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Design & Technology Resources
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Light Emitting Diodes (LEDs)

Light Emitting Diodes, or LEDs, are special types of diodes that emit light when an electric current flows through them. Some of their advantages are:

 

• Long life and reliable                                                    Very unlikely to fail in use

• Wide range of colours available

• Low power consumption                                              Batteries last longer

• Range of sizes available

• Not damaged by physical shock                                 Less likely to break than filament lamps

• Some LEDs can produce more than one colour

Current Limiting Resistors
Standard LEDs

Standard LEDs emit a single colour light when a current flows through it. It has two connections: ANODE and CATHODE. Current will only flow through the LED when it is forward biased, this means that:

 

• The Anode must be connected to a positive voltage

 

• The Cathode must be connected to a negative voltage

 

Anode

Cathode

Anode
(positive)

Longer lead
Cathode
(negative)

Shorter lead
Flat side
The cathode is identified in two different ways:

• The shorter lead
• The lat side on the body of the LED

A circuit that will not work because

• The anode is connected to the negative and the

• Cathode is connected to the positive.

In the circuits above you will notice that a resistor is included in the circuit. The resistor is used to limit the amount of current that can flow through the LED. If too much current is allowed to flow through the LED then it will be damaged. In the exam you may be asked to calculate what value the resistor should be to limit the amount of current to a certain value.

 

LED Forward Voltage (volt-drop)

All LEDs have a property known as forward voltage or volt-drop. This is where a certain amount of the voltage is dropped or “lost” inside the diode. Different colour LEDs have different forward voltages.

 

 

 

2 Volts

2.00V

An example exam question:

 

Calculate the value of R1 in the circuit diagram shown to the right

 

The LED has a forward voltage of 2V and draw 20mA

 

Answer

R1 = battery voltage - forward voltage

      LED current

20mA = 0.02A

Bi-colour LEDs

LEDs are available the contain two different colour LEDs in one package. These are often found on televisions to indicate whether it is on or on standby.

 

The colour of the LED is determined by its polarity. Only one LED can light up at a time because only one can be forward biased at a time due to them being wired up back to back.

 

The animation shows how reversing the polarity can be used to control the colour of the bi-colour LED.

Tri-colour LEDs

Tri-colour LEDs also contain two LEDs in one package.

 

Unlike the bi-colour LEDs however both LEDs can be turned on together because it has two anodes.

 

The LED shown can turn on the red LED, the green LED and both together producing a yellow light.

Flashing LEDs

Flashing LEDs contain a tiny integrated circuit which flashes the LED on and off at a rate of about 1 Hertz (once a second).

 

Although they have to be connected the right way around they do no need a current limiting resistor as this is built into the integrated circuit.

 

The circuit below shows how one flashing LED can be made to flash more than one standard LEDs.

7-Segment LED Displays

7-Segment LEDs contain seven rectangular LEDs and one small round LED to represent a decimal point.

 

The type we need to know about have one cathode and seven anodes. By switching the anodes on in the right patterns the number 0 to 9 can be represented.

 

They can be used with a CMOS 4026 integrated circuit to operate as a decimal counter. Click on the link to see how.

4026 7-Segment Counter
4026

A circuit that will work because

• The anode is connected to the positive and the

• Cathode is connected to the negative.