Index

Optoelectronics and optics page

    Optoelectronics components

      LEDs

      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 electricityis flowing through them. Invented in 1967, the LED had been used strictly as an indicator device.

      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.

      LEDs are current-driven devices. The brightness is essentially proportional to the current through an LED. There is usually a minimum current where no noticable light comes out and then the maximum continuous current that LED can handle without overheating. You can quite freely select the current in that range to match you needs, what give the brigtness and power consumption that works in your application. Usually, a voltage source provides this current through a current-limiting resistor at a low cost. Most LEDs, regardless of efficiency, have a nominal current drive of 20 mA; some units can use a maximum drive of 70 mA. Regardless of current level, you need to maintain the drive current within specification in all operating conditions. Conventional (generally older) LEDs needare designed for 20 mA current. At this current they are specified to reach their rated intensity. Many newer low current high intensity LEDs are specified to reach their rated intensity at 10 mA. Nowadays there are also very low current high intensity LEDs available, those maybe able to operate at their rated intensity with a current of 2 mA.

      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.

      Nowadays there are special LEDs that give out white light. White LEDs are normally simply blue LEDs with fluorescent material to make up the remaining spectrum (more or lesswell, depends on LED). The blue light pumps the fluorescent material. Thus, there is usually no electrical difference betweenblue LEDs and white ones.

      General voltage drops ratings of different LEDs at usual operatingcurrents ("usual" currents around 20 mA):

      • Infrared: around 1.4-1.9V
      • 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
      The polarity markings on LEDs (Light Emitting Diodes) seem to be a common cause of confusion. The polarity may be indicated by a flat section on one side of the body or by one lead being shorter than the other, but different manufacturers are not consistent as to which lead they are marking! Rectangular and other shaped LEDs often rely on lead lengths as the only indication. To test LED polarity you could connect the LED to a battery via a suitable resistor. it will light on one direction and does not light on other direction. When using battery please be careful on the voltage you use. The maximum rated reverse voltage for a typical LED is about 4V, so you could damage it if you are using a 9V battery for testing.

      When powering LED from a DC source, power it through a series resistor that limits current to what is suitable for LED. use theLED's data sheet to determine how much current it likes and how muchvoltage it drops when that much current is flowing. The value of theseries resistor, in ohms, is: 

       resistance = (supplyVoltage - LEDvoltage) / LEDcurrent

      The voltage is in volts, current is in amperes and resistance is in ohms.If you're using AC to power a LED, add a series rectifier and series resistor. LED's are diodes, but make poor rectifiers. If you are using AC, the simplest appriach is to put a diode across it to take the inverse voltage.

        LED circuits

        • 1.5 Volt LED Flashers - four circuits with description    Rate this link
        • 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    Rate this link
        • 7 by 10 LED Moving Sign - LED matrix sign circuit which does not use any microprocessor    Rate this link
        • 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.    Rate this link
        • Circuit efficiently switches bipolar LED - This article represents two methods to switch a bipolar, two-color LED using an SPDT mechanical switch or relay.    Rate this link
        • 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.    Rate this link
        • Low Power LED Flasher - based on LM3909    Rate this link
        • 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    Rate this link
        • 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    Rate this link
        • Mains LED - Ever come across an application where all that is needed is some indication the power is applied. Incandescents at mains voltages are a pain as they generate incredible levels of heat and neons, well, they just go black after a while. An LED would be perfect as they have proved their exceptional reliability. A current limiting resistor at 230VAC, even with a super-bright LED running at 10mA, will consume a massive 2.3W. There is another means to generate the required current without dissipating heat - a series capacitor. At first it may appear to yield the same results, but running an LED from the mains with a capacitor is incredibly interesting.    Rate this link

      PIR sensors

      Infrared motion detectors

      • Pyroelectric Infrared (PIR) Motion Sensor - The document referenced here includes an excellent design as well as an excellent tutorial on PIR devices. You've probably seen these devices in homes and businesses and maybe wondered what they were. Now you can find out what they are, how they work, and how you can build one.    Rate this link

<[email protected]>

Back to ePanorama main page ??
 
| Submit site | Info | Advertise | Feedback | Disclaimer | Privacy | Ad. FAQ | Legal Notice |
Copyright © 1994-2011