As indicated in the figure above, the IR Shield has four QRE1113 sensors that output to the 3DoT board. The shield’s QRE1113 IR reflectance sensor is comprised of two parts – an IR emitting LED and an IR sensitive phototransistor. When you apply power to the VCC and GND pins, the IR LED inside the sensor will illuminate. A 100Ω resistor is on-board and placed in series with the LED to limit current. A 10kΩ resistor pulls the output pin high, but when the light from the LED is reflected back onto the phototransistor, the voltage at the output will begin to go decrease. The more IR light sensed by the phototransistor, the closer to zero it will go.

These sensors are widely used in line following robots because of how the output varies based on the reflective properties of the material below it. In general, white surfaces reflect much more infrared light than black, so, when directed towards a white surface, the voltage output will be lower than that on a black surface. This is due to the black surfaces absorbing the IR light, which results in the 3DoT board interpreting it as an output of 1 (digital logic HIGH) . When on a white surface, the output will be seen as a 0 (digital logic LOW).

With this in mind, you may see what needs to be done in order to implement the wall following logic that was discussed in the Lab 1 design challenge. To prepare you for Lab 3, please answer the following questions.

Question 1 – As only two IR sensors will be needed for a simplistic wall following algorithm, which motor driver pins should they be connected to? Consider which pins will require the least additional code to implement. Mention any extra steps needed for your choice to work. We want the robot to correct its movement if it runs into a wall and to keep going forward.

Question 2 – What assembly instruction can be used to save a specific bit value from a register? Where is that information saved to?