Figure 4. Circuit diagram for the infrared emitter/detector line
sensor for the MIT Handy Board. If you plan on making the
sensor from color LEDs and Cds photocells, replace the IR
emitter with a red LED and the IR detector with a Cds photocell.
Click here for a circuit diagram if the sensor is used with other
microcontrollers. Note the subtle difference in the way the IR
detector is configured for this design.
moreBuilding the sensor
Here is the electronic circuit of the LDR based line sensor we
used in our robots in the Robocon 2007 competition. As you
can see it is composed of eight cells, each one resembling the
cell in figure 3.B. There are many reasons to choose to build
a sensor with exactly eight cells, no more, no less: Eight can
provide enough precision, it connects directly to one port of
the microcontroller, and is represented by one single Byte of
data, making it easier to implement in the programming and
in the memory of an 8 bit microcontroller.
more
Photo Interrupters
The LED emits infrared radiation. If the Photo Interrupter passes
over a white surface, the infrared light is reflected back and is
detected by the phototransistor. Let us assume that this is the
case for the right (shown in red) photo interrupter assembly. The
result is that the emitter-collector resistance drops and a large
current flows through the red 27k resistor. This created a large
voltage drop across this resistor and VR is reduced to near zero.
If at the same time, the left photo interrupter assembly (shown
in blue) is over the black line, the radiation is not reflected back
to the phototransistor and the emitter-collector resistance
remains high. This results in a small current through the blue 27k
resistor. The resulting small voltage drop across this resistor
leads to a high value of ~9 V for VL. We send these two voltages
to a comparator, which then makes one of the two wheels of the
robot rotate in such a fashion that it turns to the left.
Photodiode and Robot Line Tracking
