Project
by Gerard Fonte
Measure the Speed
of Light
This Month”s
Projects
Speed of Light . . . . . 38
Pulse Generator . . . 46
Relay Board . . . . . . . 50
The Fuzzball
Rating System
To find out the level
of difficulty for
each of these
projects, turn to
Fuzzball for
the answers.
The scale is from
1-4, with four
Fuzzballs being
the more difficult
or advanced
projects. Just look
for the Fuzzballs in
the opening header.
You’ll also find
information included
in each article on
any special tools
or skills you’ll
need to complete
the project.
Let the
soldering begin!
Clocking the Cosmos for Less Than $20.00
The speed of light is the fastest thing
there is. Most people know that light
travels at around 186,000 miles per
second in a vacuum, but that speed is really
incomprehensible. Compare it to a rocket that
only needs to reach seven miles per second to
escape from the Earth’s gravity. To use a
more common unit of measure, light travels at
670 million miles per hour; light travels just
about one foot in one nanosecond (a billionth
of a second). That’s pretty quick. This project
will allow you to measure the speed of light
simply and inexpensively, as the basic parts
cost well under $20.00. However, you will need
a good oscilloscope.
The Basics
There are a number of ways to measure
the speed of light, and our approach here was
chosen for a number of reasons. The first
requirement was to use visible light. This
makes optical alignment much easier.
Besides, it only seems proper to be able to see
what you are measuring. A second consideration was the need to keep it simple and small.
Photo 1. This is the transmitter and some parts
that require “field modification.”
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The “optical bench” is only 12 inches square.
The transmitter uses only a single integrated
circuit (IC) and the receiver uses only two ICs.
The third requirement was to keep it inexpensive. If you buy all new parts, the cost should
be under $20.00. Most of my parts came from
my junk box, so the cost was much less. A
good oscilloscope will be needed; the bandwidth should be greater than 100 MHz for
good results (mine was 300 MHz). The design
goal is one nanosecond (ns) of resolution (not
accuracy). Lastly, this approach will introduce
you to a number of optical principles and
characteristics.
Fundamentally, the idea is very simple. A
clock creates regular pulses. A pulse is used
to create a laser light pulse that is sent away
from the apparatus and then reflected back.
This causes the returned pulse to be delayed
very slightly. If we “subtract” the original pulse
from the delayed pulse, we will get a short
pulse that is due only to the delay. This delay
corresponds to the distance the light pulse
traveled. Of course, things aren’t quite that
easy in practice.
Let’s look at Figure 1. The clock is just a
crystal oscillator, and the frequency is not
really important. The choice of 4 MHz is fairly
arbitrary. It is useful to have crystal stability
because jitter on the oscilloscope trace will
make it hard to read. This clock drives a semiconductor laser (which is just a cheap laser
pointer). We use the laser because it creates
a beam of light that’s intense and narrow.
The laser pulse travels out and bounces off a
corner-reflector.
The corner-reflector is just a red bicycle
reflector, but it has the very useful property of
reflecting light back from the direction it
came from, regardless of the angle. Trying to
use an ordinary mirror requires a significant
alignment effort (although it could be done
that way). Using the corner-reflector is
extremely easy, as it will always reflect directly back toward the source. However, this
APRIL 2005
