Measure the Speed of Light
reflection is slightly scattered and is not a tight beam.
For that reason, a lens is needed to focus the laser light
onto the receiver. This lens is an inexpensive plastic
The beam splitter (just a piece of clear plastic) reflects
only the returning light from the lens toward the receiver.
Otherwise, the receiver would have to be in the same position as the laser. (We’ll discuss the beam splitter and lens
in more detail later.) The receiver creates an electrical
pulse that is a duplicate of the transmitted pulse, but slightly delayed. These two signals are combined with an exclu-sive-OR (XOR) gate that “subtracts” one pulse from the
other, leaving only those parts that are different. These differences are the results of the distance/delay and are displayed on the oscilloscope.
Figure 2 shows the schematic diagram of the laser
pulse generator or transmitter. As you can see, it’s very
simple. The first section of a complementary metal-oxide
semiconductor (CMOS) inverter is used as a crystal oscillator. A second section is used to buffer the oscillator, and
the last four sections are used in parallel for increased
power to drive the laser diode. The rectifier diode, D2, does
double duty. It prevents problems if the power is reversed,
and it drops the six volts to about 5. 3 volts so the inverter
won’t get upset. Resister R3 and LED D1 are optional and
are used as a power-on indicator. (It’s just a habit of mine
to include them.)
There are a few notes. The inverter can be almost any
CMOS family (74H04, 74HC04, 74HCT04, etc.), but bipolar inverters won’t work (7404, 74LS04, 74S04). You can
certainly use a commercial oscillator instead of the crystal
oscillator circuit. (In that case, you can eliminate R1, R2,
C2, C3, and Y1. Connect your oscillator output to U1 Pin
3, and ground U1 Pin 1.) While the oscillator circuit is very
robust and will work with nearly any crystal, keep the value
around 4 MHz to match the receiver delay circuit (to be
discussed later). A 3.57-MHz color-burst crystal should
Photo 1 shows the transmitter and some parts that
require “field modification.” I put the transmitter in a box
because I have a feeling that I will find a number of uses
for a laser transmitter that can be modulated at up
to 20 MHz. As you can see, the actual electronics fits on a
small, perforated circuit board, and point-to-point wiring
The laser I used was a key-chain type that used three
1.5-volt button batteries (right center of Photo 1). I chose
to remove the laser head from the body for ease of use.
(You don’t have to, but you will have to make power connections somehow.) I carefully used a hacksaw to remove
the head assembly. You can see the spiral cut in the body
(left center). The laser head consists of a lens, laser, and a
tiny PC board (center). I attached power wires that
Figure 1. 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).
bypassed the switch so I wouldn’t have to press the button
for the laser to work. The long leads were only for testing.
(The two parts in the lower right of Photo 1 are the optical
receiver and will be discussed later.)
with the Ethernet Connection Development Kit.
RCM3720 RabbitCore™ with 512K Flash,
256K SRAM, 1MB Serial Flash, 33 digital I/O.
Complete development software
(not a trial version).
Royalty-free TCP/IP stack and sample
Development board with prototyping area. Limited
AC adapter and complete documentation.
Free Book With Kit
“Embedded Systems Design using
the Rabbit 3000® Microprocessor”
Circle #94 on the Reader Service Card.