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3.2 Light Sensors and Detectors

Phototransistors were used to find the light of the candle and follow the white line that runs throughout the course. These sensors are connected to the analog-todigital converter on the Lego board, returning values ranging from 0 to 255 (corresponding to 0 to 5 Volts).
The sensors that are used to follow the line are mounted with an LED parallel to them facing the same direction. This gives the sensor a better range of values by increasing the ambient light that the white tape reflects. Three of these sensors are oriented in a straight line (see figure 3.2.1).

Figure 3.2.1 Light Sensors (bottom view)

This set-up allows the robot’s orientation with regard to the white line to be known at all times. If the middle sensor is the only one reading the white line, then the line is in the middle of the robot, and if the right sensor reads the line, then the line is to the right, etc. Using just two sensors would be inadequate; the position of the white line would be undetectable when it’s located in between the two sensors.
The use of more sensors would have been beneficial; however, the analog-todigital converter on the Lego v6 board only has eight channels, limiting the total number of sensors that can be used. The phototransistors located on the front of the robot are used to guide the robot to the candle once it is in the proper room. These sensors are built using the same phototransistors previously discussed. These sensors do not require any additional light as the candle emits light. Three sensors were used, one in the
center of the robot, and one on each side of the robot facing forward. These sensors can be seen in the following picture, figure 3.2.2.

Figure 3.2.2 Light Sensors (top view)

With such a configuration, the robot is able to easily determine the position of the candle and move into position to deploy the candle. The position of each sensor is important as well, the middle sensor is at a height to detect the light from the lower LEDs on the candle and the adjacent two are at a height to detect the higher LEDs on the candle. We found the sensors are able to ‘see’ the light of the candle at a distance of up to 3 feet. With the sensors organized in this manner, the robot can easily make adjustments to its trajectory and guide itself directly at the candle while moving towards it. Again, the use of more sensors on the front of the robot would have been beneficial, but do to the limitations of the Lego board used, was unfeasible. Three sensors were used because they read the light much better when it shines directly into them. If, for instance, only 2 sensors had been used, it would be rather difficult to determine the exact bearing of the candle because of its circular form.

3.3 Distance sensor

The distance sensor was built using an infrared light emitting diode and phototransistor.4 The infrared signal emitted from the diode bounces off of any solid surface and the phototransistor detects the strength of the signal. Depending on the intensity of the signal we can determine how far the object is from the front of the robot. The analog-to-digital converter on the Lego board makes it easy to check the values since the intensity of the detected signal is nicely converted to numerical values ranging from 0 to 255 (corresponding to 0 to 5 Volts). The problem with using this approach to determine the distance is interference from the surrounding lighting. To avoid this interference, we use the fact that theambient light is a modulated light. Therefore, we created an oscillator that oscillates at a low frequency. This forces the diode to emit the infrared light at that frequency. On the receiving end, the signal from the phototransistor is sent through filters that only allow a signal at the frequency of the emitter pass through. For the design of the infrared emitting diode with the oscillator and phototransistor with filters see figure 3.3.1 and 3.3.2.

Figure 3.3.1 Infrared emitting diode with the oscillator

Figure 3.3.2 Phototransistor with filters

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