An optocoupler combines a light-emitting device with a light-sensitive device in one package. One of the simplest examples is an LED packaged with a phototransistor. The LEO is illuminated by an input circuit and the photolransistor responding to the light drives an output circuit. Thus the input and output circuits are coupled by light energy alone. The principal advantage of this arrangement is the excellent electrical isolation it provides between the input and output. In fact, these devices are often called optoisolators. Other examples of optocouplcrs include LEOs packaged with photo darling tons and LASeRs.
Figure 18-40 shows the circuit symbol and two methods used to construct an optocoupler consisting of an LED and a phototransistor. 111 each structure the LEO is actually a GaA; infrared emitter producing light with wavelengths in the vicinity of’O.9 “< JO .m. In one device, the coupling medium is a special “infrared glass” while the other uses an air gap for better electrical isolation.
r n important specification for an optocoupler is its current-transfer ratio: theratio of output (phototransistor) current to input (LEO) current, often expressed as a percent, The ratio varies with the value of LEO CUI rent and can range from (1.I or IcS’i to several hundred in photodarlington devices. Figure IH-41 shows a set of spccilicati ,11 sheets for the TI UII, -114. -110. and -117 series of optocoup Noh: the high voltage isolation I lese devices provide: lip to 2500 V. ote also tha
the base terminal of the photo transistor is externally accessible, so the coupler can be operated in a photo diode mode by taking the output between. the base and collector terminals as shown in Test Circuit B. The specifications for switching characteristics show that the response time is improved by a factor of 5 when in photo diode operation.
Find the current-transfer ratio (CTR) of the TILl 17 optocoupler when the diode current is 20 mA and the collector-to-emitter voltage of the phototransistor is 10 V. Assume that I 0
Referring to the log-log plot of “collector current versus input-diode
forward current.” shown in the “typical characteristics” section of the specifications in Figure !R-41. we see that lc ::::20 mA when I,.. = 20 mA (from the TILl!7 characteristic). Note that the characteristic applies to the case when VCE == 10 V and 1/1 == O. Thus.
Liquid Crystal Displays (LCDs)
Liquid crystals have been called “the fourth state of mailer” (after solids. liquids and gases) because they have certain crystal properties normally found in solids, yet Ilow like liquids. In recent years they have found wide application ill the construction of low-power visual displays such as those found in watches and calculators. Unlike LEDs and other electroluminescent devices LCDs do not generate light energy but simply -alter or control existing light to make selected areas appear bright or dark.
There are two fundamental ways in which liquid crystals are used to control properties of light and thereby alter its appearance. In the dynamic-scattering method, the molecules of the liquid crystal acquire a random orientation by virtue of an externally applied electric potential. As a result, light passing through the material is reflected in many different directions and has a bright frosty appearance as it emerges. In the absorption method, the molecules arc oriented in such a way that they alter the polarization of light passing through the material. Polarizing arc used to absorb or \’lass the light, depending, on the polarization it has been given so light is visible only in those regions where it can emerge from the filter.
The basic construction of a liquid-crystal display is shown in Figure 18-42. A thin layer of liquid crystal, 10-20 ,um thick, is encapsulated between two glass sheets. To enable application of an electric field across the crystal, a transparent conducting material is deposited on the inside surface of each glass sheet. The conducting material forms the electrodes to which an external voltage is connected when it is desired to alter the molecular structure of the liquid crystal. The electrodes are etched in patterns or in individually accessible segments that can be selectively energized to create a desired display similar to a seven-segment LED display. Materials used to form the electrodes include stannicoxide (Sn02) and indium
Some liquid-crystal displays are designed so that light passes completely through them while others have a mirrored surface that reflects light back to the viewer. In the first instance called the transmissive mode of operation light originates from a light source on one side of the crystal and is altered in the desired pattern as it passes through to emerge on the other side. In the reflective mode light enters one side is altered within the material and is reflected by a mirror to emerge from the same side it entered. This mode is particularly useful for small displays such as watches where ambient light is present on one side only.
Figure 18-43 shows a dynamic-scattering LCD operated in the transmissive mode. In the region where no electric field is applied the rod-shaped molecules have the common alignment shown and in the region that has been activated by an externally applied field the molecules have random orientations. The light source is displaced in such a way that the un activated region appears dark. In the activated region the molecules cause light to be reflected in many directions (scattered) and it escapes with a bright diffuse appearance. Thus the pattern appears bright on a dark background. A special light-control film can be placed on the illuminated glass to permit· the use of a diffuse light source
A dynamic-scattering LCD operated in the reflective mode has essentially the .samc construction shown in Figure 18-43, except that a mirrored surface replaces or is added behind one of the glass sheets. However, unwanted reflections limit the readability of displays of this type, which must therefore be fitted with special retroreflectors in practical applications.
Liquid-crystal displays that depend on light absorption rather than light scattering are constructed using so-called twisted-nematic crystals. The electrodes in these displays are coated so that the molecules of the liquid crystal undergo a 90° rotation from one side of the cell to the other. As a result, light passing through the material
undergoes a 90° change in polarization. However if the crystal is activated by an externally applied electric licld:the molecules are reoriented in such a wny that the change in polarization does not occur. The absorption-type LCD operates on the principle that horizontally polarized light is not visible through a vertically polarizing filter. As shown in Figure IR-44. light enters the (transmissive) LCD through II vertical polarizer and can exit through another vertical polarizer provided the liquid crystal is activated, In the region where the crystal is not activated. the vertically polarized light becomes horizontally polarized and is absorbed in the vertical filter. Thus. the unactivatcd region appears dark and the activated region appears bright. Since it is the electric field resulting from an externally applied voltage that reorients the molecule. the twisted-nematic display isoften culled a effect type.
Figure 18-45 shows a light-absorbing.field-effect LCD operated in the reflective mode. As in the transmissive LCD, incident light is vertically polarized. hut in this case a horizontal polarizer is placed between one of the glass sheets and a reflector. The vertically polarized light becomes horizontally polarized when it passes through an unactivutcd region of the crystal. This light can pass through the horizontal polarizer and is-reflected back into the crystal. The unnctivutcd crystal then shifts the polarization back to vertical. so the light emerges through the vertical polarizer where it entered. The activated region of the crystal does not alter the vertical polarization, so light passing through it cannot pass the horizontal polarizer and is not reflected. As a consequence. the pattern created in the activated region appears dark on a light background.
Practical liquid-crystal displays arc activated by ac wave forms because de activation causes a plating effect that shortens their lifetime. The dynamic-scattering type requires a relatively high voltage, 25-30 V at 50-60 Hz. However, LCOs consume very lillie power in comparison to LED displays. One method used to activate the crystal is to drive each of the opposing electrodes with a square wave. If the square waves arc in phase, then the net voltage across the crystal is zero and it remains unactivated. When the driver circuitry causes the square waves to be Hmo out of phase. the net voltage across the electrodes is twice the peak-to-peak value of either wave, and the crystal is activated.
One disadvantage of a liquid-crystal display is its relatively long response time i.e .. the time required for the display to appear or disappear. LCOs are much slower in that respect than LED displays. However. in practical applications such as watches, the long response time docs not impose a limitation on their usefulness. Table 18-1 shows a comparison of the principal characteristics of dynamic-scattering and twisted-nematic LCOs. Note that the twisted-nematic type requires about onetenth the voltage and current of the dynamic-scattering type, has better contrast, but is about four times slower