Manufacturers of electronic equipment use LCDs for calculators, watches, mini-videogames, and pagers, for example. In comparison with LED-based displays,which consume power on the order of tens of milliwatts, an LCD consumes onlya few microwatts. The LCD thus saves power by a factor of approximately 1000. Checking an LED is as simple as checking a semiconductor diode but, in the case of LCDs, involves some added complexity. An LCD requires an ac electric field to excite the organic compound in the display. Applying a dc voltage could permanently damage the LCD.
The circuit in Figure is a simple configuration to test the performance of an LCD. The circuit produces biphase square waves with negligible dc content. The circuit is based on a CD40106 hex Schmitt-trigger inverter. The circuit comprises an oscillator, IC1A; a phase splitter, IC1B; and a pair of buffer/drivers comprising IC1C/IC1D and IC1E/IC1F.
The buffers and drivers connect to test probes through 47-k series resistors, which protect the IC in the event of shortcircuits. With the component values shown in Figure , oscillator IC1A provides a square-wave frequency of approximately 45 Hz. The circuit can operate from 3 to 5V. To test any segment of an LCD, touch the backplane using either of the two test probes while touching the segment with the other probe. If the segment under test is operational, it will light up. If the LCD under test is a multiplexed type, then all segments, which are connected, will glow if they are operational. Usually, the rightmost or leftmost connection is the backplane of the LCD. If it is not, you have to find it by trial and error.