pipe) will work just fine.
Begin construction by taping one end
of the wire to the form approximately one
half to one inch from the end, leaving a six
inch length of wire. Wind the turns with
tension applied such that the windings are
not loose and spaced as close together as
possible as shown in Figure 3.
Cut small segments of tape ahead of
time to use for any breaks in the winding
process. When finished winding the
secondary coil, tape the end winding in the same
way as the beginning winding. Remove any
temporary tape used in between the first and last
winding, and spray the coil with an aerosol clear
acrylic and let it dry. (Most acrylic sprays require
about 30 minutes of dry time.) Drill three small
holes at either end of the tube to create a strain
relief that will allow connection of stranded wire to
the magnet wire as
shown in Figure 4.
The primary coil is very easy to construct. Surface
area is the most important metric which can be
accomplished using either large diameter insulated wire or
copper tubing. The position of the primary can be at any
point around the secondary.
Looking at the schematic, you can see that the
primary and secondary coils are polarized. If you wind the
secondary in a clockwise manner, the primary coil will
need to be wound clockwise also for induction to occur.
Look in the PVC section of your local hardware for
primary coil forms. A good choice for the form would be
an expansion or reducing coupling of about two to three
inches. Wrap two to three turns of wire used for 120V
power cord around the form as shown in Figure 5.
How Does the
Self-Tuning Work?
The coil pair is made to resonate by the oscillating DC
potential applied to the primary coil. This is accomplished
through the collector connection made to one end of the
primary to the circuit input power supply. Power is created
by regulating the output of a 24 volt transformer, and is
used to power the
primary coil. The
base of the driving
transistor is
switched on and off
by pulse width
modulation (PWM)
using the C2 pin on
the PIC. A simple
program running
on the PIC sweeps
a frequency range as it samples the output of a small
ferrite core inductor mounted under the secondary.
Current is induced in the small pickup coil which is
located within the EM field of the secondary, acting as a
power receiver as shown in Figure 6. Feedback from the
voltage that develops on the small inductor is fed to an
ADC (analog-to-digital controller) pin on the controller.
The supply voltage acts as a reference to the incoming
value and divides this analog voltage into a digital range
from zero to 255. If the supply is three volts, an incoming
voltage of 1.5 volts reads 127, or half the reference
voltage. As soon as the regulated output from the receiver
; FIGURE 3.
; FIGURE 4.
; FIGURE 5.
; FIGURE 6.
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