Chapter 4 Visible Light Sources (VLS)

2 Duty Cycle Controller circuits contain at least one of the following: 1. transistor IC 3. operational amplifier (op amp)

3 Duty Cycle Controller circuits are used for:

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5 Diffused LEDs Non-Diffused LEDs

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Clip-Lite LED Lens Clip-Lite LED solves the point effect of some LED’s narrow light radiation pattern. It allows you to diffuse any kind of LED’s light without suffering significant attenuation of its apparent intensity. It has striated lines (Laterally grooved) and Fresnel lens to produce a short focal length wile brightly and uniformly dispersing the light. It increases apparent power up to 125% and the viewing angle for both diffused and non-diffused LEDs up to 180 o. They also guard against Electrostaic discharge (ESD).

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10 LED Sizes

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LEDs with Silver Leads Silver reacts with sulfur to form silver sulfide (Ag 2 S) or tarnish. Hard to solder. Need more active flux to remove it. Can store LEDs in plastic zip-lock bags with mothballs. The napthelene will evaporate and create vapor pressure in the bag which excludes air.

15 Competing VLS devices Incadescent Lamps Neon Lamps Halogen Lamps

16 it. Figure 4- Thomas Edison did not invent the light bulb, but he vastly improved it. The light bulb lasted only 150 hours previously, but after Edison’s improvements, it lasted 1200 hours. Light bulbs today only last about 1500 hours.

17 Exposure Value

18 One of a neon lamp's greatest beneficial features is its ability to absorb voltage surges and transients without damage! leaded

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26 Power Strobing LEDs

27 Power Strobing LEDs

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Dual LEDs Usual colors are Red and Green (pg. 98) Want to have closely matched visible output color intensities for both colors (CIE curve). Current flow in one direction will give Red. Current flow in the opposite direction will give Green Can obtain mix of the two colors, by switching between Red and Green at 30 Hz or faster. The eye perceives a continuous color when the switching rate is 22 to 30 Hz or faster. Color mixing resolution of a dual LED’s colors can be increased by allowing more steps in continuous duty cycle variations in one direction followed by the other.

30 An LED’s radiant energy decreases with temperature at -1% / o C. The voltage across signal diodes decreases with increase in temperature. Placing small signal silicon diodes in series with LEDs will stabilize the LED’s temperature dependence. The diodes have about 0.65 V drop. The voltage drop is proportional to temperature. An LED’s luminance intensity is inversely proportional to temperature. If temperature increases, current will be increased to offset the decrease in luminance intensity.

7-Segment LED display 7447 display driver One annoying trait is incompletely forming the “6” and the “9”. Another annoyance is its inability to decode and display hexadecimal equivalents of 10 through 15 (A to F). (A, b, C, d, E, F are used instead)

Common anode is more often used than common cathode. Anode is positive side. Cathode is negative side. Common anode is tied to +5V, so external power source supplies current to light the LEDs is current sink. Common cathode is tied to ground, so 7447 must supply current to light the LEDs is current source.

33 Labeled in clockwise direction starting at the top.

5 x 7 dot matrix Can create all the 128 possible ASCII characters and symbols. Generating the matrix uses LED power strobing and scanning.

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