7 Seg LCD's from Calulators

[VISN]

I have got a 7 segment LCDisplay pulled out from a large calulator with a wired header, and am wondering how to proceed with using it.

I am finding a too much noise using google, but from my memory these operate by sending a low current inverting voltage to one pinand using the other pins to switch the segments. I want to play around with thisbut am having ahopeless time with google.

Should I hookup a 555or pic to each of the pins in turn and try sending alogic signal to the remainder, in order to figure it out, or will it blow something this way? Or does it work some other way? I remember the 7106 vaguely, will google those now ...

{edit} ~multiplexing, its about testing then electrically(not logically) interfacing these salvaged components, yep kb is broke. {\edit}

 

[Pommie]

Google multiplexing and check your space bar, it seems to be broke.

Mike.

 

[mvs sarma]

I have got a 7 segment LCDisplay pulled out from a large calculator with a wired header, and am wondering how to proceed with using it.
...................

Please tell the specific component number of the display under concern, and from which brand and model number it was taken. This will help to further the discussion, I feel.

 

[on1aag]

Hi VISN,

Build an astable multivibrator with a CD4047. See to it that it runs
at about 30 Hz at the outputs of the flip-flop, pins "10" and "11".

https://www.alldatasheet.com/datasheet-pdf/pdf/66430/INTERSIL/CD4047.html

Connect these outputs "somewhere" to the display. Multiple segments
will "light", continue to do so until you have found two connections
where only one segment will light. Now you have found the backplane . . .
The backplane is usually located at the extremes of the display, left,
right or even at both sides.

Have much fun !

on1aag.

 

[VISN]

Thanks! The word backplane triggered the word antiphase, and I found this
https://www.chatzones.co.uk/discus/messages/1218/2326.html (2nd post)
& this
" LCD segments must not be subjected to a DC voltage or else they will die.
That is why the "on" segments and backplane are driven "out of phase" by the
two bicore outputs which generates a 30 Hz AC voltage equal to 3V RMS
between the "on" segments and the backplane. An "off" segment is
connected to the same bicore output as the backplane and therefore the voltage
between an "off" segment and the backplane is 0V"
on this (huge) page **broken link removed**
which is a good start.
Now that I have a toe in I will google around.

 

[ecerfoglio]

..................

" LCD segments must not be subjected to a DC voltage or else they will die.
That is why the "on" segments and backplane are driven "out of phase" by the
two bicore outputs which generates a 30 Hz AC voltage equal to 3V RMS
between the "on" segments and the backplane. An "off" segment is
connected to the same bicore output as the backplane and therefore the voltage
between an "off" segment and the backplane is 0V"

...........................

One way to drive individual segments is with an exclusive or (XOR) gate per segment. One of the input pins ("a") go to the backplane, the other ("b") to the control signal and the output to the segment.

When the "B" input is low, the output will be in phase with the other input ("a") and the segment will be off.

When the "B" input is high, the output will be in "antiphase" with the other input ("a") and the segment will be on.

Of course, at 4 gates per chip, this method uses a lot of ic's to drive an entire display. Years ago I used this to drive the decimal points in the display of an ADC (Intersil ICL 7106), while the 7106 drove the remaining segments and provided the backplane signal.

To drive an entire display you may use a microcontroler with enough i/o pins.

 

[k7elp60]

I have got a 7 segment LCDisplay pulled out from a large calulator with a wired header, and am wondering how to proceed with using it.
}

If the display has a wire heater it is probably a flourscent display. If so it works like a vacuum tube.

 

[Oznog]

Many people ask about how to drive a bare LCD display. It's always the most useful-looking thing in a junked device.

Truth is, they're generally impractical work out a driver for them and thus pretty much useless. It makes more sense to get a cheap LCD module with a driver on it with a parallel interface and those are easy to drive.