Working Principle of LCD

LCD operate from a low-voltage (typically 3 to 15V rms), low frequency (30Hz to 60 Hz) AC signal and draws very little current. They are often arranged as 7 segment displays for numerical readouts. The AC voltage is applied between the segment and a common element is called the backplane (bp). The segment and backplane from a capacitor draw very low current as long as the frequency of the AC signal is kept low. An LCD does not emit light energy like an LED, and so it requires an external source of light.

Each segment in the display is driven by an exclusive-OR gate with one input connected to the output of a seven-segment decoder and the other input connected to the signal source. A 40Hz square wave is applied to the backplane and also to the input of a CMOS 4070 exclusive-OR gate. The Working Principle of the LCD diagram is given below:

When the control input of the EX-OR gate is LOW, its output will be the same as the 40Hz square wave, so that the signals applied to the segment and backplane are equal. Since there is no difference in voltage, the segment will be OFF. When the control input is HIGH, the EX-OR output is a square wave that is 180° out of phase with the signal applied to the backplane. As a result, the segment voltage will be alternatively at +5V and -5V relative to the backplane.

The same logic can be applied to a complete 7 segment LCD as shown below figure:

In this case, the CMOS 4511B-BCD to 7 segment decoder/driver supplies the control signals to each of the seven EX-OR gates for the seven segments. The IC 4511 has active HIGH outputs since a HIGH is required to turn ON the segment.