilted square wave. I switched to a (possibly more stable) twin-T design which I managed to get to work with as little as 3 volts.
I thought that I could try using MOSFETs but I couldn't figure out how to get them to work.
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I met a guy named Dave who referred me to an idea for driving LED's with a microcontroller using a shift-register and latch with a technique called "bit modulation." It's a derivative of pulse-width modulation except it's much better suited to driving multiple outputs concurrently.
In traditional pulse-width modulation (I'll assume it's all for LED's although it can apply to any averaged output) the LED is driven at full-brightness for some percentage of the time at a relatively high frequency (enough that the human eye's persistence of vision can't detect the blinking.) So, for instance, to create the illusion of 10% brightness, the LED is turned on for 1 unit of time and then off for 9 units of time. Usually the frequency of the wave remains the same and only the duty-cycle changes.
In bit-modulation, the desired modulation is assumed to be a binary value. The least significant bit (2^0) is read and the LED is turned on or off depending on its value for 1 unit (2^0 units) of time. The same is done for bit 1 (2^1) only for 2 units of time (2^1.) This continues for successive bits. The resulting brightness output averages to the value over 2^(n+1) clock cycles — the illusion works as long as the LED is on for [duty-cycle]% of the time and off for 100%-[duty-cycle]% regardless of how many times the LED is turned on or off during a cycle.
The huge advantage here is that a shift-register with a latch can be used to set the values for any number of LED's in a very short amount of time. That is, if you have X LED's, the same bit from each of the X LED's desired duty-cycles can be shifted in to the register, the latch can be triggered, and then the CPU will wait for (2^bit) time units until the next bit is loaded.
Unfortunately, I'm using a system where the red, green, and blue anodes are on three wires and the common-cathode of each LED cluster is brought back on one wire — so for X LED's I need 3+X wires. I dismissed the idea — although really cool — because it just wasn't practical for what I wanted to do.
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I finally had a chance to put everything together with the office chairs and check it out. Some of the angles of the mounting points are off by a bit, but otherwise everything went together fine.
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I assembled the pupils of the eyes with tubing, the pipe nipples I bought, the washer/nut assemblies that make the pupils themselves, and a couple clamps. Basically, I fit the end of the bolt that's welded to the washers that make up the pupil into the end of a short piece of polyethylene tubing. I used the pipe clamp to tighten it down and then glued it into the pipe nipple on the frame. I gave it a quick test with my LED light and it seemed that it might just work.
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I added masking tape to the portions I don't want painted and brought them to Austin-Spencer Collision (2433 Brighton-Henrietta Townline Rd.) I talked with Mark who didn't give me a rosy outlook for getting done by Friday — I hustled to finish the frame so I could bring it to the ARTWalk (University Ave. from Atlantic to Merriman) Muse-a-Thon on Saturday, June 11. However, after he had his painter check it, he felt they could finish by Friday afternoon.
I took apart the wheels I had (the front rims are being painted) and reassembled the tires for the back wheels. It took a while to get the hang of assembling the wheels right — alternating the spokes the right way and all so it all fits together. I tightened up the spokes and got them "true enough:" they're still a bit warped, but good enough for what I need.
I used some office chairs I had and took them apart to use temporarily on the vehicle.
I stopped by Austin-Spencer Collision (2433 Brighton-Henrietta Townline Rd.) around 4 on Friday and by the time I left around 4:45, they still weren't done. Mark said he'd try to get there tomorrow morning around 8 and I could come by and pick it up, but that leaves me almost no time to finish building stuff, so I suggested we just call it off. I doubt that I could be ready in an hour from the time I get back with this stuff until the time that I could get to ARTWalk (University Ave. from Atlantic to Merriman) since I had to build the bike wheels, build the seat, and getting everything taken apart and put back in the car (the event was from 10 a.m. to noon.) There's just no way. I e-mailed some people I told about it and called it off.
On Monday I picked up all the parts. They look really good with the navy blue automotive clear-coat applied. Definitely much different than the scrappy metal I started with. When I assembled the front wheels, it took nearly 2 hours to make two wheels so I never would have made it to the ARTWalk event on Saturday.
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I had found a metal cage in the garbage across the street so I cut it to a smaller size and welded it together. It's for the top of the rear shelf so people are tempted to use the shelf for stuff instead of riding on. I sanded it a bit and threw on a couple coats of primer real quick — it's already mostly painted.
I applied primer to the eyes and the top rails then drilled holes and tapped top rails for the brackets to hold the light tubes. I finish-welded the seat brackets, measured them (figuring I could build the seats while it's getting painted at the shop) and ground down the sharp edges.
I created a pretty decent battery holder out of the battery cage in a dead computer UPS but did a lousy job installing it. The solid metal shelf which is welded to the frame is a bad idea. It warped even more with the additional welding and I got really unhappy about it so I had to cut off the battery cage and the shelf.
I got the front end parts stripped of rust and primered. I spent the better part of a whole day painting and finished applying primer. I made hooks for all the parts so they can be painted.
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