How Lcds Work - From Avenue Sound

The difference between static operation (direct segment control) and multiplex operation (switched segment control). As its name suggests, static operation provides each segment with its own drive, and one common electrode may be used by all the segments (and usually is).

Thus, in this respect it is like the seven segment l.EDisplays (common cathode or common anode). As opposed to multiplex operation, static operation is uncritical with regard to contrast, tolerance and temperature. Figure 8a shows a simple control circuit for a segment with a push-pull transistor stage.

The transistors are part of a CMOS inverter IC, a CD4007 or CD4009,

for instance. The inverter receives a square wave of 30-50 Hz at its input and switches at its output between +Ub and 0 V.

The peak value of the alternating current applied to the segment is equal to half the operating voltage. Capacitors are expensive and take up a lot of space when compared with IC gates, so it would be an advantage if the circuit could be built without any discrete components.

After inversion, the square wave at the rear electrode is 1800 out of phase with the one at the segment electrode. Between the two electrodes lies an alternating current with a peak voltage equal to the supply voltage Ub.

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The principle can be put into practice in an elegant manner with the aid of EXOR gates of the CMOS type (for instance, CD

4030 or CD 4070).

A gate is required for every segment. To one of the inputs of each gate and the display common, a constant low frequency alternating current is applied. The other gate input then controls the segments. lf there is a logical at the control input the square wave at the segment electrode will be out of phase (with reference to the display common) and in phase if it is a logic 0. Because the signals are in phase when the segment is powered, no difference in voltage occurs.

When they are out of phase, the AC rises with a difference in potential of twice the amplitude of the square wave (between the triggered segment electrode and the common electrode).

This must of course be taken into account when the supply voltage of the LCDisplays is fixed. On the data sheet this is usually given as the effective value of the AC waveform.

The effective value of the waveform is equal to its peak value and this is equal to the operating voltage Ub of the CMOS gates. For an LCD specifying an operating voltage of 4 to 6V the CMOS drive circuit will be fed with 5 V.


Courtesy - Elektor electronics

Article Written By Swagatam

Swagatam is a blogger at

Posted on: Last updated: 25-07-2016 97 0

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