QCould you explain the use of a degaussing coil, and tell
me if it can be used as a loading coil for an antenna?
■ Len, via Internet
■ FIGURE 3
The PIC can wake up from Sleep by any of the following events:
1. An external reset input on GP3/MCLR/Vpp pin.
2. A Watchdog Timer time-out reset (if WDT was enabled).
3. A change on input pin GP0, GP1, or GP3.
The WDT is cleared when the device wakes up from Sleep,
regardless of the wake-up source. When the Sleep instruction is
executed, the next instruction is pre-fetched. When the PIC wakes up,
that instruction is run. In cases where the execution of the instruction
following Sleep is not desirable, the user should insert a NOP after the
Now before I get flooded with PIC programming questions — I’m
not a PIC programmer. So don’t ask me to debug your code — not
even for money! I just dabble in PIC programming from time to time,
and have learned that the best way to program a PIC using
assembly language is to acquire a library of routines and make calls to
them as needed. Karl Lunt has a good library of PIC macros located at
■ FIGURE 4
ALet’s start with the degaussing coil and go from there.
All cathode ray tubes (CRT) have a metal screen
called a shadow mask mounted just behind the face
of the tube that provides the focus aperture for
the three (RGB) color guns. If the mask becomes
magnetized, the color purity and focus of the image starts looking
like a kaleidoscope or psychedelic trip from the ‘60s. Even moving
the monitor or TV set from one room to another can cause the Earth’s
magnetic field to change the magnetic properties of the shadow
When this happens, the shadow mask needs to be demagnetized using a degaussing coil, which is nothing more than a few turns
of wire that generate an alternating magnetic field. Virtually all CRT
monitors have a built-in degaussing coil that activates for a few
seconds every time you turn the set on.
However, the field that the built-in degausser generates is not
always strong enough to erase really strong magnetism from localized
areas of the mask–like when the grandkids discover your fridge
magnets. For that you need a hand-held degausser — which sells for
$40 to $60.
Fortunately, you can make one for under $10 (free if you can
scrounge the parts!). A degaussing coil is nothing more than a large
coil of wire that you plug into the AC wall outlet. The coil is made by
winding many turns of magnetic wire (the kind found in a discarded washing machine motor) in a circle. The wire size should be 22
gauge (AWG) or larger, and can be purchased from RadioShack
(part number 278-1345). Wind the coil around a five-inch form —
something like a small stove pot. A precise diameter isn’t important;
anything close will work.
Remove the coil from the form using a few strips of electrical
tape to keep it from unraveling.
Solder two wires to the end of the
coil — lamp cord works fine —
and insulate the splice with shrink
tubing or electrical tape. Now
completely cover the coil with a
layer of electrical tape to protect
yourself from electrical shock.
Leave no bare wire showing! Because the wire has a very low resistance, you will need some kind
of current limiter in series with the
coil. I find a 100W light bulb to be
just perfect (Figure 3).
Finally, install a line switch
and a wall plug. An extension
cord from the local grocer works
To use the degausser, plug
PWM AC Lamp Dimmer
18 NUTS & VOLTS November 2005