16 July 2016
n WITH TIM BROWN
• Digital Clock Backlight Problems
• Perpetual Motion Machines
Digital Clock Backlight Problems
QI have a Philips Model AJ3936/17 AM/FM radio/disc/alarm clock. It has a yellow backlight hat lights up the clock numbers at night with a selector switch that changes from off, to
medium intensity, and full brightness. It’s gotten where the
light just flickers off and on.
Sometimes I can get it to come on by clicking that
switch numerous times. It will stay lit for awhile and then
go off. Sometimes if you rap it a time or two it will come
on like there’s a loose component or bad solder joint. It
keeps time perfectly though; it’s just the lighting feature.
Looks like the module that has the clock numbers and
lighting has about 10 or so leads that are soldered into the
motherboard. I don’t know if it’s sort of an argon gas setup
or what. Do you have any suggestions other than touching
up the solder joints on the motherboard?
AI have included a link in the Q&A Sidelines for the Philips Model AJ3936/17 Owners’ Manual to help with our discussion (schematic not available). (Figure 1 is the image of the unit
as shown in the manual.) I do not see the microswitch
you refer to on this model. “Button 14: REPEAT ALARM/
BRIGHTNESS CONTROL” is used to adjust the brightness
of the display by: (1) Setting the clock control (Button 1)
to the CLOCK position; and then ( 2) pressing Button 14
once or more to change the brightness of the display in the
sequence: LOW, MEDIUM, BRIGHT, LOW.
Unfortunately, manufacturers hardly ever include
schematics with electronic devices anymore. There are still
some of us in the electronics hobby that love to “tear” into
malfunctioning units in hopes of repairing the particular
device — even if it costs us more than a new one (you have
to factor in such things as enjoyment and learning which
can more than justify the additional costs).
Before I discuss the backlight problem, let’s discuss
how a Liquid Crystal Display (LCD) works. I will discuss the
twisted nematic liquid crystal theory since these are the
types most often used in electronic displays.
All substances exist as solids, liquids, or gasses. Solids
that have their molecules or atoms arranged in an orderly,
repeating, and rigid pattern are called crystals (table salt,
iron alloys, etc.). Liquids have molecules or atoms which
take the shape of the container in which they are placed,
and their molecules or atoms can easily move over
each other. Liquid crystals occupy the crossover area of
properties between a crystalline solid and a liquid.
In a liquid crystal, the molecules are oriented in a
very specific direction which gives the crystal optical
properties. Figure 2 shows a series of molecules arranged
as a “director” so that the light passing perpendicular to the
“n-vector” is rotated in the polarization of its electric field.
[Light — like radio waves — is made up of waves which are a
series of electric and magnetic fields perpendicular to each
other. The electric polarization of light sources other than
For those with an interest in chemistry, Figure 3 shows
the molecule 4-cyanobenzylidene- 4’-n-octyloxyanaline
which can act as a liquid crystal. In this molecule, the
atoms are abbreviated as: C for carbon; H for hydrogen;
N for nitrogen with the lines between atoms as chemical
bonds; and the two hexagons with circles in them
represent benzene, each of which has six carbon atoms
and four hydrogen atoms, plus the two chemical bonds
information, search for Organic Chemistry on
your web browser.
Q & A
n FIGURE 3.
n FIGURE 1..
In this column, Tim answers questions about
all aspects of electronics, including computer hardware, software, circuits, electronic
theory, troubleshooting, and anything else of
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n FIGURE 2. In the twisted nematic liquid
crystal director, n is the direction in which the
long portion of the individual molecules point.
Light polarized in the n direction passes through
the crystal easier. Light is rotated in polarization
by the twist of the molecules in the director.