Light Emitting Diodes (LEDs)
FunctionLEDs emit light when an electric current passes through them.
Connecting and solderingLEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or - for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).
LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow.
No special precautions are needed for soldering most LEDs.
Testing an LEDNever connect an LED directly to a battery or power supply!
It will be destroyed almost instantly because too much current will pass through and burn it out.
LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!
For an accurate value please see Calculating an LED resistor value below.
Colours of LEDsLEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours.
The colour of an LED is determined by the semiconductor material, not by the colouring
of the 'package' (the plastic body). LEDs of all colours are available in uncoloured
packages which may be diffused (milky) or clear (often described as 'water clear').
The coloured packages are also available as diffused (the standard type) or transparent.
Tri-colour LEDsThe most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on.
The diagram shows the construction of a tri-colour LED. Note the different
lengths of the three leads. The centre lead (k) is the common cathode for
both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing
each one to be lit separately, or both together to give the third colour.
Bi-colour LEDsA bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.
Sizes, Shapes and Viewing angles of LEDs
Round cross-section LEDs are frequently used and they are very easy to install on boxes by drilling a hole of the LED diameter, adding a spot of glue will help to hold the LED if necessary. LED clips are also available to secure LEDs in holes. Other cross-section shapes include square, rectangular and triangular.
As well as a variety of colours, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less.
Rapid Electronics stock a particularly wide selection of LEDs and their website is a good guide to the extensive range available including the latest high power LEDs.
Calculating an LED resistor valueAn LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.
The resistor value, R is given by:
VS = supply voltage
If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose. In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.
For exampleIf the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA = 0.020A,
R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).
Working out the LED resistor formula using Ohm's lawOhm's law says that the resistance of the resistor, R = V/I, where:
V = voltage across the resistor (= VS - VL in this case)
I = the current through the resistor
So R = (VS - VL) / I
For more information on the calculations please see the
Ohm's Law page.
Connecting LEDs in seriesIf you wish to have several LEDs on at the same time it may be possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED.
All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for VL.
Avoid connecting LEDs in parallel!Connecting several LEDs in parallel with just one resistor shared between them is generally a bad idea.
If the LEDs require slightly different voltages only the lowest voltage LED will light and it
may be destroyed by the larger current flowing through it. Although identical LEDs can be
successfully connected in parallel with one resistor this rarely offers any useful benefit
because resistors are very cheap and the current used is the same as connecting the LEDs
If LEDs are in parallel each one should have its own resistor.
Reading a table of technical data for LEDsSuppliers' websites and catalogues usually provide tables of technical data for components such as LEDs. These tables contain a good deal of useful information in a compact form but they can be difficult to understand if you are not familiar with the abbreviations used.
The table below shows typical technical data for some 5mm diameter round LEDs with diffused packages (plastic bodies). Only three columns are important and these are shown in bold. Please see below for explanations of the quantities.
Flashing LEDsFlashing LEDs look like ordinary LEDs but they contain an integrated circuit (IC) as well as the LED itself. The IC flashes the LED at a low frequency, for example 3Hz (3 flashes per second). Flashing LEDs are designed to be connected directly to a particular supply voltage such as 5V or 12V without a series resistor. Check with the supplier to find the safe supply voltage range for a particular flashing LED. The flash frequency is fixed so their use is limited and you may prefer to build your own circuit to flash an ordinary LED, for example the Flashing LED project which uses a 555 astable circuit.
LED DisplaysLED displays are packages of many LEDs arranged in a pattern, the most familiar pattern being the 7-segment displays for showing numbers (digits 0-9). The pictures below illustrate some of the popular designs:
Pin connections of LED displays
Also see: Display Drivers.
Rapid Electronics have kindly allowed me to use their images on this page. Rapid stock a wide range of components, tools and materials for electronics. I am happy to recommend them as a supplier for individuals and education. In my experience their standard delivery really is rapid!
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