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Light Sensors

Topics
Light Sensors

Preparation
Take several large poster boards and add alternating black and green lines at regular intervals. (Electrical tape works well for this)

Attach light sensors to each robot and upload a simple program that uses a light sensor and run it once. I recommend a go light, a light sensor wait-for and a stop light in that order. Do this with all the robots. The reason is that the RCX will not recognize the light sensor unless it has been used once. The beginning of this lesson has the students using the view panel on the RCX without programming anything so the light sensor needs to be set.

Handouts
Light Sensor Handout

Lesson
Begin by explaining that you have attached light sensors to all the robots. Show them a robot with the light sensor attached and explain what it is and how it works. The light sensor uses infrared light to determine how bright something is. The light sensor produces a value between 0 and 100. 100 being complete light and 0 being complete dark.

Now show the students how to view what the light sensor is currently reading. This is done by turning on the RCX and pressing the view button until the arrow at the top of the screen is under the port the light sensor is plugged into. For example my light sensor is connected to port 2. I would turn on the RCX and press the View button twice. Next to the image of the robot man is a number between 0 and 100, this is the value being read by the light sensor.

Have the students try the light sensor on a nearby object like their desk. Write the numbers each student got on the board. (Note these numbers should be different) If possible have the posters you made earlier out on the desks and have the students try the light sensor on the white, black and green sections. Write down what each group got on the board. Again these number should be similar but not the same.

Ask the class why the numbers arn't all the same.
The answer is do to the differences in lighting and coloration. One student's desk may be slightly darker than the other. One student may be near a window or directly under a light. To show this turn off or dim the lights in the room and have the students see how the values change. (They should go down)

Now that they understand why the numbers are different you need to explain how they are similar. The numbers in each column should be similar to each other and different from the other columns. So for example black should always have smaller numbers than white. You might try showing this by mapping the numbers on a line to show that all the black readings are at one end, green in the middle and white at the opposite end from black.

Building on the above example you need to go over boundary values. How to pick a number that marks the difference between black and white. So for black and white show them how to pick a number so that every number greater than that number is white and everything darker is black. (ignoring green for now) Then add green back in and have them come up with a boundary between black and green, and green and white.

Now you are finally ready to re-introduce if/else, loops and wait-fors.
Starting with the wait-fors, draw this program on the board and ask students what they think it will do. (Note: you may have to change 40 to a different number to accommodate lighting in your room) `

This program causes the robot to go forward until the light sensor reads a value less than 40. (less than 40 means darker than 40) Then stops. The modifiers attached to the motors are power level modifiers. Power levels control the speed at which the motor moves, 5 is the fastest, 1 is the slowest. The light sensor takes readings at regular intervals. If the robot moves too fast it may pass the dark line before the light sensor can register it. To fix the problem this program uses a low power level. Next is the wait-for-dark icon. This icon waits for the light sensor attached to port 2 to read a number smaller than 40.

Once the students understand the above program show them this one:

This program checks the light sensor the moment it starts. If it reads lighter then 40 ( greater than 40) then the robot beeps twice and ends the program. Else the program beeps once and stops. Generally this program is just an example of using the if/else with a light sensor. Don.t spend too much time explaining the program.

This last program is an example of a loop using the light sensor. This program will cause the robot to go forward till it hits a black line then stop. The light sensor loop in this program loops while the light sensor reads a value greater than (lighter than) 40.

After explaining all this give the students the handout and let them work the challenges.

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