Section A.2 - Application Notes.Design Examples
Version 1.3 - 2009-12
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A.2.3 LED and Switch Design
A.2.3.1 BLUE
Signal
The BLUE logic LED signal is not multiplexed with any other functionality, which makes the
design more straightforward (for more information see Figure 17). There are two important
notes:
-
A blue LED requires about 3.5V forward voltage drop (cannot be used in a 3.3V system).
-
The BLUE signal can be used to detect if the module is connected or not. The BLUE signal
flashes when the module is sending and receiving data (see Chapter 3, Table 8). See Figure
18 for an example on how to suppress the flashes.
Figure 17: A blue LED can be connected directly to the module if the LED current is below 4mA. A high
state (active low) makes the voltage drop over the LED to 2.2V. This is not enough to light the LED.
Figure 18: A Low pass filter and a 74HC08 suppress the flashes when the module is sending and receiving
data. The Blue LED can be removed if not required. The values are not suited in all applications and need to
be verified in a specific application.
