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Geek Culture / Calculator LCD's

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Dark Java Dude 64
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Posted: 18th Apr 2011 00:00 Edited at: 18th Apr 2011 00:01
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Yes, it's another calculator question! Either way, a few days ago i opened up a pocket calculator i have (the Canon LS-270H) and looked around inside. I was looking at the screen ribbon connector and noticed it only has 28 connections on it, and only 27 were actually connected to the PCB. However, the screen itself has 72 individual segments, seven on each of the eight digits, one decimal point for each of the eight digits, five commas, and a memory, error, and negative sign. I cant for the life of me figure out or find out how only 27 connections can control 72 digits! So does anyone here know, if so tell me! I have researched all over... And BTW the LCD doesnt appear to have any IC's of its own at all

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Interplanetary Funk
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Posted: 18th Apr 2011 00:26
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It may not look like it has any ICs, but it likely does, OR, it might be using a matrix approach like with an LED dot matrix which uses 16 pins (and 1 ground pin, which is likely what your unconnected pin is) to control 64 LEDs.

An integer walks into a bar, the barman says "Sorry, we don't serve your type here"
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Dark Java Dude 64
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Posted: 18th Apr 2011 00:58
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Interesting, im guessing that your first idea was right, it is an LCD, certainly not LED... And if it was a dot matrix only one LED would be lit at a time if im right, and that would mean the circuit would have to by synchronous and there is no timer on the PCB. Then were would the IC be on the LCD?? It's just what looks like a simple glass screen with a reflective back. But it does have a lump on the side but it doesnt look there is anything inside that bump though...

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Interplanetary Funk
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Posted: 18th Apr 2011 01:32 Edited at: 18th Apr 2011 01:34
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if it's using a Matrix approach the MCU controlling the calculator wont necessarily need to have a timing component, it simply needs to remember the state the screens in and constantly activate the segments until the state changes at which point it just loops back to updating the screen until the state changes.

Also, most MCUs have a built in clock or it is determined by a crystal.

Edit:
also, you can get LCD dot matrices, and a simple IC for a display driver could be only nano meters thick, so it might be built directly behind the screen (maybe behind the reflective surface)

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Dark Java Dude 64
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Posted: 18th Apr 2011 01:44 Edited at: 18th Apr 2011 01:47
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Ok that clears stuff up Im still thinking it's IC controlled, and the ribbon goes right into the center of the screen. Thanks!

Edit

This is a bit off topic, but inside the calc is a red, flat diode with a notch in one corner. Im trying to figure out which side id the cathode..

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Interplanetary Funk
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Posted: 18th Apr 2011 01:56
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try using an multimeter, I think some can check diode polarity, but I'm not sure.

An integer walks into a bar, the barman says "Sorry, we don't serve your type here"
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Dark Java Dude 64
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Posted: 18th Apr 2011 02:07
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Yeh mine can do transistor testing but im not sure about diode polarity. I could try hooking up some wires to it but the current might leak into the surrounding circuitry and fry the PCB... The reason i want to know is because i like to follow the traces between the solar panel and battery and whatnot

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the_winch
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Posted: 18th Apr 2011 02:58
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If something costs < 5$ and is made in huge numbers you can be pretty sure the manufacturer has put a lot of effort into cutting manufacturing costs.
A cheap calculator is just going to use a raw LCD panel and integrate the LCD "driver" circuitry into the main IC.

By way of demonstration, he emitted a batlike squeak that was indeed bothersome.
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Dark Java Dude 64
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Posted: 18th Apr 2011 07:06
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Haha did you read the main post?

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