Hardware Reference
In-Depth Information
Figure 9.31
Driving an LED from a buffered digital I/O port: (a) logic 1 to
illuminate the LED; (b) logic 0 to illuminate the LED
thus ideal for providing visual status and warning displays. LEDs are available
in a variety of styles and colours, and 'high brightness' types can be employed
where high-intensity displays are required.
A typical red LED requires a current of around 10 mA to provide a reasonably
bright display and such a device may be directly driven from a buffered digital
output port. Different connections are used depending upon whether the LED
is to be illuminated for a logic 0 or logic 1 state. Several possibilities are shown
in Figure 9.31.
Where a buffered output port is not available, an auxiliary transistor may be
employed as shown in Figure 9.32. The LED will operate when the output from a
PC expansion card is taken to logic 1 and the operating current should be approx-
imately 15 mA (thereby providing a brighter display than the arrangements
previously described). The value of LED series resistance will be dependent
upon the supply voltage and should be selected from the table shown below:
Series resistance
Voltage (V)
(all 0.25 W)
3to4
100
4to5
150
5to8
220
8to12
470
12 to 15
820
1.2 k
15 to 20
20 to 28
1.5 k
Driving LCD displays
A number of process-control applications require the generation of status mes-
sages and operator prompts. These can be easily produced using a conventional
alphanumeric dot-matrix LCD display. Such displays are commonly available
in a variety of formats ranging from 16 characters
1 line to 40 characters
4
lines and can usually display the full ASCII character set as well as user-defined
