Light-Emitting Diodes (LEDs)
Reference: http://en.wikipedia.org/wiki/Light-emitting_diode
http://en.wikipedia.org/wiki/LED_circuits
LEDs are used extensively in electronic devices as indicators and displays. As shown in the second
reference, a LED circuit consists of a LED in series with a resistor. The value of the resistance is
given by:
R = (VCC – VLED) / ILED
Example: The specification for a red LED indicate a typical forward voltage of 2.1 V and a current of
15 mA. What resistance would you use if the supply voltage is 5V?
R = (5 – 2.1) / 15 * 1000 = 193Ω
Example: The specification for a red LED indicate a maximum current of 25 mA and a forward
voltage of 1.7V (min), 2.1V (typ) and 2.8V (max). What preferred value resistance would you use if
the supply voltage is 5V?
R = (5 – 1.7) / 25 * 1000 = 132Ω
Use 150Ω
(1.7V gives “worst case”)
(120Ω would exceed current rating)
Typical circuit
When using LEDs with TTL
circuits it is normal practice to
use the configuration shown.
Input
R
LED
+5
V
Cathode
Anode
As a TTL output can sink a larger
current than it can source this is
An inverter is shown, but any gate type may be used,
the preferred arrangement.
Seven-Segment displays
LEDs are often arranged in a single package known as a 7-segment display to allow for alphanumeric characters. Typically the anodes of all LEDs are connected together (common anode) to
allow them to be externally connected to VCC. The inputs to segments a to g can then be connected to
driver circuits which may be TTL gates or a BCD to 7-segment decoder/driver such as the 7446 (see
data sheet below).
+VCC
Resistors
BCD
Input
BCD to
7-segment
decoder/ driver
e.g. 7446
Cathodes
Common Anode
a
f
b
g
e
c
d
Common Cathode Seven-Segment displays
The 7446 is shown driving a common anode display. Another type of 7-segment display is the
common cathode arrangement. The driver for this type of display must have active-HIGH outputs.
With some exceptions CMOS devices are not normally used to drive the display directly, and TTL
devices are unable to source large currents. A transistor circuit may be used to interface between a
TTL decoder chip and the common cathode display.
Liquid-Crystal Displays (LCDs)
Reference: http://en.wikipedia.org/wiki/Liquid_crystal_display
An LCD controls the passage of light by “twisting” it as it passes through polarising filters. The
available light may be reflected, or the display may be back-lit. Their power consumption is extremely
low compared to LEDs – particularly important in battery operated equipment such as calculators or
digital watches. However the display is not as bright as LEDs which may make it difficult to read in
sunlight.
An LCD segment is non-activated (transparent) when no voltage is applied, and activated (opaque or
dark) when a low frequency AC voltage is applied.
Driving an LCD
A segment may be turned ON by “gating” a low frequency AC voltage with an XOR gate used as a
selective inverter.
Control
40Hz AC
Segment
Backplane
Example
Complete the following waveform diagram, then the table.
Control
AC input
XOR
output
Segment
voltage
Control
LOW
HIGH
Segment (ON/OFF)
Driving a 7-segment display
D
C
B
BCD to 7segment
Decoder/
Driver
a
f
e.g. 4511
A
b
g
e
c
d
Backplane
40 Hz
For another circuit using a 4511 see http://www.doctronics.co.uk/4511.htm
Note that the 4511 has active HIGH outputs, while the 7446 has active LOW outputs. When driving
LEDs the 4511 can drive a common cathode 7-segment display directly, or a common anode display
via a transistor interface (see data sheet). Compare this with a TTL chip driving a common cathode
display.
7446 BCD to 7-Segment Decoder/Driver (Data Sheet)
CD4511BC BCD-to-7 Segment Latch/Decoder/Driver (Data Sheet)