LEGO Rotation or Angle Sensor:
The interface for the Rotation sensor looks just like the Light sensor. About 8V is applied for 3ms and then a voltage is read while 5V is applied through a 10,000ohm resistor for 0.1ms. The sensor only outputs 4 analog values of 1.8v, 2.6v, 3.8v and 5.1v. The figure below is the voltage sequence of the for a Rotation sensor with increasing values. Each voltage corresponds to 22.5 degrees of rotation so there are 16 counts per one rotation. You can see that the pattern of voltages allows the RCX to tell direction from any point. The total range of the RCX is -32767 to +32767 and it can keep up even when directly connected to the shaft of a motor. It would be nice if you could use the Light sensor to duplicate this function with a wheel or barcode, but I have not been able to fool the RCX yet.
In the realm of "We have the technology" here is a circuit that will create a fake Rotation sensor. I'm not sure what you would do with this, but it will count clock edges up to 300Hz. It will work with standard RCX Code.
Philippe Hurban has an excellent page showing the internals of theLEGO Rotation sensor.
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