images with only changes in intensity.
This is sensation flicker. Flicker ceases
above a certain critical rate. This is the
critical flicker frequency (CFF).
The Ferry-Porter Law
The Ferry-Porter Law states that
CFF is proportional to the logarithm of
the luminance of the flickering stimulus (L). Equation 1 states that:
EQUATION 1:
CFF = a log L + b
■ FIGURE 5. The project’s
block diagram theory of
operation.
of the compass. This acted like a
lodestar, or direction finding star referenced to e.g., Polaris. The name comes
from lodestone (magnetite, Fe3O4) since
lode meant “way” in Middle English, and
magnet evolved from Magnesia.
In 1269, Frenchmen Peter
Peregrinus and Pierre de Maricourt,
used a compass and a spherical lodestone to discover invisible lines of force
surrounding the sphere at opposite
ends. Maricourt called these points the
North and South Poles. In 1600,
William Gilbert systematically studied
terrestrial magnetism and demonstrated that the Earth itself is a large magnet. In 1820, Hans Christian Oersted
proved a relationship between magnetism and electricity, and in 1825,
William Sturgeon invented the electro-
magnet. This prompted Michael
Faraday’s theories on electromagnetic
induction and development of the
transformer, alternator, and dynamo.
In 1821, Oersted noticed the flow
of the electric current in the
wire-deflected compass needles.
Andre-Marie Ampere pursued this to
discover that magnetism was quite
different from popular belief. Ampere
concluded it was a force between
electric currents: two parallel currents
in the same direction that attract, and
in opposite directions that repel.
Here, a and b are constants. This
relationship is valid over a wide range.
The above equation implies that when
you plot CFF as a function of log L, the
straight line identifies the region where
the Ferry-Porter Law is valid. Increasing
the intensity of your test stimulus also
increases your flicker perception. This is
why when your computer monitor flickers, decreasing the intensity eliminates
this flicker. In fact, the reason NTSC
television signals use a 30 Hz frame
rate with two interleaved halves to get
a 60 Hz rate is to minimize flicker.
Theory of Operation
The Human Eye as
a Light Sensor
CRITICAL FREQUENCY
FLICKER (CFF)
CFF is the transition point of an intermittent light source where the flickering
light ceases and appears as a continuous
light. Marks and Bornstein (1973) found
that flicker frequencies ranged from 20 to
35 Hz in normal healthy human eyes.
This is where you start to experience a
compromised ability to detect flicker and
start to incorrectly interpret it as a
continuous light source. You experience
this here since 60 Hz appears to you as a
continuously on light source (LED).
Various factors determine humans
perception of flicker. These include the
intensity and size of the stimulus.
Our eyes must function over a
wide range of luminance levels (light
intensity per given area). During a
normal day, our eyes continuously
sample information as images
projected onto our retinas (the light
sensitive nerve tissue in your eye
that converts images from your eye’s
optical system into electrical impulses sent by the optic nerve to your
brain). Our brain integrates or averages these objects to make them
appear stable or smoothly moving.
The time our brains require to
collect and process these images
limits our eyes’ or visual system’s
ability to tolerate rapid change. If
the rate at which our eyes see
intermittent stimuli is slow, our
eyes perceive this as acceptable
The I/O Basics
There are two outputs — the visible
LED and the transformer — that produce
the tingling sensation you feel on the
back hinges. We flipped the transformer
around and applied the input to its secondary, forming a step-up transformer.
Figure 5 is a functional block diagram.
Each ouput has one-second pulses of
60 Hz up to seven times (see the sidebar on CFF). When you view the LED, it
appears to be on for the entire pulse
because the human eye cannot
distinguish 60 pulses a second. There
are either four or six Hall effect sensor
circuits as inputs. (Refer to Figure 6.)
The Selection Process
The 16-to-1 multiplexer selects
one channel for each state of the four-bit counter. The first three states select
the 60 Hz oscillator directly. The
fourth, sixth, eighth, 10th, 12th, 14th,
and 16th channels are tied to ground
48
May 2007
