February 16, 2004
April 16, 2004
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LEGO MINDSTORMS RCX output Multiplexer (MUX)

outmux1April 2004: I developed a simple yet effective motor multiplexer for RCX, to raise the number of motors it can drive from 3 to 6. It’s really ugly, I know. I didn’t have enough LEGO electric plates to sacrifice, so I had to insert those screws into the 4×6 plate.

The output MUX must be connected to all three RCX output ports to obtain 6 independent outputs.

  • Port A controls motors 1, 2 and 3.
  • Port C controls motors 4, 5 and 6.
  • Port B make the channel selection:
    • No power: motors 1 and 4 move as you want;
    • Negative power: relay2 is on, motors 2 and 5 move as you want;
    • Positive power: relay1 is on, motors 3 and 6 move as you want.

In any state, when a pair of motors work, the other are disconnected, and the electric brake does not work. Here’s the schematic of the device:


I fitted the board with two tiny relays and four diodes inside the volume of two stacked 2×6 bricks. It was challenging. I used this MUX for my Johnny 5 replica based on RCX.


Since I didn’t want to sacrifice the LEGO 9V electric cables, I made them from scratch. Again, ugly, but functional.

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