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A number of simple electronic devices can be used as eyes for your robot. These include the following:
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I Photoresistors. These are typically a cadmium-sulfide (CdS) cell (often referred to sim-
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ply as a photocell). A CdS cell acts like a light-dependent resistor: the resistance of the cell varies depending on the intensity of the light striking it. When no light strikes
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the cell, the device exhibits very high resistance, typically in the high 100 kilohms, or even megohms. Light reduces the resistance, usually significantly (a few hundreds or thousands of ohms). CdS cells are very easy to interface to other electronics, but they are somewhat slow reacting and are unable to discern when light flashes more than 20 or 30 times per second. This trait actually comes in handy because it means CdS cells basically ignore the on/off flashes of AC-operated lights. I Phototransistors. These are very much like regular transistors with their metal or plastic tops removed. A glass or plastic cover protects the delicate transistor substrate inside. Unlike CdS cells, phototransistors are very quick acting and are able to sense tens of thousands of flashes of light per second. The output of a phototransistor is not linear ; that is, there is a disproportionate change in the output of a phototransistor as more and more light strikes it. A phototransistor can become easily swamped with too much light. Even as more light shines on the device, the phototransistor is not able to detect any more change. I Photodiodes. These are the simpler diode versions of phototransistors. Like phototransistors, they are made with a glass or plastic cover to protect the semiconductor material inside them. And like phototransistors, photodiodes are very fast acting and can become swamped when exposed to a certain threshold of light. One common characteristic of most photodiodes is that their output is rather low, even when fully exposed to bright light. This means that to be effective the output of the photodiode must usually be connected to a small amplifier. Photoresistors, photodiodes, and phototransistors are connected to other electronics in about the same way: you place a resistor between the device and either V or ground. The point between the device and the resistor is the output, as shown in Fig. 37.1. With this arrangement, all three devices therefore output a varying voltage. The exact arrangement of the connection determines if the voltage output increases or decreases when more light strikes the sensor. Light-sensitive devices differ in their spectral response, which is the span of the visible and near-infrared light region of the electromagnetic spectrum that they are most sensitive to. CdS cells exhibit a spectral response very close to that of the human eye, with the greatest degree of sensitivity in the green or yellow-green region (see Fig. 37.2). Both phototransistors and photodiodes have peak spectral responses in the infrared and near-infrared regions. In addition, some phototransistors and photodiodes incorporate optical filtration to decrease their sensitivity to the visible light spectrum. This filtration makes the sensors more sensitive to infrared and near-infrared light.
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FIGURE 37.1 The basic connection scheme for phototransistors, photodiodes, and photoresistors uses a discrete resistor to form a voltage divider. The output is a varying voltage, which can go from 0 to V depending on the sensor.
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Photodiode Human eye
CdS cell
300 nm
700 nm
1100 nm
FIGURE 37.2 Light sensors vary in their sensitivity to different colors of the electromagnetic spectrum. The color sensitivity of CdS cells is very similar to that of the human eye.
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A single light-sensitive photocell is all your robot needs to sense the presence of light. The photocell is a variable resistor that works much like a potentiometer but has no control shaft. You vary its resistance by increasing or decreasing the light. Connect the photocell as shown in Fig. 37.3. Note that, as explained in the previous section, a resistor is placed in series with the photocell and that the output tap is between the cell and resistor. This converts the output of the photocell from resistance to voltage, the latter of which is easier to use in a practical circuit. The value of the resistor is given at 3.3K ohms but is open to experimentation. You can vary the sensitivity of the cell by substituting a higher or lower value. For experimental purposes, connect a 1K resistor in series with a 50K pot (in place of the 3.3K ohm resistor) and try using the cell at various settings of the wiper. Test the cell output by connecting a volt-ohm meter to the ground and output terminals. So far, you have a nice light-to-voltage sensor, and when you think about it there are numerous ways to interface this ultrasimple circuit to a robot. One way is to connect the output of the sensor to the input of a comparator. (The LM339 quad comparator IC is a good choice, but you can use just about any comparator.) The output of the comparator changes state when the voltage at its input goes beyond or below a certain trip point. In the circuit shown in Fig. 37.4 (refer to the parts list in Table 37.1), the comparator is hooked up so the noninverting input serves as a voltage reference. Adjust the potentiometer to set the trip point. To begin, set it midway, then adjust the trip point higher or lower as required. The output of the photocell circuit is connected to the inverting input of the comparator. When the voltage at this pin rises above or below the set point, the output of the comparator changes state. One practical application of this circuit is to detect light levels that are higher than the ambient light in the room. Doing so enables your robot to ignore the background light level and respond only to the higher intensity light. To begin, set the trip point pot so the circuit
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