Applications of the Photoemitter or LED.


The most common application of the LED is in visual indicators, they are everywhere from small portable radios to car instrumentation panels. They provide a low current source of illumination, and although an LED cannot provide as much illumination as an incandescent bulb it does last for many years. Most  visible light LEDs are used as visual indicators to report a status. Some are simply glowing lights to show what a device is doing, some are in more complex arrangements.

 The diagrams and explanation below shows how an LED based clock performs its visual function:



There are in fact six bar-shaped LEDs arranged in a rectangle with a seventh horizontal and through the centre. Each LED is known as a segment.  Some have a decimal point but we will ignore this for the purposes of this study. The assembly is known as an array or panel and has either a common cathode or a common anode to which all the LEDs are joined. By powering some and not others, any decimal number can be made. For instance, to show the number two, ABGED need to be powered, F and C must be kept dark. The number six would be AFEDCG powered and B dark. The sense of this is shown below:


Of course we would need four of these for a time display, and if we only want a 12 hour clock we could get away with a two-segment panel leftmost as it only ever needs to show  1 or be completely dark. For this we need an array of drivers, four counters and a timing circuit as well as their complimenting components (these are now available in one integrated circuit). It is truly amazing how far such a display can be clearly seen.

Not all LEDs emit visible light and a range emitting infra-red are available. Such devices can be used in control systems. Take for example a supermarket checkout conveyor. Formerly these were activated by means of the operator putting his or her foot on a pedal to bring the goods forward. This system was simple and effective, but after four hours the operator would get cramps and aching ankles and knees. the answer is to have a  "beam" of infrared cross the conveyor belt at its end point, where it shines through a lens to a photodiode. To improve the conditions in bright light, the photodiode is made sensitive to the wavelength of the infra-red. If no obstruction is encountered,  the diode receives the IR light and a relay further down the circuit is energised and moves to contact, allowing the motor to run which through a gear train and rollers move the belt forward. if an obstruction is encountered no IR light is received by the diode, the relay de-energises, the motor receives no supply and the belt stops.

These are the general applications of photo-emitters, as indicators or control systems, and this is but two examples. There are literally thousands more, but the base principles remain largely the same.



This is the end of the Optoelectronics Section .

Written by Ian Lang Oct 2010 Last update Oct 2010