NXT Technic clock

I built this clock, starting with wanting to try this new idea I had of using differentials for setting the time.

The clock uses one RCX motor to power it (very efficient, and quiet). It uses an RCX rotation sensor to know how to adjust it’s motor speed to be at the right time. It is based on absolute timing (from NXT current tick). If you stop the motor, and then let it go again, it will “make up lost time”. Because it is based off of the NXT current tick, it will not drift.

The gear design uses differentials in such a way that it allows you to easily add or subtract to the output, thus allowing you to set the time to the real time. The differentials also provide a 2:1 gear ratio, which is almost a necessity in a lego clock. So that the knobs for setting the time don’t spin freely (corrupting the time), I made a clicking mechanism with a 12 tooth gear, a Bionicle tooth, and a 2×3 spring box (auto centering).

As you can see, there are 4 hands on the clock. The left most is the “second hand”. It shows individual seconds (spins once per second). The next one is the “seconds hand”, as you would see on a “normal” analog clock. The minutes and hours hands are also standard.

The gear ratio from the motor to the second hand is as follows:

      1:1 2/3 (20:12)
      5:1 (8:40)

The gear ratio from the second hand to the seconds hand is as follows:

      2:1 (12:24)
      3:1 (8:24)
      2:1 (differential)
      1:1 (differential to axle, also correcting direction)
      5:1 (8:40)

The gear ratio for the minute hand is the same as for the seconds.

The gear ratio for the hours hand is as follows, starting with the differential line of the minutes (minutes*5):

      2:1 (12:24)
      1:1 (24:24 for direction and as a spacer)
      3:1 (8:24)
      2:1 (differential)
      1:1 (differential to axle, also correcting direction)
      5:1 (8:40)
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