LEGO MINDSTORMS RCX Input expander

April 2004: With this wonderful device, you can connect up to 8 active sensors to one single RCX Input port. It works bad only for Lego rotation sensors, because of their particular signal, but it works perfectly with temperature sensors, light sensors, touch sensors and so on. My device is derived from an idea by Michael Gasperi.

This multiplexer (MUX) does not modify readings from sensors; the sensors are toggled by rapidly switching the input type from passive (touch sensor) to active (light sensor) in the RCX program. When the port is kept in passive mode for more than 200 ms, the MUX is automatically reset, enabling the first sensor. This way you know always which sensor is being reading.

It’s incredibly useful and easy to use in programs. The display will not be present in the final version, because it distorts readings, forcing output value to a high value. I used the display only for the prototype, to test the power-on-reset net. You can see the schematic below.