The laser key chain I had on hand was very convenient. It worked on 4. 5 volts, so a 5.3-volt, 50 percent duty-cycle signal didn’t overload it.
There was no actual driver circuit for the laser.
There was only a 62-ohm surface-mount resistor that
limited current. This meant that I could drive it with a
very fast signal without problems. A good laser pointer
with a real current control circuit probably won’t be
able to handle the high-speed signals. If you use a laser
that runs on three volts (or other voltage), be sure to
use a suitable current-limiting resistor. My oscillator circuit used an additional 14 mA with the laser on. This
means that the peak current was 28 mA ( 50 percent
duty cycle). Since most of the red laser diodes are
very similar, a 62- to 75-ohm current-limiting resistor
supplying about 25 mA DC should be okay for a
pulsed 5.3-volt operation. Also, the head assembly
acts as a heatsink. It is critical that the heatsink be
functional; otherwise, the laser will quickly overheat
and destroy itself.
The Optical Bench
When you work with optics, it’s important to be able
to keep components stable and fixed. It’s also important
to be able to remove a component and then replace it
easily. The tool used for this is called an optical bench. A
professional optical bench is typically made of metal,
about two by three feet, and has holes every inch (sort of
like large perf-board). Optical component carriers hold
NUTS & VOLTS
the optical elements, while the base of the carrier
plugs into one or more holes in the bench. Optical
benches can cost $1,000.00 or more. That’s a little
pricey for us, so we will create a single-use optical
bench out of wood.
There are many approaches to building the
bench. I’ll explain how and why I built mine the way I
did. (Feel free to choose the methods that you feel
most comfortable with.) Photo 2 shows the basic layout. The base is a 12 x 12-inch piece of particle board.
The transmitter case is snugly held in place with
pieces of 3/4 x 1-inch wood screwed to the base. The
beam splitter rests in a slot I cut out of a scrap of 2 x
3/4-inch wood. The wood is also screwed to the base.
I used shims cut from a business card to keep the plastic
snug in the slot.
A similar procedure is used for the Fresnel lens. The
optical receiver PC board is raised to the approximate
height with a wooden base. For fine tuning of the height,
I used long screws as feet. By adjusting the screws, I
could position the receiver with precision. The receiver
was held in place with high-tech, one-dimensional organic tension straps (otherwise known as rubberbands). The
corner-reflector shown in Photo 2 is just taped to an
empty reel of wire to keep it vertical. In actual use, it
is not part of the bench assembly and is placed several
There are a few things to discuss about the beam
splitter and the Fresnel lens. The beam splitter is just a
scrap piece of clear plastic. It’s about 4 x 4 x 1/8-inch
thick. Any piece of clear plastic should be okay. Its distance from the laser and the mounting angle are not too
important. Give yourself room for the other components.
(I guessed, not measured, around 45 degrees.) You can
use glass for your beam splitter, but, obviously, glass is
sharp and fragile. Additionally, it’s hard to drill a hole in
glass. There is a hole in the center of the beam splitter,
but it’s very hard to see in the photo. It’s not absolutely
necessary to have the hole, but it helps. Here’s why: If we
split the beam on its outward leg, we lose about 50 percent of the light. Obviously, we want as much light reflected as possible, hence the hole. There is also a hole in the
Fresnel lens, but this hole is critical. If the outgoing laser
beam passes through the lens, the beam will spread out
The Fresnel lens I used was a
credit card magnifier (or “wallet”
magnifier) that was a free promotional item I got from some company years ago. Its placement isn’t
too critical. Just remember that the
focal length is about 3. 5 inches
(yours might be different).
Therefore, the distance from the
lens to the beam splitter to the optical receiver IC will be that length.
Figure 2. This is the schematic diagram of the laser pulse
generator or transmitter.
Figure 3. This is the circuit of the Sharp GP1FA551RZ fiber-optic receiver.
For under $2.50 it is fast, with a response of 13.2 MHz.