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Author Topic: Feasible to digitize 7-segment display by connecting to underlying circuit?  (Read 3781 times)
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I'm currently working on a small project which uses a webcam to take a picture of an LCD display and OCR it with a custom made PC program. I'd like to know if it's feasible to have the Arduino read and decipher the electrical signals from the LCD directly, thereby avoiding the requirement of a PC and webcam all together. This solution would be better simpler, require fewer parts and be very fast compared to taking a picture and OCRing.

I have a few questions in this regard:
  • 1. Is it feasible?
  • 2. How would I go about reading the signals?
  • 3. If, by a miracle, I find out how to read the signals, can anything be soldered  by hand on to the tiny contacts?

Here is a picture of the LCD in question:


Here is a picture of the underlying circuitry, 36 contacts driving 6 digits with 28 segments or 8 digits with 42 segments. (The codes displayed are only 6-digits, but the physical display is wide enough for 8 digits. It's a standard 7-segment display.)


Here is a description from the supplier:
http://www.vasco.com/products/digipass/digipass_go_range/digipass_go3.aspx

Here is my forum post where I presented my problem and suggested solution using a PC and webcam:
http://arduino.cc/forum/index.php/topic,66396.0.html
« Last Edit: September 26, 2011, 08:50:24 am by murple » Logged

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I wrote an article back in 1987 about how to do this:-
http://www.doc.mmu.ac.uk/STAFF/A.Wiseman/Acorn/BodyBuild/BB87.html

However, it is very messy and you need one analogue input per segment because the multiplexing is a many voltage level signal.
The diagrams for it are here:-
http://www.flickr.com/photos/33177304@N03/5162443812/in/photostream

Sadly I would not recommend it.
« Last Edit: September 26, 2011, 09:20:50 am by Grumpy_Mike » Logged

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Grumpy_Mike: Wow, that was very interesting and definitely open up my eyes on this subject. Thank you very much for sharing. Basically, what you're saying (or said in 1987) is that my egg most probably is not worth the vast amount of work required, but that said, it is possible.

I'm still curious about one thing. You write that the number of connections for the LCD display can be calculated by either 7*N+1 or 3*N+3, given my display has either 6 or 8 digits there should be 21, 27, 43 or 57 connections, yet I count 36? My best guess is that it's a triplex display and that several of the connections on the circuit are not in use. Does that make sense?

Seeing as I have no information about the connections (which are in use, which drive the backplanes etc.) and I cannot actually read the digits and the connection voltages at the same time. I'll revert back to plan B which was actually plan A namely to use a PC and webcam. There's more hardware involved, but I've already prototyped the software and it works like a charm.

I don't suppose the supplier will have any interest of disclosing any information...

Grumpy_Mike: Thank you again very much for that in-depth article, right on the money!
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Quote
Does that make sense?
It does, although given that I have not looked at the problem since 1987 there might be some newer ways of multiplexing LCD displays that I have not considered.
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Understood. Cheers!
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