Again, assuming 3Vf for each of the diodes @ 20mA, each RGB LED requires 3V @ 60mA, for a total of 180mA from a 3V supply.
Based on experimentation with our geared DC motors, we were already going to have 8AA batteries (in series) on the diaper cake. Standard NiMh batteries output a nominal 1.2V per cell, so we'd have ~9.6V to work with. The least expensive RGB LED's we could find had a nominal voltage drop of 3.2V (4V max), so we'd only be able to drive 2 (maybe 3) LED's in series, or use more batteries. Because of the size of the lower tier, we'd need around 8LED's to light it evenly, plus another 4 for the top tier. So, series LED's would either require adding batteries in series, or more wiring and more electronics. Since the current requirement for all 12 LED's (20mA * 12 * 3) was still a fairly modest at around 720mA, so large current wouldn't be a concern.
An audio amplifier circuit based on an LM386 was included (but remains completely untested). The circuit is simply an implementation of one of the reference designs from the datasheet. The circuitry was never populated due to a listing mistake from Mouser - they had a SMD description with a through-hole part. We made the board based on the SMT package, but received a PTH component. Due to time constraints, the PCB wasn't re-spun.
The mechanics of the moving fish are straight forward - a motor rotates a small wheel which turns a large diameter "belt" that moves around the outer diameter of a tier. The belt is captivated by a groove in the tier, an outer spacer, and a bottom cap. For aesthetics , we wanted to hide as much of the belt as possible, which is why it is inverted (otherwise the cap wouldn't have been completely necessary since gravity would be helping us out). While all of this sounds fairly straight-forward - fabricating it was more challenging than it should have been.
The materials chosen were hanger strap and some particle board. The particle board was chosen over MDF because it was a bit lighter - in retrospect, MDF should have been used because it has much better machinability than particle board.
The hanger strap was chosen because it was on-hand and initial proof-of-concept tests with it were promising. A timing belt approach was also suggested and considered, but abandoned because of the anticipated cost.
The motors needed some sort of wheel with a large enough diameter that would allow the motor to sit back far enough from the belt to provide clearance for the wires that would be used to mount the fish. The drive wheels were made using a CNC mill. There were also several smaller diameter wheels made to sit opposite the drive wheel. The idea here was to reduce the amount of friction instead of having the drive wheel press the belt into the side of the groove, it would roll between a couple of other wheels.
As expected, there was a fair amount of tear out with the particle board when the channel was routed. This uneven surface most likely created friction with all of the holes in the hanging strap. As a result, the motion wasn't nearly as smooth as it could have been. Couple this with the geared DC motor that wasn't geared down quite enough and we wound up with some fairly undesirable motion. Several variations on the motor drive mechanism were tried, what wound up working the best was simply using a rubber band to maintain tension between the motor drive wheel and the hanger strap.
Putting It All Together
The electronics etched poorly, but were usable. The poor etching was primarily due to a very old laser printer that's printed over half a million pages on the original toner fuser and doesn't put a thick enough coating of toner on the copper.
The final implementation of the motor mount and drive wound up looking like this. The string of LED's underneath the tier are also visible. The individual LED carrier boards were soldered together with ribbon cable and simply hot glued to the particle board.
Solid copper wires were soldered onto the track and bent to bring the fish closer to the outside of the tiers. A small length of copper wire was pushed and glued to each fish adn then soldered onto the piece hanging from the track. This provided a solid way for the fish to ride around the track without spining around swaying enough to get caught on anything (there was only an 1/8" clearance in some spots).
Overall, things turned out fairly well, but the motion of the fish was a lot rougher than intended - but hey, fish don't swim the same speed all the time anyway, right!
Here's a complete gallery of images. Click on any image to see a larger copy as well as some descriptions of what you're actually looking at.
Last Updated on Tuesday, 04 September 2012 14:14