Simple tester for checking LCD display

Electronic equipment manufacturers use liquid crystal displays (LCDs) in calculators, electronic watches, small video game consoles, and pagers. LCDs consume only a few microwatts of power, compared to LED (light-emitting diode) displays, which consume tens of milliwatts. As a result, LCDs are very energy-efficient, consuming only about one thousandth of the power of LED displays. Testing LEDs is as simple as testing semiconductor diodes, but testing LCDs is more complicated. LCDs require an AC electric field to excite the organic compounds in the LCD. Applying a DC voltage to an LCD will permanently damage the LCD. The circuit shown in Figure 1 is a simple configuration for testing LCD performance. The circuit can generate a two-phase square wave with negligible DC components. It is based on the CD40106 hex Schmitt-trigger inverter. The circuit includes an oscillator IC1A, a phase splitter IC1B, and a pair of buffer/drivers consisting of IC1C/IC1B and IC1E/IC1F, respectively.

The buffer and driver are connected to the test probe through a 47Ω series resistor. The 47Ω series resistor protects the CD40106 in the event of a short circuit. If the component parameter values ​​are as shown in Figure 1, the square wave frequency generated by the oscillator IC1A is about 45Hz. The operating voltage of this circuit is 3~5V. To test any display segment of the LCD, touch the back electrode with one of the two test probes and touch the display segment with the other test probe. If the display segment under test works properly, it will glow. If the LCD under test is a multiplexed transmission type, all the display segments can be connected, and they will glow if they work properly. The leftmost or rightmost connection point is usually the back electrode of the LCD. If not, you must repeat the test to find the back electrode.

Figure 1 This simple circuit generates a two-phase square wave with no DC component, allowing LCD testing.