Photointerrupters and photoreflectors can be bought prepackaged or constructed separately from an infrared LED and a photodiode or phototransistor, after making certain the detector is sensitive to the wavelength produced by the LED. Photoreflector units are also available with more advanced position sensitive detectors (PSDs), which report the location of infrared light incident on the sensing surface (Fig. 19.7). The fixed location of the LED relative to the PSD, as well as the location of the image of the infrared light on the PSD, allows the FIGURE 19.5 The Fairchild semiconductor QVA11234 photointerrupter. FIGURE 19.6 The Fairchild semiconductor QRB1114 photoreflective sensor. FIGURE 19.7 A position sensitive detector (PSD), UDT Sensors, Inc. ©2002 CRC Press LLC Photointerrupters and photoreflectors can be bought prepackaged or constructed separately from an infrared LED and a photodiode or phototransistor, after making certain the detector is sensitive to the wavelength produced by the LED. Photoreflector units are also available with more advanced position sensitive detectors (PSDs), which report the location of infrared light incident on the sensing surface (Fig. 19.7). The fixed location of the LED relative to the PSD, as well as the location of the image of the infrared light on the PSD, allows the FIGURE 19.5 The Fairchild semiconductor QVA11234 photointerrupter. FIGURE 19.6 The Fairchild semiconductor QRB1114 photoreflective sensor. FIGURE 19.7 A position sensitive detector (PSD), UDT Sensors, Inc. ©2002 CRC Press LLC . semiconductor QVA1 1 23 4 photointerrupter. FIGURE 19.6 The Fairchild semiconductor QRB1114 photoreflective sensor. FIGURE 19.7 A position sensitive detector (PSD), UDT Sensors, Inc. 20 02 CRC Press. semiconductor QVA1 1 23 4 photointerrupter. FIGURE 19.6 The Fairchild semiconductor QRB1114 photoreflective sensor. FIGURE 19.7 A position sensitive detector (PSD), UDT Sensors, Inc. 20 02 CRC Press. photodiode or phototransistor, after making certain the detector is sensitive to the wavelength produced by the LED. Photoreflector units are also available with more advanced position sensitive detectors