[HELP WANTED] Driving a calculator LCD

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[HELP WANTED] Driving a calculator LCD

Postby Atoss » Tue Apr 11, 2017 1:04 pm

So I have gutted this Citizen SDC-810N pocket calculator out of both interest and the false assumption it was dead. What interested me the most was whether I could re-use it's little 7-segment LCD. As you'd expect from such a calc, it's very primitive - you know the type, reflective, 10x 7-segment digits plus 10 decimal points, 3 thousand separators (apostrophes, I guess), M, E and a negative number indicator. Passive matrix, I guess. No controller on board for sure. 34-pin connection to the PCB. No markings whatsoever on the display - no model number, manufacturer or serial number, and googling for phrases like "generic calculator LCD" or "Citizen calculator LCD" reveal nothing.
Probing with a multimeter around the pins indicates that it runs on 1.5V - again, not surprising, given that this calc runs on either a single coin cell or a small photovoltaic panel. Applying that voltage to random pins of the LCD briefly lights up (or should I say darkens down??) random segments, and they fade out a second or two later. The polarity seems to be irrelevant - either way, I can't seem to get more than a couple of seconds out of the segments.

So, in the end, ladies and gentlemen, how do I control this little display? Seems to me that it's a bit more involved than just applying Vcc to the right pins.
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Re: [HELP WANTED] Driving a calculator LCD

Postby Valen » Sun Apr 16, 2017 5:26 am

Essentially those segments are like capacitors. They need an alternating voltage (low frequency) applied to keep the segments dark. DC voltage degrades them.
These two application notes explain a bit how they work.

http://ww1.microchip.com/downloads/en/A ... 00563c.pdf
http://www.atmel.com/Images/doc2569.pdf

In addition there is this Arduino based article. http://www.arduinoos.com/2014/08/lcd-direct-driving/
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Re: [HELP WANTED] Driving a calculator LCD

Postby rony albert » Sun Apr 30, 2017 6:33 am

Would you please share some visuals to make it more easy to understand for a layman. Thanks!
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Re: [HELP WANTED] Driving a calculator LCD

Postby Valen » Mon May 01, 2017 4:27 am

In what way do the provided links not suffice? Btw, that was just a google search away (ignoring any links to how to interface to ready made lcd displays.)
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