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Dot Matrix Clock

A Mindstorms clock using axles and pin holes as a grid dot display.


Completion date: 31/01/2024
Power: electric (Mindstorms brick)
Language: Scratch
Bricks: 1
Motors: 5 x Spike Prime M
Sensors: none

Another clock project, this was inspired by the new Technic plates released in black in the 10323: PAC-MAN Arcade set. I found that if you’re using digits that are 3×4 studs big, the plate is just the right size to fit four such digits with a colon between.

The idea is best understood by watching the video, but in essence it came down to using belts with bricks on them to make “impressions” of digits on freely sliding axles, thus making them show up or disappear from the “faceplate” of the clock. The faceplate consisted of two tilted Technic plates with a gap and then a third Technic plate. Inside the gap, there were four identical digits, each made up of 13 “dots” created using a 180° axle connector with 4L axles coming out of both ends. The connector not only held the axles together but also prevented them from rotating and from sliding out of the tilted faceplate through the pin holes – the angle was such that the axles would slide backwards under their own weight, and the thickness of the rear Technic plate allowed the rear axles to stick out. And since the 4L axle facing the front of the clock was red in every “dot”, pushing the rear axle forward resulted in red “+” appearing in the pin hole inside the front black Technic plate.

Every digit was moved fully back by default, and not visible from the front. Behind the faceplate, there were four belts build using LEGO 5L tracks, controlled by four separate motors and pushed forward or backward by a fifth motor. This mechanism acted like a stamp – the tracks had bricks sticking out of them, forming 5×3 digits, and they would be repeatedly pushed into the faceplate, making the desired “dots” of the each digit appear visible on the clock, then pushed back and the next digit would be rolled into position behind the faceplate. So the clock would go e.g. from 11:58 to 11:59, then 12:00 and so on. There were no seconds indicator, just hours and minutes, so this was supposed to happen once per minute but I sped it up for the video. Also, since the fifth motor pushed all tracks once per “minute”, I added two red axles to the assembly holding the four sliding tracks together, so that they axles would form a colon between hours and minutes every time the digits were pushed.

It was a typical “proof of concept” build and only cared about its working, not its appearance. It was relatively simple, the only real challenge being ensuring that the “stamps” always lined up with the backs of the “dots” correctly. I found that maintaining constant tension of each track was crucial here, which is why my initial prototype had only the front sprockets of each tracks sliding, so that track would get pulled and then loosened, but I ended up sliding the entire track assembly forward and backward, with each motor directly connected to the sprocket and sliding together with it. It was also difficult to make the fifth motor sufficient for pushing forward the entire assembly of four tracks with motors – I had to find the angle which was steep enough for the “dots” to slide back into the faceplate when released, but not so steep that the motor would struggle to push the tracks forwards. Also, using red axles wasn’t the best solution in terms of visibility, but when I experimented with yellow ones, I found them too visible – they would remain partially visible even when retracted into the faceplate.


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