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How It Works
The Parallax Line Follower module is composed of two distinct sections: an IR emitter/detector array and
a threshold comparator circuit. A simplified diagram of each emitter/detector is shown in Figure 2.
Figure 2: IR Emitter/Detector Circuit
The IR LED is activated by making the associated BASIC Stamp output pin low (see Figure 4 for
connections). The active-low configuration is used because the BASIC Stamp can sink more current per
pin than it can source. When the LED is active, reflected IR light from the course surface will strike of
the IR detector transistor, affecting the current flow through it. More reflected IR causes greater current
to flow through the transistor.
Notice that the IR transistor is placed in series with a 10 kΩ fixed resistor and that the IR Detect output is
taken at collector of the transistor. As the current flow through the transistor increases the voltage
across the 10 kΩ resistor also increases, causing the voltage at the output to decrease. The greater the
reflected IR, the lower the output voltage.
This circuit forms a voltage divider with the output dependant on current flow through the transistor.
The greater the IR reflection from the course surface, the greater the current flow through the transistor.
This causes the voltage at the output to go lower because it "looks" like a smaller resistance. When
there is little or no reflection, the current flow through the transistor is reduced making it look like a very
large resistance in the circuit, causing the output voltage to increase.
The second portion of the Line Follower circuit is the threshold comparator (see Figure 3). The purpose
of this circuit is to compare the output from the IR detector with the level setting on the threshold
potentiometer.
Figure 3: Threshold Comparator Circuit
Parallax, Inc. • Parallax Line Follower (#29115) • 12/2001
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