[HELP WANTED] Driving a calculator LCD

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[HELP WANTED] Driving a calculator LCD

Postby Atoss » April 11th, 2017, 2:32 pm

Hello ladies and gentlemen,

I seem to have a problem with a primitive LCD that I extracted from a Citizen SDC-810N calculator, based on the assumption that the calc was dead (which proved false later, but I was already too interested in the display to just put it back together).
The LCD in question is rather primitive. 10x 7-segment digits, 10 decimal points, 3 thousand separators, M(emory), E(rror) and a negative number indicator. Reflective. Passive matrix, I suppose. 34 pin connection to the calc PCB. No controller whatsoever. No model number or manufacturer either. Searching for "generic calculator LCD" or "Citizen LCD" gives no useful results.
Some probing around with a multimeter revealed that it runs on 1.5V - not surprising given that the calc itself runs on either a single coin cell or a small photovoltaic panel. Applying this voltage to random pins did indeed light up (or is it "darken down" in this context?) some random segments, however they faded a second or two after. Also, attempting the same with reverse polarity yielded the exact same results.

So it looks that the process of controlling this is a bit more involved than just applying that potential to this pin. Does anyone have any idea of how exactly it works?
Atoss
 
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Re: [HELP WANTED] Driving a calculator LCD

Postby k-ww » April 11th, 2017, 3:55 pm

The voltages and wave forms applied to a LCD display can be quite complex - from 1 to 4 different wave forms/voltages to the 'rear' [for want of a better term] and ones opposite or the same to the segments on the 'front'.

To get a handle on how these work and how complex they can be, the best place to look may be a microprocessor that can run this type of display.

Since I'm a PIC person [no aspersions on other brands], I would suggest you download the PDF file of [as an example] the PIC16F1937 at:

http://ww1.microchip.com/downloads/en/D ... 41364E.pdf

Section 27, specifically section 27.9 deals with the type of wave forms needed based on the type of LCD display.

Be warned, it is not something for the faint of heart.
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Re: [HELP WANTED] Driving a calculator LCD

Postby Atoss » April 17th, 2017, 3:32 pm

Thank you, k-ww, that PDF was very useful, gave me a few keywords to search by among other information.

Given that this LCD is so simple, I presume it's of the static drive variety, with a single common plane. According to the PDF you linked to, and few others I stumbled upon, waveforms for those are not that horribly complex - two identical square waves, one offset from the other. Assuming that I understand this correctly, they should look like the ones in the attached image, Vcc being 1.5V, right?
Attachments
Waveform1_scaled.jpg
The driving waveform, I suppose.
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Re: [HELP WANTED] Driving a calculator LCD

Postby k-ww » April 17th, 2017, 4:27 pm

Well, the best way to tell I to see how many pins the lcd has and how many segments the display has-
for a basic display with one backplane, and 6 7 segment digits with decimal point, you would need
at least 49 pins.

I suspect that the display has at least 2 backplane segments, if not more.
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