Tap into 7 seg LCD driver...

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What would be the most simple and safe way to derive a clean 'Logic High'
(for CMOS 4000 series etc), from each segment of any functioning device
which has an LCD display?
In this case, i'm seeing an AC square wave of about nine volts p.p. per
An Intersil ICL7136 chip is driving the LCD display (as it is designed to
do) and the system is running off a nine volt battery.

I have a half built 'Veroboard Lashup' I started over ten years ago as an
attempt to do the task using a nice little four pin bridge rectifier for
each seg. followed by a smoothing cap and it was a good enough signal to
present to the CMOS Digital part of the project.

But I really doubt if this is the best approach in terms of the ICL7136's
safety or simplicity in the work required to finish this one-off gadget.

I've done a reasonably large amount of searching, but ended up here (as
usual) for these sorts of answers.

Any Ideas?

Re: Tap into 7 seg LCD driver...

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Gooogle  "Driving LCD segments directly"

There are too challanges to overcome.

a) The LCD segments are matrixed - not a static waveform (pulsed)

b) The LCD waveforms are not static (AC waveform)

So your logic will need to detect the the suttle differences in the AC
waveforms as well as the pulsed waveform.

Might be easier to to do it another way


Re: Tap into 7 seg LCD driver...

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On page 9 of this PDF http://www.intersil.com/data/fn/fn3086.pdf there is
a circuit to select the decimal point.  My thought is to XOR a segment with
the backplane signal and latch the data in a FF when the backplane is not

You might eliminate the XOR by latching the segment when NOT backplane.

Re: Tap into 7 seg LCD driver...

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Feed the segment outputs into a latch (eg: several HEF4044) and clock the latch
from the backplane
pin. (possibly with a slight phase shift and/ or signal inversion interposed)

or you could feed the segment outputs straight into MM74C915 if you're using
them, and
use their internal latches, (same scheme for clocking the latches)

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