# Found an old LCD in an old radio, how to test it?

I found this here LCD in an old radio, it has no number or anything on it, so I have no idea where to find a specsheet.

Is there any safe way to see how this baby works?

• Just try a low voltage battery like 1.5V and randomly attach it to any two terminals - at some point you'll likely find that one of the display segments shows some sign of showing itself. Note down which segments are activated by which connection. In all probability there will be one wire which is common to all segments and the other 13 wires will activate individual segments. Can you tell by holding it to the light what it can display. This would be a small step in the right direction. Dec 23, 2013 at 23:58
• @Andy aka. Make this an answer? Dec 24, 2013 at 1:48
• No, do not just connect a battery between wires. DC can damage LCDs. Dec 24, 2013 at 18:06

It is probable that there are more segments than wires.

If it is a 4 or 6 digit display, there will be 1 connection per digit and 1 per segment common to all digits. So when you get one segment to darken, move one connection around. Either different segments on the same digit will flash, or the same segment on different digits. The other connection will have the opposite function (if the first changed digits, the second will change segments) Quickly you can identify and write down the pattern. There may be a "spare" segment bit or two that light up odd bits of display (AM, FM, colon for clock, etc) according to which digit is driven.

A basic LCD is actually AC driven, so if you can arrange about 3V AC (square wave is best) at 50 or 60Hz, say from 2 Arduino pins, (or one pin via a capacitor) that will be a better test signal than Andy's 1.5V battery (though the battery will work well enough for quick tests)

Having identified the pattern, writing a program to drive the LCD will be an interesting exercise...

• edit and change it. Dec 24, 2013 at 12:01
• normally a small AC with a DC bias. Am I right? Dec 24, 2013 at 12:01
• @sandundhammika: Any DC bias is incidental. LCDs are capacitive devices. Dec 24, 2013 at 12:08
• +1 to counter the unexplained -1 :-) Dec 24, 2013 at 12:19
• @AnindoGhosh I think someone gave -1 because of the mistyped "LED" word but I don't get it either, why not edit the post instead? Dec 24, 2013 at 12:22

Bare LCDs, as this appears to be, need to be handled very carefully. DO NOT just connect a battery between segments and see what you get. LCDs are damaged by DC. A second or two may be OK, but its better not to push it in the first place. Always put AC with no net DC on LCD pins.

You can create a appropriate AC signal quite easily. Use a function generator or a microcontroller or something to create a digital square wave of a few 100 Hz. Use a resistor divider if necessary to get this down to the 2-3 Volt peak to peak range, then put a capacitor in series with it to block any DC. A 100 nF ceramic should do fine. LCD segments are quite high impedance.

Now you probe around looking for pairs of pins that cause segments to be solidly actuated. This is a small LCD so you probably only have one or two commons. It will take some experimenting, but eventually you should be able to identify the commons, and then map out what segments are actuated when each of the other wires are driven for each common.

This sounds a lot more complicated than it is.

• your right sir, NO DC ! DC would damage the LCD. Dec 30, 2013 at 17:54
• And sir is that actual LCD driver outputs does have that kind of coupling capacitor there? link to a reference design would be a fine. Dec 30, 2013 at 18:00

Just try a low voltage battery like 1.5V and randomly attach it to any two terminals - at some point you'll likely find that one of the display segments shows some sign of flickering. Note down which segments are activated by which connection.

In all probability there will be one wire which is common to all segments and the other 13 wires will activate individual segments. It may be a dot-matrix LCD and 14 wires forms a 7x7 matrix of dots.

Can you tell by holding it to the light what it is capable of displaying? This would be a small step in the right direction.

• Gentle pressure will also cause the display polarization to scramble, revealing the segments. Gentle pressure. Dec 24, 2013 at 11:23
• @IgnacioVazquez-Abrams Very true but be gentle!! Dec 24, 2013 at 11:27