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Anode (+)                     Cathode (-)

A Light Emitting Diode (LED) is a diode that is capable of producing light at the PN junction between the anode & cathode as electrons jump across a wire bond. The material the wire is made from determines the colour or wavelength of light that is produced.

Flashing LED's can be used whereby an internal IC will control the LED and cause it to flash automatically at a set frequency (Hz). They are also available as other flashing patterns and as Rainbow LED's which have three wire bonds - RED, BLUE and GREEN and an internal IC

​The Anode is determined by a longer component leg and the cathode by a flat on the side of the casing. The LED will not light if the polarity is incorrectly wired as it performs like a diode.

An LED is an incredibly efficient replacement for traditional incandescent light and is responsible for a wide range of modern innovations


There are two other types of LED; Bi-colour and Tri-colour. 

  • The bi-colour LED has two bonded wires inside the casing which allow it to produce two colours of light. To change the colour the polarity of the LED is switched.
  • The tri-colour LED has 3 component legs and two bonded wires. They share a cathode but depending on the anode that is charged the colour will change. The third colour is achieved by the blending of both colours being on at the same time.

Infrared light can also be produced by an LED and are often found in remote control technology. Although invisible to the human eye the light can be used to communicate with infrared receivers and wirelessly transfer information. Infrared is used in Opto-Isolators for switching

LED's can also be purchased in flexible strips of 12V SMD (surface mount) LED which can be cut to the required length (every 3 LED's). These are available in a wide range of colours and lengths and require minimal soldering/design and can even be remote controlled. The advantage of this method is the speed and convenience at which you can add lighting to a project with limited practical skill and circuit design knowledge 

An Opto-Isolator is a component that allows the switching between circuits with differing voltages much like a Relay. Encased in a 4/6/8 Pin DIL IC is a infrared LED and an infrared phototransistor, separated by a small gap. When the LED is on the phototransistor conducts allowing the attached circuit to become active. As with a relay, no electrical connection is made between the two circuits

Advantages over a electromagnetic relay -

  • Incredibly fast switching (speed of light) compared to relay
  • Low voltage/current required - higher efficiency than relay
  • No fatigue (wear) as there are no moving parts 

The disadvantage is that they are not suitable for high current switching ​so are only suitable for low voltage electronics such as computing

Flashing LED

Component Theory - LEDs