The RCX Input Mux 

The way you switch between inputs is by toggling sensor type temporarily between Light, Touch and back to Light. Going to Touch type drops the 8V supply. This is picked up by the D7, D8, C2 & R2 circuit which clocks the 4017 counter. To figure out where the counter is on startup, the input for count 0 is a special case. (Special thanks to Paul Haas for this idea.) By using a 2.2k resistor and the input tied to ground the RCX will read 100 for this channel. The other channels use a 3.9k resistor. This means their readings will never exceed about 91 even with 0v. Remember 0v = large readings and 5v = 0. Once the RCX knows where the counter is, it can just toggle the type and read values. Four more channels can be easily added by adding another 4066 quad analog switch hooked up to O4 through O7 of U1 and move RESET to O8.

You can hook up Touch type sensors between the inputs and ground and get 0 for an open sensor and about 88 for one that is pushed. You cannot hook up Light or Rotation sensors to these inputs because they actually require power to operate and this has been stripped off to run the circuit. There isn't enough power available at an input to power more than one Light sensor anyway. Finally a Temperature sensor can be hooked up, but the values won't be in C or F.