From: Gary Tait
Subject: Re: LCD Question
References: <email@example.com> <3DC294B0.firstname.lastname@example.org>
X-Newsreader: Forte Free Agent 1.92/32.572
Date: Fri, 01 Nov 2002 13:46:11 -0500
NNTP-Posting-Date: Fri, 01 Nov 2002 13:46:30 EST
Organization: WorldCom Canada Ltd. News Reader Service
On Fri, 01 Nov 2002 14:49:10 GMT, Wafer
> Typically, a 7 segment display is used for clocks, both for LCD and
>LED. There is usually a common connection for one side of each digit's
>segments, so a 4 segment display gives you 4 connections. All the other
>ends of the segments are actually connected together. Only one digit
>will actually be "on" at any given time, via the common. The pulses are
>timed such that only the proper segments get juice to form the proper
>number for the respective digit, all in such rapid succession as to look
>like all the digits are "on" simultaniously. Your display may look
>something like this. Each "*" indicates one digit, hence one connection.
>Then the 7 segments give an additional 7 connections, for a total of 13.
> _ _ _
>|[_] . [_][_] AM
>|[_] . [_][_]
>* * * * * *
More typically, all the ancilliary segments (like the 10s hours,
colon, and AM indicator are on one matrix, making it a 4x7 matrix,
even more typically for LCDs, it is not even on a digit basis.
The LCD commons are known as Backplanes, and for a typical 7
segment LCD, such as yours, there may be 1 to 4 backplanes. I'd guess
three for yours.
You can see what terminals are the backplanes by looking adjacent to
the glass connector, as there will be a small white dot where the
terminal on that side of the glass conects to the backplane on the
The way LCDs are driven, is that each terminal on the LCD panel is fed
a square wave, typically 100Hz or so, where if the signal on the
backplane were to be in phase with the signal on the segment line, the
segment will be disabled, if it were to be out of phase, the segment
would be active.
> The backlight is an electroluminescent strip, like the ones sold by
>All Electronics. Here is a link to one:
>Feel free to email me with any questions. Have fun!
>Jack B. Pollack wrote:
>> I have several questions about a LCD display in a portable clock that I
>> There are 13 'pad' connections on the PC board that connect the LCD display
>> to the PCB and 'blob chip'. If I look at these connections with a scope
>> there is a square wave on each of them.
>> The display has no electronics in it, how is the data transferred to the
>> display and decoded? How are the individual segments on the display
>> The second question is about the LCD backlight:
>> There is a thin film (looks like a piece of paper) with 2 contacts on it.
>> When 50V AC is applied the film glows blue and backlights the LCD. I would
>> like to find out more about this type of backlight technology and how to
>> build a similar driver circuit for the backlight.
>> Any insight would be appreciated.