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In the past there have been a number of designs
for battery operated and mains operated lasers
but this is the first design using a visible laser
diode. Using this device, a small PC board and
three AA-size cells, you can make a very
effective laser pointer.
By LEO SIMPSON
Every year, laser diodes are produced in quantities which run into
the millions. They are used in CD and
video disc players, CD ROM drives
and laser printers. The vast majority
of these laser diodes produce their
output in the infrared region, at around
780 nanometres. The laser diode used
in this project is normally intended
for use in bar code readers, as used in
supermarkets and libraries. It produces its output as red light, at around
670 nanometres.
The laser diode is, of course, a spe60
SILICON CHIP
cial type of light emitting diode. And
since light emitting diodes do not normally produce a very narrow beam,
the laser diode is fitted with a collimator lens assembly which also doubles as a heatsink.
The laser diode assembly is mounted on a small PC board together with
its drive circuitry which is a constant
current source. The PC board and AA
batteries can then be packaged in a
short length of electrical conduit to
make a handy laser pointer.
This can be used at lectures and
slide shows and has the advantage of
being a little more bulky than commercial units which are more expensive. Since when is bulk an advantage? It makes the unit easier to point
more precisely and the larger size
means that it is less likely to be misplaced.
Besides being used as a laser pointer, the unit could also be used for a
range of optical experiments, including data transmission. It could even
possibly be used as the basis for a surveyor's level, although higher power
gas lasers are usually used in this last
application.
The circuit
Now let's talk about the circuit
which is shown in Fig. l.
The laser diode is connected to the
battery supply via a constant current
source comprising transistors Q1 and
QZ and diode Dl. The battery supply
is shown as having of range of3.5-5V.
It could be supplied from three 1.5V
AA alkaline cells or four 1.2V nickel
S1
+3.5-5VC>------e
_ _ _ _...__....__ _ _____,....__ _
....__-o
22ll
~·
100
B2n
3.9pf
OLLIMATING LENS
LOOSEN
SET SCREW
ANO SLIDE
TO ADJUST
SET
60-100mA
01
2N2219A
E
GND<:r---il>----------------u
LASER POINTER
VIEWED FROM BELOW
cadmium rechargeable cells.
You might expect that a laser diode
circuit would be little more than a
battery and a resistor since it is essentially just a light emitting diode. However, the operating current is a good
deal higher than for typical LEDs.
Whereas LEDs typically operate with
a current of 10 milliamps (depending
on their application), a laser diode
usually requires somewhere between
60 and 100 milliamps. This means
that a current limiting resistor is impractical and a constant current source
is required. The circuit works as follows.
Q1 and D1 work together to provide a constant voltage of close to
1.2V between Q1 's collector and emitter. This voltage will remain constant
for quite large variations of the bat-
Fig.1 (left): the circuit uses Ql & Q2 as a constant current
source so that the laser diode is never overdriven. Fig.2 (above)
shows the laser diode assembly.
tery supply voltage. The voltage from
the collector of Q1 is applied directly
to the base of Q2 which functions as
an emitter follower. This means that
the voltage at the emitter of Q2 will be
constant at around 0.6 volts.
Because the voltage at the emitter
ofQ2 is constant and because its emitter load resistor is fixed, it follows
that the emitter and collector currents
of Q2 are constant. Hence, the laser
diode current, supplied from the collector ofQ2, is constant. Trimpot VR1
is used to set the current to about
80mA. We'll talk about this setting
later in the article.
Note that there are two diodes inside the laser diode 3-lead package.
The laser diode itself is between pins
1 & 2 while another diode, a photodetector diode, is connected between
The Laser Pointer is powered by three AA cells installed in a 4-cell holder (one
battery position is shorted out). Keep the parts leads as short as possible.
pins 2 & 3. This is shown with a lOOQ
resistor connected across it although
in this circuit it is superfluous. Its
real purpose is to monitor the light
output of the laser diode and to use
the voltage signal to control a feedback loop which ensures that the
laser light level is constant over a long
period of time.
This is important if the laser diode
is used with optical fibres or barcode
scanners, or in other data transmission applications.
Construction
The Laser Pointer circuit is assembled onto a small PC board measuring
85 x 26mm. The laser diode and the
collimating lens assembly are supplied separately and need to be assembled together. The first step is to
short all three leads of the laser diode
together to avoid the possibility of
damage to it via static discharges.
The assembly is as shown in Fig.2.
Remove the end plate by undoing the
two screws, taking care not to lose the
two spring washers (not shown in
Fig.2). This done, insert the laser diode and reassemble the unit. While it
is possible to do so, we do not recommend that you try to adjust the collimating lens - it is not an easy job and
the unit comes preset anyway.
Now that you have finished the
laser diode assembly, put it aside and
proceed to assemble the PC board. Do
not install the laser diode on the circuit board until it has been powered
up and the trim pot set for the correct
current. If you connect the laser diode
into circuit before you are sure it is
working properly, you could easily
DECEMBER 1991
61
Fig.3: install the parts on the PC board as shown here.
Note that pin 2 of the laser diode is connected to the metal
case & thus to the collimating lens assembly. The entire
assembly slides into a conduit housing as shown at right.
damage the dio de - a very costly exercise.
No special order needs to be followed in assembling the board. Just
install the com ponents as shown
while making sure that their values
are correct and that the transistors
and electrolytic capacitors are correctly oriented.
Check your work carefully When
you have finished assembling all the
components onto the board (with the
exception of the laser diode). You now
need a source of 4.5V DC or thereabouts. You can use a DC pow er supply or the battery pack. If the latter,
you will need to wire it up. If you
intend using alkaline cells (wh ich put
out a nominal 1.5V), you will need to
wire the 4-cell holder so that one cell
PARTS LIST
1 Toshiba ROLD9200 red laser
diode
1 collimating lens assembly to
suit laser
1 PC board, 85 x 25mm
2 2N2219A NPN transistors
(Q1,Q2)
1 1 N4148 signal diode (D1)
1 SPST pushbutton momentar·y
contact switch (S1)
1 4-cell AA battery holder and
snap connector
2 100µF 16VW electrolytic
capacitors
1 3.9pF ceramic disc capacitor
1 100 trimpot (VR1)
Resistors (0.25W, 5%)
1 1kQ
1 100Q
1 82Q
1 22Q
2 10Q
1 1Q
Miscellaneous
Electrical conduit for laser housing ,
foam rubber, insu lated hookup
wire, black spray paint , laser
warning label, solder.
62
SILICON CH IP
position is shorted out. Once this has
been done, insert the three cells and
measure the output voltage across the
snap connector. It should be about
4.5V.
Connect the supply to the board
and short the collector of Q 2 to the
+4.5V rail. Now connect a digital multimeter across the lQ resistor and adjust trim pot VRl for a reading of 85mV.
This corresponds to a constant current of 85 milliamps.
Now you are ready to connect the
laser into circuit. Mount the laser assembly onto the PC board and secure
it with the supplied screws and spring
washers. Solder short leads to pins 1
& 3 of the laser dio de and sleeve them
to avoid shorts to pin 2, then solder
the two leads to the PC board.
Now apply power and the laser diode should, but may not necessarily,
put out laser light. If the current
through the laser diode is not quite
high enough , it will emit red light in a
focused beam but it will not be laser
light because the energy input is insufficient. If this is the case , you will
have to change the setting of VRl to
increase the current through Q2.
To do this, reconnect your digital
multimeter across the l Q resistor and
rotate VRl so th e reading increases.
By the time the reading is 90m V, corresponding to a laser diode current of
90mA, the light should have that characteristic "speckle" appearance of
laser light.
Do not , under any circumstances,
increase the laser current beyond
lO0mA, otherwise there is a strong
chance that you will damage the laser diode.
Having confirmed that the laser diode is able to "lase", you can finish
the assembly. The laser housing is
made from a couple of lengths of
40mm OD electrical conduit joined
together with a short length of 40mm
ID conduit. One end of the resulting
tube assembly is blocked off with a
disc of wood or plastic while the other
end has a similar disc with a 6mm
hole drilled in it for the laser light
output. The photos show how the tube
assembly goes together.
The battery pack is mounted in one
half of the tube assembly, together
with a piece of foam rubber packed in
so that it does not slide about. Similarly, the laser PC board assembly goes
in the other half, again with a piece of
foam rubber so that it does not rattle
about.
Finally, a momentary contact pushbutton switch (Sl) is mounted in the
laser tube assembly to function as the
ON switch. It is simply wired in series
with the positive lead from the battery pack.
SC
Where to buy the kit
The complete kit for this project, including laser diode, collimating lens &
laser warning label , is available from Oatley Electronics for $179 plus $6 for
certified postage and packing.
Oatley Electronics can also supply infrared laser diodes and gas laser
tubes. Their address is PO Box 89, Oatley, 2223. Phone (02) 579 4985.
Note : copyright of the PC board associated with this project is retained by
Oatley Electronics.
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