LEDs (Light Emitting Diodes) are those little colored lights you see
in electronic equipment, household appliances, toys, on signs, and many other places. Red,
blue and green ones are the most common, since they have been around the longest. Discrete
LEDs of practically all colors are available nowadays. There are LEDs available from
infrared up to ultraviolet range. Technically LED is basically a really fancy diode, which
has the unique "side effect" of producing light while electricity is flowing
through them.
Light-emitting diodes (LED) emit light in proportion to the forward
current through the diode. LEDs are low voltage devices that have a longer life than
incandescent lamps. They respond quickly to changes in current (many can easily go up to
10 MHz). LEDs have applications as visible indicators in devices and in optical-fiber
communication. LEDs produce a narrow spectrum of visible )many colors available) or
infrared light that can be well collimated.
White LEDs are simply blue LEDs with fluorescent material to make up
the remaining spectrum (more or less well, depends on LED). The blue light pumps the
fluorescent material. Thus, there is no electrical difference between blue LEDs and white
ones.
Short LED history: Inrared GaAs LED was original commercially
successful LED. Original Visible Red, GaAsP on a GaAs Substrate, was the original
commercially successful visible type (this was RED led). Then came yellow LEDs. After that
green LEDs became available. Then came the blue LEDs. After introduction of that UV and
"white light" LEDs became available.
General voltage drops ratings of different LEDs at usual operating
currents ("usual" currents around 20 mA):
- Infrared: around 1.9V
- Red: around 1.8 V
- Yellow: Around 2.0 V
- Green: around 2.1 V
- Blue: around 3 to 3.6 volts
- White: around 3 to 3.6 volts
General
LED circuits
- 1.5
Volt LED Flashers - four circuits with description
- 1.5V
LED flasher version A - uses only a single inexpensive C-MOS IC and flashes the LED
for a full year on a single 1.5 volt AA alkaline battery
- 7 by
10 LED Moving Sign - LED matrix sign circuit which does not use any microprocessor
- AC Line powered
LEDs - The circuit below illustrats powering a LED (or two) from the 120 volt AC line
using a capacitor to drop the voltage and a small resistor to limit the inrush current.
- Circuit
efficiently switches bipolar LED - This article represents two methods to switch a
bipolar, two-color LED using an SPDT mechanical switch or relay.
- LED
doubles as emitter and detector - Every junction diode exhibits some degree of
photosensitivity when it receives light comprising an appropriate range of wavelengths.
LEDs can serve as narrowband photodetectors. Here, the LED links two embedded systems via
a fiber-optic cable or a short-distance, line-of-sight coupling path.
- Low Power
LED Flasher - based on LM3909
- Pulsed
LED test circuit - designed to test visible and infrared LEDs in pulsed mode
operations, can drive the LED with peak currents in excess of 10 amps, pdf file
- Modulated
laser diode tester - circuit can either be used to test laser diodes or as a general
purpose modulated light source, adjusted from below 30KHz to above 50MHz, powered from 9V
battery, pdf file
PIR sensors
Infrared motion detectors
General
Semiconductor laser driving circuits
- Laser-diode
driver stabilizes sensitivity parameters - stable laser-diode driver with an optional
modulation-input facility
- Micro
power 40 KHz burst laser diode driver - laser tag or simulated combat games can use
this circuit to send short bursts of modulated laser light at the opponent's vest,
equipped with a matching light receiver, operates from three 1.5v cells (4.5v) that should
provide enough energy for about 200,000 shots, pdf file
- Modulated
laser diode tester - circuit can either be used to test laser diodes or as a general
purpose modulated light source, adjusted from below 30KHz to above 50MHz, powered from 9V
battery, pdf file
Laser communications
Optocouplers
The optoisolator, sometimes called an optocoupler, is an assembly
that contains a light emitting diode and a solid state photosensitive device. These are
placed in close proximity to each other so that light generated by the LED will be
impressed upon the photosensitive device, which may be a transistor, SCR or triac that is
normally non-conducting. An input signal fed to the LED causes it to glow, emitting light.
When the light energy is impressed upon the solid state device it becomes conductive,
allowing the output circuit to be energized. Since the coupling medium is light, the
optoisolator can be designed to attain an isolation voltage rating of several thousands of
volts.
Guides for use
Technical data
Optoisolator circuits
- Analogue Opto Isolator -
use a standard, low quality opto isolator to transfer an analogue signal with reasonable
linearity and without complicated feedback loops to monitor and linearise it
- Circuit
compensates optocoupler temperature coefficient - When using an optocoupler in a
linear application, you should consider its gain drift with temperature. Traditional
single- and dual-transistor-output devices have a notable gain drift with temperature.
This circuits helps to solve this problem.
- Circuit
optimizes phototransistor bandwidth - a simple circuit can improve the dynamic
performance of a phototransistor for use in low- to medium-speed applications as fast as
100 kbps, such as optical isolation of an RS-232C serial line
- Light
powers isolation amplifier - self-powered isolation amplifiers, which need no external
isolated power supply
Components and component datasheets
