December 2017 63
are lots of other free video lectures
available for free from MIT open
courseware as well at http://ocw.mit.
#2 My best attempt to explain a
transformer: Any time you move a
magnet near a wire, you generate
electricity in the wire. Especially so if
the magnetic field lines “cut through”
the wire as they are moving.
The secondary (output) windings
on the transformer are constantly
being “cut” by the moving magnetic
field from the primary, so they
generate electricity. The job of the
primary (input) windings is to set up
the moving magnetic field in the core
and in the secondary. The primary is
acting as an electromagnet.
The “movement” results from the
fact that the current in the primary
is AC and constantly changing. A
transformer will work on AC but not
on DC. That’s it. Fools rush in where
angels fear to tread.
#3 Magnetics can be really
fascinating. I could sit here and type,
but if you Google "left hand motor
rule," it has nice diagrams to explain
the functions. Motors are different, so
you need the right hand rule.
I was very confused as you are,
but an instructor in the Marines
demonstrated the hand rules and it
then made sense. Good luck and
#4 The magnetic field is a force
field that exists around any current-carrying conductor. When the wire is
wound as a solenoid, the fields add
and it acts like a magnet if the current
is DC. If the current is AC, the north
and south poles continuously reverse.
The force field is continuous;
there are no lines, but it's convenient
to measure the density of the field in
Webers per square meter or Gauss,
depending on the system. The density
of the field is sometimes called flux.
[#9173 - September 2017]
My wife and I are hearing
impaired. We need an AVC amplifier
for TV audio to keep the level
constant going into Wi-Fi hearing aids.
Commercial or build-it-yourself (no
surface-mount); analog not digital.
#1 What you want is to
dynamically increase the volume
of the soft passages, and diminish
the volume of the loudest passages.
The circuit to do this is called a
You don’t want a simple passive
circuit because when you are hearing
impaired, you need to retain the
clarities and minimize the distortion of
the higher frequencies to distinguish
consonants and articulation of
speech, and you don’t want to muddy
the lower frequencies so you can
catch inflection and vowels.
Radio stations (and to some
extent, TV stations) employ this to
maximize modulation (so their signal
comes in stronger), and yet not so
loud as to over-modulate per their
As you experience in TV,
marketing companies use separate
circuits for each narrow band of
frequencies to blast out commercials
as loud as possible over the program
material. Now you’ve got program
material with softer speech passages,
and louder music passages and
loud commercials. This is where the
compressor/limiter comes in for
hearing impaired use.
There are numerous examples
available of all varying complexities,
such as this one from Georgia Tech
sp08dp06.pdf, or this one based on
an Automatic Gain Control circuit
by Jim Keith on Electro-Schematics:
I myself am using a a circuit
based on the TDA1054 IC as
described here by P. Marion: www.
#2 Right now, there are 304 “audio
compressors” listed on eBay. Pick one
within your price range and go for it.
[#9174 - September 2017]
Door Chime Protection Circuit
I need a way to protect my
hardwired door chimes from being
accidentally “burnt up” when the
doorbell button gets stuck.
A couple of times when the
button got stuck, I was home and
able to fix it before damage occurred.
The last time it happened, however,
nobody was home. Someone came to
the door, and we arrived home to the
smell of burnt electronics.
I’ve replaced the button more
times then I can count. I guess the way
the weather hits it — eventually — it
becomes stuck. I am now on my third
set of replacement chimes.
I have TWO 16 volt chimes
(10VA) wired to a single button (LED
lamp) on a 16V 30VA transformer. The
transformer is much larger than normal
to accommodate the two chimes.
I would like to protect the chimes
in one of two ways:
1) Simple protection — Inline fuse
that would blow if the chimes were
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