Applications of LEDs

The LED is a low-power device. The power rating of a LED is of the order of milliwatts.
This means that it is useful as an indicator but not good for illumination. Probably the
two most common applications for visible LEDs are (i) as a power indicator (ii) seven-
segment display.

(i) As a power indicator-

A LED can be use to indicate whether the power
is on or not. Fig.1 shows the simple use of the LED as a power
indicator.

When the switch S is close, power is apply to the load. At the
same time current also flows through the LED which lights, indicating
power is on. The resistor R S in series with the LED ensures that current
rating of the LED is not exceed.

(ii) Seven-segment display-

LEDs are often group to form seven-segment
display. Fig.2(i) shows the front of a seven segment-display. It contains
seven LEDs (A, B, C, D, E, F and G) shaped in a figure of *8. Each LED is
called a **segment. If a particular LED is forward biase, that LEDs, it is
possible to display any number from 0 to 9. For example, if LEDs A, B, C,
D, and G are lit (by forward biasing them), the display will show the
number 3. Similarly, if LEDs C, D, E, F, A, and G are lit, the display will
show the number 6. To get the number 0, all segments except G are lit.
Fig. 2(ii) shows the schematic diagram of seven-segment display. External
series resistances are include to limit currents to safe levels. Note that the anodes of
all seven LEDs are connect to a
…………………………………………………………………………………………………………………………

Note that LEDs A, B, C, D, E and F are arrange clockwise from the top with LED G in
the middle.

** Each LED is call a segment because it forms part of the character being displayed.

………………………………………………………………………………………………………………………………………..
common positive voltage source of +5 V. This arrangement is know as *common-
anode type. In order to light a particular LED, say A, we ground the point A in Fig.2
(ii). It forward biases the LED A which will be lit.

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