LCDs

An LCD is a type of display that can display numeric, alphanumeric, or graphic data. These displays provide higher contrast than LED displays, but need a backlight to be able to be read in low ambient light.

LCDs, first off, do not illuminate on their own. They require a backlight, either LED or EL, to illuminate the panel.

Secondly, these displays need an AC input, not a DC input. If you were to apply DC to the display, it would damage the liquid crystals that make up the display's segments. Use a 5 volt AC signal for maximum contrast.

To gain control of your LCD, design a clock signal generator that will output a square wave at a frequency of 50 to 100 hertz. This will go to the common backplane of your display.

Next, connect exclusive-OR gates to the segments. Two 74LS86 packages for a single-digit LCD will do the trick. The backplane clock signal goes to one input of all gates, and the segment signal goes to the other input. How this works is that with the signal input low, the clock signal from the other side of the gate is in phase with the backplane, thus showing no segment, and with the signal pulled high, the clock signal is shifted out of phase, showing the segment.

Now, add driver circuitry, such as a 74LS248, to the LCD. With all connections made and segment outputs on the decoder connected to the exclusive-OR gates, the LCD should display numbers 0 to 9 as expected.