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THE 
LIGHT EMITTING
DIODE

In this set of projects, a small light emitting device is used in projects 1, 2, 3 and 4 to show that a circuit is working. It looks like a very tiny red or green globe but in fact it does not have a filament and does not get hot - so it's not a globe. It's a solid-state device that never burns out and consumes very little current. It is called a Light Emitting Diode or LED for short. 

                             5mm Red LED        3mm Red LED

It is one of the most amazing electronic devices to be invented and you will have seen it used in flashing tail lights for bicycles and as "ON" indicators for electronic appliances and in many types of 7-segment number displays. 

Even though this component looks very simple it has a number of requirements that have to be met to make it work properly and that's the purpose of these experiments. 

Here are some of the characteristics and requirements of a LED: 

A LED must have a resistor in series with one lead to prevent it burning out. 

A LED will only work when connected around the correct way. 

A LED produces a characteristic voltage across its terminals and this voltage is constant, no matter how bright the LED. The voltage is 1.7v for red LEDs, 2.1v for green and 2.3v for orange LEDs.  This voltage is slightly higher for some LEDs, depending on the manufacturer and High Bright devices have slightly different characteristic voltages.  
But for our particular devices, it remains constant. 

A LED can be turned on for a very short period of time and your eye will extend the time (due to a phenomenon called Persistence Of  Vision). That's why we can pulse a LED very briefly and repeat the process at a high frequency and the LED will appear to be ON all the time. 

The only thing you cannot get from a LED is white light. You can get red, green, yellow, orange or blue, but not white. The colour is determined by the crystalline material used in the centre of the LED. The casing or body is sometimes red, green or orange etc to help enhance the colour of the emission from the crystal and this is called a diffused 
LED. If the body is clear, the colour produced by the crystal will depend on the type of crystal giving off the illumination. White light cannot be produced by a single emission - the only way to get nearly white light is to combine red, blue and green LEDs together and the eye will merge the colours to get white. 

The symbol for a LED is shown below:

 
The line on the diagram corresponds to the cathode lead and this is generally the shorter lead. 

You cannot always be certain of this as we have found some LEDs are made in reverse or sometimes the two leads are the same length. 

When we are talking about the leads, we do not describe them as "positive" and "negative" we only say anode and cathode. Most of the time we only refer to the CATHODE lead.

A small flat on the side of the body of a LED indicates the cathode lead - this is very helpful to remember as it identifies the correct placement of the LED when fitting it to a project. 

If you do not know which lead is the cathode, connect it to a 9v battery with a 220R or 470 ohm resistor in series with one lead. The LED will illuminate when the cathode lead is connected to the negative terminal of the battery. 

The normal current required by a LED is 10mA (milliamp). The LED will still operate on currents as low as 1mA and the maximum continuous current is 25mA. 

For a particular current flow, some LEDs are brighter than others. This is due to their efficiency. Light output is measured in milli-candella. Most LEDs have an output of about 20mcd and these are used as "ON" indicators. Better quality LEDs are 100, 200 and 500mcd and these are called High Bright. Super High Bright LEDs have an output of 500mcd, 1,000mcd,  2,000mcd and 5,000mcd. 5,000mcd = 5 candella and these LEDs produce a light beam suitable for a key-light torch. 



TESTING A LED 

Before fitting the LEDs to the projects in this e-book, they can be tested to find the cathode lead. Simply connect one of the 220R or 470R resistors to one of the leads and connect to a 9v battery as shown in the diagram. 

If the LED does not illuminate, turn the LED around. This testing will not damage the LED. (Do not connect the LED directly across the 9v battery as this will damage the crystal inside the LED.) When the LED illuminates, the cathode (k) lead of the LED will be connected to the negative terminal of the battery. 



FITTING A LED 

The LED is fitted to the PC board so that the cathode lead (the shorter lead) goes down the hole marked with the line on the overlay. Refer to the diagram

Sometimes a small flat can be seen on the side of the LED but this is very hard to find on 3mm LEDs. The best is to reference from the shorter lead but if you have cut the leads, you will have to test the LED as shown above before fitting it. 

Don't forget to solder LEDs very quickly as they can be easily damaged when soldering and their light output will be reduced.