Specifications

Little Board/P5i Technical Manual
2–38
Note
Incorrect Vee polarity or voltage can damage an LCD panel. Set
the polarity and voltage on the Vee supply before connecting the
LCD panel.
Attaching an External Contrast Control
Vee controls the contrast of the LCD display. (Do not confuse this with a backlight, which illuminates
the screen using one or more fluorescent tubes. Backlights generally require a high voltage AC supply.)
An onboard control (R1) allows you to set the precise Vee voltage for the contrast you require.
However, you may want to provide a more accessible Vee control so that a user can set the display
contrast to accommodate various ambient lighting conditions. The board provides a jumper and control
signal to allow the attachment of a remote potentiometer.
To use the contrast potentiometer on the LCD Bias Supply board, install a jumper on W2 (on the LCD
Bias Supply board).
To use an external potentiometer, remove the jumper from W2, and connect a circuit as shown in Figure
2–10 between J3-47 and ground.
Figure 2–10 External Contrast Adjustment for LCD panels
Select Ra and Rb to provide the appropriate voltage range adjustment for the LCD panel you are using.
Consult your panel’s technical literature for the range of voltages you need to supply for the contrast
adjustment. Use the following formulae to calculate the resistor values (in K Ohms).
270
Ra = - 12
(Vee max/1.5) - 1
270
Rb = - 12 - Ra
(Vee min/1.5) - 1