August 2017 9
if one or more cells shorted (Figure 3). You’ll lose a little
voltage headroom in doing this, though. To do even better
would require a more sophisticated charging scheme using
delta-V monitoring, looking for the slope change in the
voltage, and/or temperature monitoring delta-T charging.
Then, you can do fast charging safely.
There’s a great little device made by Linear
Technology: the LTC4060. With a few external
components, it forms a nice delta-V charger for NiMH or
NiCd. You can find information about it at www.linear.
LCDs: How They Work
QI love to use liquid crystal displays (LCDs) in my projects involving PIC microcontrollers. When shopping for LCDs at large component retailers, I came across some terminology that I do not
understand. I’m hoping you can enlighten me.
If I understand correctly, a “reflective” LCD uses a
mirror behind, and a “transmissive” LCD has a backlight.
Can you tell me how a “transflective” LCD (which is also
backlit) is different from a transmissive display? Can you
explain what the differences are between TN (twisted
nematic), STN, FSTN+, and FSTN- LCDs? What qualities do
the different display types bring to our finished projects
that would cause us to select one over another?
Judy May W1ORO
ALet’s see what we can find out. While I know some of the answers to this question, I’m going to educate myself at the same time regarding the other parts. (That’s the best way to learn,
isn’t it?) I’ve had a long history with these devices — even
building a watch kit using some of the
earliest commercial seven-segment
displays in the mid-1970s.
Starting from the beginning, the aim
with various kinds of LCD displays is to
control the transmission of light (as you
alluded to in your question) in regions of
the display. Whether that’s pixels for a
certain color filter or just a fixed segment
in a display, the principle is the same.
This is accomplished by using the
polarization property of electromagnetic
wave propagation along with some
interesting materials. There are films
that can be used to only permit certain
polarizations of light (an example of
electromagnetic waves) to pass through.
The actual liquid crystal material
responds to an electric field and changes
the polarization properties of the liquid crystal molecules.
This, in turn, changes how much light is transmitted.
It is similar to stacking two polarizing filters and
twisting them relative to each other, and observing how
you can darken the filters. It’s actually an example of
Quantum Electrodynamics, or QED (don’t miss Feynman’s
wonderful book on this subject, titled QED).
The net effect of this is that areas where the electric
field is applied to the liquid crystal material are generally
made to appear darker. With this in mind, let’s first look at
how light is directed through the filters and material, and
then back to your eye (Figure 4).
First, without any internal light source, we have to
have light from the external environment enter through the
front surface, which is usually a polarizing filter oriented
one way. Let’s say vertically for this discussion (layer 1).
Then, there is an electrode layer (layer 2). This is a clever
QUESTIONS and ANSWERS
Post comments on this article and find any associated files and/or downloads at
n FIGURE 4. LCD optical layers
(source Wikipedia, CC BY-SA 3.0,
user Ed g2s).
n FIGURE 3. NiCd trickle
charger with current source.