Chapter 4
Visible Light Sources (VLS)
Duty Cycle Controller circuits contain
at least one of the following:
1. transistor
2. 555 IC
3. operational amplifier (op amp)
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Duty Cycle Controller circuits are used for:
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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 180o.
They also guard against Electrostaic
discharge (ESD).
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LED Sizes
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LEDs with Silver Leads
Silver reacts with sulfur to form silver sulfide
(Ag2S) 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.
Competing VLS devices
Incadescent Lamps
Neon Lamps
Halogen Lamps
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it.
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.
Figure 4-
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Exposure Value
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leaded
One of a neon lamp's greatest beneficial
features is its ability to absorb voltage
surges and transients without damage!
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Power Strobing LEDs
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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.
An LED’s radiant energy decreases with temperature at -1% / oC.
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.
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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. 7447 is current sink.
Common cathode is tied to ground, so 7447 must supply
current to light the LEDs. 7447 is current source.
Labeled in clockwise direction starting at the top.
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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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