Specifications

Chapter 6 - Samples
Previous page Table of contents Chapter overview Next page
Samples
Light-Emitting Diodes - LEDs
LEDs are surely one of the most commonly used elements in electronics. LED is an abbreviation
for 'Light Emitting Diode'. When choosing a LED, several parameters should be looked at:
diameter, which is usually 3 or 5 mm (millimeters), working current which is usually about 10mA
(It can be as low as 2mA for LEDs with high efficiency - high light output), and color of course,
which can be red or green though there are also orange, blue, yellow....
LEDs must be connected around the correct way, in order to emit light and the current-limiting
resistor must be the correct value so that the LED is not damaged or burn out (overheated). The
positive of the supply is taken to the anode, and the cathode goes to the negative or ground of the
project (circuit). In order to identify each lead, the cathode is the shorter lead and the LED "bulb"
usually has a cut or "flat" on the cathode side. Diodes will emit light only if current is flowing from
anode to cathode. Otherwise, its PN junction is reverse biased and current won't flow. In order to
connect a LED correctly, a resistor must be added in series that to limit the amount of current
through the diode, so that it does not burn out. The value of the resistor is determined by the
amount of current you want to flow through the LED. Maximum current flow trough LED was
defined by manufacturer. High-efficiency LEDs can produce a very good output with a current as
low as 2mA.
To determine the value of the dropper-resistor, we need to
know the value of the supply voltage. From this we subtract
the characteristic voltage drop of a LED. This value will range
from 1.2v to 1.6v depending on the color of the LED. The
answer is the value of Ur. Using this value and the current we
want to flow through the LED (0.002A to 0.01A) we can work
out the value of the resistor from the formula R=Ur/I.
LEDs are connected to a microcontroller in two ways. One is to turn them on with logic zero, and
other to turn them on with logic one. The first is called NEGATIVE logic and the other is called
POSITIVE logic. The above diagram shows how they are connected for POSITIVE logic. Since
POSITIVE logic provides a voltage of +5V to the diode and dropper resistor, it will emit light each
time a pin of port B is provided with a logic 1 (1 = HIGH output). NEGATIVE logic requires the LED
to be turned around the other way and the anodes connected together to the positive supply.
When a LOW output from the microcontroller is delivered to the cathode and resistor, the LED will
illuminate.
http://www.mikroelektronika.co.yu/english/product/books/PICbook/6_03Poglavlje.htm (1 of 3) [4/2/2003 16:18:45]