vb.net read barcode from camera Copyright 2006, 2001, 1987 by The McGraw-Hill Companies, Inc. Click here for terms of use. in Software

Creation QR Code in Software Copyright 2006, 2001, 1987 by The McGraw-Hill Companies, Inc. Click here for terms of use.

Copyright 2006, 2001, 1987 by The McGraw-Hill Companies, Inc. Click here for terms of use.
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(a few hundreds or thousands of ohms). CdS cells are very easy to interface to other electronics, but they react somewhat slowly and are unable to discern when the light level changes 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. 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 act very quickly and are able to sense tens of thousands of changes in light level 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. In this condition, even as more light shines on the device, the phototransistor is not able to detect any change. 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 act very fast 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.
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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. 32-1. 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. 32-2). 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 32-1 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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FIGURE 32-2 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.
32.2 One-Cell Cyclops
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. 32-3. 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 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 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.
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