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186 Masterpiece 4 • PneumADDic II

The Sum Sensor

This sum sensor combines
a small pneumatic piston,
a touch sensor and a
Potentiometer Brick sub-assembly.

The RCX feeds the values you want added to the pneumatic adding circuit two bits at a
time.The pneumatic adding circuit provides one output bit that is the result of adding
(also called summing).The RCX needs to be able to read this sum bit and remember it, so
it can display the final answer on its numeric display.The Sum Sensor sub-assembly you
are about to build combines a small pneumatic piston with a touch sensor.The small
piston is controlled by the AND Gate sub-assembly, which is also part of the pneumatic
adding circuit. When the output of the adding circuit is zero, the Piston sub-assembly
contracts, and does not press the touch sensor button. When the adding circuit outputs a
one, the Piston sub-assembly expands, and presses the touch sensor button.The RCX
reads the state of the touch sensor button using an input port.
Since the Sum Sensor sub-assembly shares an RCX input port with another
sensor (the Pressure Sensor sub-assembly), the Sum Sensor sub-assembly requires a
Potentiometer Brick sub-assembly. I will explain the potentiometer brick/touch sensor
circuit in further detail when we connect the sensors to the RCX.

Sum Sensor Step 0