FM Transmitter
Sistemas Electrónicos S.A EM 100 DIG
Technical Manual - v1.2 - November 2005 51
9 - ON THE AIR indicator LED.
10 - External control keyboard.
11 - CANCEL key, to suppress any order or command.
12 - OK data entering key, to confirm any order or command.
13 - Two-rows, backlighted alphanumeric LCD display.
The keyboard switch set is sequentially interrogated one hundred times in a second to determine if a
key was pushed. IC5,a serial to parallel converter, drives the front-panel LEDs and the display
backlighting with TR2.
The alphanumeric display is a separate module, connected to the board by a small flat-ribbon
cable, as shown in Figure 3-6 above. Eleven digital lines incoming from the Microcontroller drive
this module. The internal board potentiometer RT1 regulates the LCD contrast and may be used to
change it for different situations. A separate power supply current for the backlight LEDs is provided by
R41 and R42: these resistors become quite hot when the display is full on and their heat someway
influence the internal temperature read by TR3.
No other regulation is provided on the board.The precision of the measurements is guaranteed by
design by the precision of the components and the reference voltage source IC1.
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