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220 Masterpiece 4 • PneumADDic II

Four distinct actions occur during the process:

■ When the touch sensor for Row 0 is pressed, the electricity flows through the
touch sensor and directly back into the RCX.The RCX senses that all the elec-
tricity it has sent out has been returned.

■ When the touch sensor for Row 1 is pressed, electricity flows through the touch
sensor, but must travel back through Potentiometer 0, before it can get back to
the RCX. Potentiometer 0 converts a portion of the electricity that flows
through it into heat, therefore the RCX reads that less electricity is coming
back, and receives a lower analog reading for Row 1 than it received for Row 0.

■ When the touch sensor for Row 2 is pressed, electricity must travel through
Potentiometer 0 and Potentiometer 1 before completing the circuit to the
RCX.The reading that the RCX receives for Row 2 is lower than the readings
for Row 1 and Row 0.
■ Following this logic, the touch sensor for Row 3 receives an even lower reading
than Rows 0 through 3, because the electricity in this circuit must pass through
three potentiometer bricks.

As you can see, the potentiometer bricks provide a simple way of merging the four
touch sensors onto a single RCX port. In electronics terms, the electricity must travel in
series through a touch sensor, and then zero or more potentiometer bricks to return to
the RCX. Series resistor networks are relatively simple to work with. We can use a series
resistor networks for our Keyboard inputs because only one row and column touch
sensor is pressed at a time (unlike the Sum Sensor sub-assembly and the Digital Pressure
Sensor sub-assembly that can both be pressed at the same time).
When we connect the Sum Sensor sub-assembly and the Digital Pressure Sensor sub-
assembly together onto a single RCX input, things become more complicated.The Sum
Sensor and Pressure Sensors’ touch sensor buttons can both be pressed at the same time.
However, we will address this issue, and how to prevent this problem, when we get to
that step in the building instructions. First, let’s build the keyboard.