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9952Re: Seven Segment Displays

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  • urrossum@att.net
    Apr 4, 2012
      > LCDs are harder to drive than LEDs. You can drive an LED with anything,
      > but most LCDs need special display driver chips. Multiplexed LCDs (as
      > found in calculators and almost everything else) require an unusual
      > multi-level AC waveform.

      I don't know about the multi-level AC voltages. For simple segment type displays (not graphic screens - I don't know much about those), each segment is generally brought out to the edge connector. The connector may just be fingers of deposited metal along the edge of the glass, which is contacted using a spongy rubber-like multi-conductor bridge clamped between the LCD and its associated circuit board.

      The other plate of glass is also covered with a transparent conductive surface, which is also brought out to the edge connector. Normally the contained liquid crystal segments are transparent, but when a voltage is applied between a segment electrode and the common surface, the segment turns opaque.

      The problem is that electrolysis sets in and soon destroys the liquid crystal medium. To prevent this, the polarities of the segments and the back- (usually actually front-) plane are alternated. For many of the smaller displays, the voltages are only a couple or few volts, so it's easily possible to achieve this drive directly off the pins of a microprocessor. (Contrast adjustment, if necessary at all, consists of adjusting the excitation voltage.)

      Maxim, and several others, make LCD driver chips that do all the AC switching for you, so you need only I2C or SPI in the segments you want to turn on, and the chip does the rest. These usually provide a separate power input pin as well for contrast adjustment.

      Many-segment LCD displays (such as 5x7 alpha-numeric displays) use several front planes as well as the segments, so that they can be multiplexed just like other display technologies. However, if the applied waveform gets too far away from a square wave, the contrast begins to decrease. This was the issue with old passive and "dual-scan" (just two passive areas on one piece of glass) laptop displays (versus TFT displays, which use thin-film transistors throughout the glass area).

      > The LCD displays themselves also tend to be customized. If you have the
      > money to pay for custom tooling, it can display almost anything you
      > want. But there aren't many generic LCDs suitable for one-off homebuilt
      > projects.

      It turns out that it's remarkably inexpensive (it seemed to me, anyway) to have a custom LCD developed. I checked into this for a project a few years ago, and at that time it was about $2,500 to develop a custom 1.5" x 3" 48 segment display, which included 10 prototypes. In quantities of 1000, the displays (had the project ever come to fruition, which it didn't; see "augered in" below) would have cost about $4.00 each.

      > When I was designing the Membership Card, I did a few "test case"
      > designs using LCDs. There was very little interest in pursuing them, so
      > I gave up and went with LEDs instead.

      It's not going to be directly suitable for the Membership Card, but it occurs to me that a simple 2-digit LCD display, with logic, that could plug into a DIP socket and just continuously display the hex value of 8 data lines might be a fairly handy thing. To that end, I've purchased a few of these to play with:

      I'm thinking of a small board, with a small processor (should be a PIC for low power, but I'll probably use my favorite little 8051, since I've got so many available) with a 16-pin DIP footprint of pins sticking out the bottom. The 8 pins on one side would be the data lines, and pin 9 and 16 would be ground and power, respectively. Plug it in, and voila! - instant Hex display! And the whole thing should only draw a few milliamps.

      I'm currently marooned out of town, but when I get back, I'll throw this on the heap of projects...

      For a similar (but *much* more sophisticated) project idea, check out this little oscilloscope:

      > It'd say your best bet for an LCD display is to buy a prebuilt module
      > that includes the driver chip, and write software to talk to it.

      This is certainly an easy way to go, and displays with parallel interfaces are frequently very cheap on the surplus market. You can also get more sophisticated displays that have a built in processor and talk serial; these are usually made as backpacks onto standard parallel displays, typically by smaller firms like Parallax, and targeted at the hobbyist and prototyping markets. I haven't used theirs, but have had good luck with CrystalFontz, and bad (as in "augered in a project" bad) with Matrix Orbital. Noritake also has an extra-cost serial option for some of their (already expensive, but very nice looking) VFD displays.

      The advantage of the serial displays is that they're easier to control in software (once you have a way of sending out serial characters), and only require one output pin. Parallel is pretty easy also, though, requiring only 5 pins usually (most displays have a 4-bit mode, plus a write line). The problem with any of these, of course, is that *some* software is required - you can't just toss them on a data bus and watch the values go by like the LEDs (or TIL-311s).
      Mark Moulding
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