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LEDs! Hmm, they are great. Whatever shines
is not gold but it is nice. RF engineers are keen on
relays; they enjoy hearing them as they turn on. We
like LEDs, they patiently stand at the corner of our
board and they tell us that something is happening.
Our circuit is powered up; our microcontroller is running
or it is getting programmed. We use LEDs for all sorts
of thinks, for indicating status or for displaying stuff.
If you think to get started with prototyping you will
find that adding a couple of LEDs to your board will
always be handy. As I said, even the greatest project
of all times most likely started with a blinking LED.
Well, LEDs are simple diodes and, without
getting into the physics or the socioeconomics of it
we may keep in mind that LEDs are polarized devices,
they have a positive and a negative pole, the anode(+)
and cathode(-) and, current may flow only from the positive
to the negative terminal, this is the forward direction.
When you hold one of these creatures, to identify which
terminal is which, you have two indicators, the terminal
length (the long one is the positive one) and the shape
of the LED’s base that resembles (to some extend)
the diode’s symbol.
Another thing we should have in mind is
that LEDs are not linear devices like resistors, the
V-I graph is a curve and there is no simple equation
that will give you the V-I relationship. So, where can
you get it? Look at the datasheet.
The graph extracted from a datasheet tells
us that this LED will develop a potential difference
of 1.6 to 2.4 V when we supply it with current. How
much current? Up to 50mA and this is a high value, if
you give more than that you will burn the poor LED or
you will overheat it and shorten its life span. A reasonable
value pair would be 20mA and 2V however you may be happy
with 10mA and 1.85V. Always try to minimize the energy
you consume, you may for example use batteries for your
circuit; you don’t want them to flatten fast.
When you go shopping for LEDs you will find that
they come in many different flavors. You should look at the
electrical characteristics first to ensure that your
LED will fit in your circuit. There are many options
regarding the mechanical characteristics (even the viewing
angle) and the price. The factor you would possibly
like to maximize for you budget is intensity. We measure
luminous intensity in candelas (cd) or you will possibly
find it as millicandela rating (mcd). Don’t let
the wavelength trick you, it just indicates the LED’s
color. If you are very optimistic about your circuit
look also to the life span, do you reckon your circuit
will be needed after 100,000 hours of operation?
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