What does it do?
| The LED (light-emitting diode) subsystem converts the input signal into light. |
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How does it operate?
LEDs are diodes that emit light when a current passes through them. The current enters the anode and leaves the diode at the cathode.
Some process units provide enough current to drive LEDs. Typical LEDs require currents in the range 2 – 25mA.
If CMOS ICs or a higher current LED are used then a driver (transistor, Darlington or MOSFET is needed to boost the current.
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LED circuit
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
The circuit on the left shows the circuit needed with a driver. The LED, in series with a current-limiting resistor R1, is connected between the supply rail (+Vs) and the input signal from the driver. The LED and resistor act as a load on the driver.
When the input signal coming into the LED subsystem is low, a potential difference across the LED causes current to flow. It is this current that causes the LED to glow. |
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LED circuit for use with higher current process units
Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
PICs, 555 Timer ICs and the LM324 op-amp can provide higher currents and can drive some LEDs without needing a driver unit.
Check the data for the LED and the process unit to make sure that the process unit can provide more current than is needed by the LED.
If this is possible, the LED is connected to the 0V rail (as on the left) rather than to +Vs |
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LEDs come in a variety of colours and sizes. Flashing LEDs are available.
Infrared (IR) LEDs produce infrared radiation which we cannot see with our eyes, but which is used in infrared communication systems. The infrared can be ‘seen’ with a digital camera. |
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Rainbow LEDs e.g the Rapid 55-1904, cycle through the colours red, green, blue and white for a few seconds each. |
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'Full colour' LEDs e.g. the Rapid 56-0672 contain a red, green and blue LED (the primary colours for light) and so, by turning different LEDs on and off, the full spectrum of colours can be displayed. |
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The series resistor R1 is used to limit the current passing through the LED. Using Ohm's law the value of this current-limiting resistor can be calculated:
The voltage Vf across the diode is typically about 2V and the forward current If is about 10mA = 0.01A. So, for a 5V power supply:
The nearest preferred (E12) value would be 330R. |
Brightness
The ‘brightness’ (scientifically this is called the ‘luminous intensity’) of a LED, as perceived by the human eye, is expressed in candelas or milli-candelas (mcd). A low cost LED might have a luminous intensity of 10 – 100mcd. High brightness LEDs can have luminous intensities of 10,000 – 100,000mcd, which is as bright as a 60W light bulb.
The ‘viewing angle’ of a LED expresses how sharply concentrated the beam is. A narrow viewing angle is suitable for illumination but a wide viewing angle is needed for a LED being used as an indicator.
Possible applications
- Acting as a signal
- Providing illumination
- Decorative or mood lighting
Making
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A LED must be connected the right way round, with its anode positive and its cathode negative. Otherwise it will not work, and it might be damaged.
There are two ways to spot the cathode. Usually one side of the LED is flat, and this on the same side as the cathode.
The cathode leg is also shorter than the anode leg. |
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How part of the PCB might look
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Normally a LED would be mounted on the PCB, as shown on the example on the left. |
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It is also possible to mount LEDs in the product case in a panel clip as on the left. In this case a terminal block is mounted on the PCB and wires from this are connected to the LED. |
Build and test the unit that will provide the driving input signal before adding the LED.
Testing
Make sure that the LED switches on and off as power is applied from the driver unit.
Fault finding
If there is a fault, check the voltage across the LED when it should be on (it should be about 2V). Check that the LED is the right way round. Check the value of the resistor.
Alternatives
- A bulb can provide more light but is larger and more expensive
- Electro-luminescent displays can be used to produce more sophisticated visual displays but are more expensive.
Web links
Return to list of datasheets
