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Project:
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How-To: Parallel Port LCD Character Display Guide |
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Guest Author:
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Jonathan Hamilton
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Date:
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December 23rd, 2003
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Introduction:
Like most of you reading this article, I was interested in adding an LCD display to a new computer I am
building. The plan was simple, build a parallel port LCD display following a basic schematic that most
LCD how-to articles were based on.
Click image for larger view
I purchased a 40x2 LCD with LED backlight from BG Micro
and built the display according to the diagram above. When I plugged the display into the computer and
powered it up it displayed nothing. Most displays, when powered, will display all block characters across the
first line until they receive some sort of signal. I saw no blocks, no smoke, and had no funny smells. The LED
backlight was working which made me feel a little better. I checked and re-checked my connections a dozen times
before I caught a slight glimpse of characters. When I held the display at an angle I could see faint characters
on the display. I was relieved that it was working but confused why the characters were so dim.
BG Micro supplied a technical sheet with the LCD which identified the LCD as being manufactured by
Tianma. After searching through their website I discovered
they offered their LCD's in both standard and extended temperature models. I don't really know what the extended
temperature means but the documentation stated that the contrast controls require negative voltage for control
rather than the simple ground used in the diagram above.
NOTE: Before we begin with this guide, I should note that proper precautions should be taken when attempting
your own project and as with all warnings, you are responsible for your own actions. MODTHEBOX.COM nor myself can
be held responsible for any damage caused to your case or computer equipment by not following the necessary safety
procedures.
Planning:
Now the problem was how to create negative voltage from +5 volts and a ground. Luckily, I stumbled onto an
article on the EIO
website which included this diagram as a method of converting voltage.
Since I was going to add an extra circuit board I decided to include the ability to power the backlight on/off through
the parallel port. This is a feature of LCD Smartie,
the software I am using to control the LCD. Below is the diagram supplied on their website to control the backlight. I
still wanted to be able to control the brightness so I will be replacing the 100 ohm resistor with a 100 ohm potentiometer.
After building the circuit, I discovered that the specified transistor could not handle the demands of this LCD. After
burning up two transistors I decided to omit this piece and use a potentiometer and switch to control the backlight. If
you are using a different LCD you may have better luck than I did or find someone that knows about transistors to find
a larger capacitiy model.
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