LEGO MINDSTORMS Robots Manual page 199

The 8V that is supplied to the active sensor is an approximate number; the actual value
depends on the juice in your batteries. The difference between fresh and used batteries can
produce any voltage in the range from about 6.5V to 8.5V. Many electronic circuits are
happy with 5V or more, so this may not be an issue for you. But it's something you should
be aware of.
Signal Splitter
The input wires carry power and sensor signals at the same time. Furthermore, the polarity of the signals may be reversed, depending on how the sensor is attached to the RCX. Remember how the
direction of the motors depended on how they were attached to the RCX? The same problem applies to sensors and sensor circuits, but a special circuit makes it irrelevant which way the sensor is
hooked up.
Six diodes are sufficient to separate out the power and signal on an input. These six diodes make up a circuit called a signal splitter. The circuit makes sure that all the electrons go in the right
directions so your sensor circuit gets power and the RCX gets a sensor reading. Diodes are used to steer the electrons the correct way
so that it doesn't matter which way the active sensor is attached to the input on the RCX. Figure 11-5 shows how to set up the diodes.∗
Remember, power is only applied for 3 ms at a time, punctuated by .1 ms sensor readings. You will need to smooth out the power supply with a capacitor, as shown in Figure 11-5.
The Touchless Touch Sensor
This section describes how to build an interface circuit for a Hall effect sensor. A Hall effect sensor is triggered by the presence of a magnetic field. If you place a small magnet near the sensor in
the proper orientation, it will trigger. The sensor itself is very small and looks like a transistor. Figure 11-6 shows a photograph of a Hall effect sensor.
Figure 11-5.
A signal splitter
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