This keyboard is connected to RCX input ports using resistor ladder network. One row button and one column touch sensor are pressed by a single keypress. The software in the RCX can tell when a key is pressed by reading the two ports' raw values and decoding the row and column raw values.
The resistor blocks are made from a 2x2 plate, sandwitched between 2 1x2 plates and 2 1x2 electric plates. When connected between a touch sensor cable and the RCX sensor input, the resistor is in series with the touch sensor.
The resistor is a 20K 10 turn potentiometer (the green box on the side of the block). The potentiometer allows greater flexibilty for reuse in other home brew sensor designs than a fixed resistance resistor would.
In the photograph of the keyboard, you can see two stacks of resistor blocks forming the resistor ladder network.
This keyboard was used to implement my pneumADDic adding machine.
