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Build A Laser
Light Show
How would you like a laser light show for
your next party? You can build your own and
it doesn’t need to be a large complex beast
with a big laser and so on. This laser display
is quite tiny yet it can project a very satisfying
display onto the walls of your lounge room.
By LEO SIMPSON
W
E DESCRIBED a motorised
laser light show in the May
1996 issue but this was a big
and bulky beast with a 100mW blue
Argon or a 10mW Helium-Neon gas
laser, a big power supply and special
circuitry for the motor-driven deflection mirrors. Both were very effective
and suitable for large venues but they
were probably overkill for anyone
who just wants a small laser display
for parties in their home.
By contrast, this laser light show
is very compact and fits into a small
instrument case on a swivel stand.
Inside, it has a tiny semiconductor
82 Silicon Chip
laser and its beam is deflected by
two mirrors driven by equally tiny
motors. Have a look at the photos
and you will see that there is almost
nothing to it.
This display system employs two
DC motors with mirrors on their shafts
to deflect the laser beam. The motor
shafts do not rotate but simply vibrate
around a central tethered position. The
level of vibration depends on the level
of audio signal applied to the input.
Two motors are provided to deflect the laser beam in the horizontal
and vertical directions. And since
a typical audio signal is more or
less random, the resulting display
is endlessly variable, with the beam
deflection being proportional to the
signal amplitude.
The motors and their mirrors are
angled in such a way as to provide
optimum deflection of the laser beam.
Due to the mass of the motor armatures and the mirrors attached to the
shafts, these small DC motors only
produce a useful response to signals
of no more than a few hundred Hertz.
Bass frequencies are quite effective but
midrange and high audio frequencies
do not produce any useful beam deflection. But the available response still
produces a very useful and interesting
range of laser patterns.
While the range of mirror deflection
is set by the ampli
tude of the low
frequency audio signals, the central
position of each mirror is fixed by a
small strip of polycarbonate film between the motor shaft and body.
Circuit description
Two audio signals are needed for
this laser drive circuit but since we
can only use bass to lower midrange
Fig.1: the circuit has an electret microphone to pick up music signals and these are used to drive two small DC motors.
frequencies there is really only one
signal present in typical program mate
rial, whether it is stereo or mono. As
an aside, most stereo tapes and CDs
have very little separation between
the left and right audio signals in the
bass region, hence there is really only
one bass signal.
This circuit gets around that problem by feeding one of the motors with
straight bass while the second motor is
fed with a signal derived from the mid
range to treble part of the spectrum.
This signal is rectified and filtered. In
effect, the derived signal is the rate of
change (or envelope) of the midrange
to treble signal.
The resultant pattern produced
by the laser simply depends on the
sound picked up by the an electret
microphone. Different types of music
and sounds tend to generate their own
unique patterns and you may find
yourself playing music chosen more
with an eye to the laser pattern rather
than how it sounds.
Looking at the lefthand side of the
circuit (Fig.1), resistors R1 & R2 and capacitor C2 provide a decoupled supply
voltage to the electret microphone. The
output from the electret microphone
is coupled via a 10µF capacitor to the
first amplifier stage involving op amp
IC1a. This is configured as an inverting
Liven up your next party with this compact laser light show. Use it to produce
endless patterns on your living room walls.
stage with a gain depending on the
setting of VR1. This can range from
unity to about 100.
Following IC1a, the amplified elec
tret signal is fed via two paths. Path
number one is via a low pass filter
consisting of resistors R6 & R7, togeth
er with capacitors C5 & C6. This filter
effectively blocks frequencies above
about 350Hz before they are fed to op
amp IC2b which has a fixed gain of 10.
The output from this stage is applied
via VR3 to IC4, an LM380 power amplifier, and this is used to drive one of
the deflection motors.
Path number two from IC1a is via a
April 1998 83
Fig.2: component overlay diagram for the PC board. Take care to
ensure that all polarised parts are correctly oriented.
high pass filter consisting of capacitors
C3 & C4 and resistors R5 & R8 and this
effectively blocks frequencies below
about 350Hz. This is the other half of
the audio spectrum from IC1a and this
is applied to op amp IC2a which also
has a fixed gain of 10.
IC2a’s output is fed to a “diode
pump” rectifier consisting of diodes
D1 & D2 and capacitors C7 & C9. The
rectifier output represents the “rate of
change of the midrange signal” and
this signal is applied via potentiometer VR2 to IC3, another LM380 power
amplifier, and this drives the second
deflection motor.
Power supply
Power for the circuit is provided by
a 13.8V DC plugpack with a capacity
of 300mA or more. Op amp IC1b,
zener diode ZD1 and their associated
components are used to derive a 12V
regulated supply, which is used as a
bias voltage for op amp stages IC1a,
IC2a & IC2b. A 7805 3-terminal regulator provides a fixed +5V rail for the
solid state laser module.
Also shown on the circuit is a DPST
switch (S2) which makes provision
to drive the motor deflection circuits
from a stereo amplifier (ext).
Construction
Fig.3: use this diagram when wiring up your laser display. Power comes from a
13.8V DC plugpack supply.
84 Silicon Chip
All the circuitry, apart from the solid
state laser module and the 3-terminal
regulator, is mounted on a PC board
measuring 96 x 47mm. This board is
divided into two sections, one involving IC1 & IC2 while the other accommodates the two power amplifiers,
IC3 & IC4. Two links between the two
sections allow you to add the DPST
switch S2. Our prototype does not
include this and provided the electret
microphone picks up adequate audio
signal, it is more convenient without
any need for audio signal cables.
The first task in assembling this
project is to assemble the PC board
and this is quite straightforward since
it comes with the component overlay
screen-printed on top – see Fig.2. Insert all the smaller components first,
followed by the trimpots, electrolytic
capacitors and lastly, the ICs. IC sockets can be regarded as optional.
When the board assembly is complete, connect up the 13.8V DC power
supply and the two motors. With no
signal, nothing much happens. However, when you speak or blow into the
electret microphone, the motor shafts
Parts List
1 plastic case, 154 x 65 x
158mm
1 swivel stand to suit
2 miniature DC motors
2 small aluminised glass mirrors
1 solid state laser module
1 finned heatsink to suit 3terminal regulator
1 PC board, 96 x 47mm
1 13.8V DC plugpack with
2.1mm DC plug
1 DC socket to suit
1 electret microphone insert
1 SPST miniature toggle switch
(S1)
1 1MΩ horizontal trimpot (VR1)
2 100kΩ horizontal trimpots (VR2)
The laser beam is deflected by the two motor-driven mirrors (top) in response to
audio signals from your music system.
The electret microphone picks up the
audio signal and feeds it to the circuit
via a shielded cable.
should vibrate rapidly back and forth.
Case work
The next step is to wire up the
3-terminal regulator. This is mounted
on a small finned heatsink and the
external capacitors are soldered between its three legs. The two power
diodes are wired in series with the
output terminal and all connections
are then secured with small-diameter
heatshrink tubing.
You will need to drill a large hole
of between 20mm and 30mm in the
front panel for the laser to be aimed
through. Ideally you should use a hole
punch for this job but if you don’t have
one, you can drill a smaller diameter
hole and then ream or neatly file it
out to size.
On the rear panel, you will need
holes to mount the 3-terminal regulator, DC socket and power switch and a
small hole for the electret microphone
cable.
On the base of the case, you will
need to drill holes to mount the PC
board, the two motors, the solid state
laser module and the swivel stand.
Fig.3 shows how all the wiring should
be run to the motors, PC board and so
on and the photographs give a further
guide to the orientation of the motors
and laser module.
Motor mounting
Earlier on, we implied that one
motor is used for vertical deflection
and the other is used for horizontal
deflection of the laser beam. They
could be arranged to do this but it is
Semiconductors
2 TL072 dual Fet-input op amps
(IC1, IC2)
2 LM380N power amplifiers (IC3,
IC4)
1 7805 5V 3-terminal regulator
(REG1)
1 12V 400mW zener diode
(ZD1)
2 1N60 small signal diodes
(D1,D2)
2 G1G rectifier diodes (D3,D4)
Capacitors
4 100µF 16VW or 25VW PC
electrolytic
2 47µF 16VW PC electrolytic
5 10µF 16VW or 25VW PC
electrolytic
5 0.1µF monolithic or MKT
polyester
4 .0033µF monolithic or MKT
polyester
2 680pF ceramic
Resistors (0.25W, 1% or 5%)
2 1MΩ
1 5.6kΩ
8 100kΩ
1 4.7kΩ
3 10kΩ
2 2.7Ω
Miscellaneous
Motor brackets, laser module
bracket, polycarbonate strip, 5minute epoxy adhesive, shielded
cable, solder.
far more convenient to mount each
motor with its major axis at 45 degrees
to the horizontal and angled in such
a way that the laser bounces off one
mirror to the next and then shines out
through the front panel hole.
April 1998 85
This close-up view shows how the two mirrors are glued and tethered to the
motors. The tethers allow the mirrors to deflect the laser beam by about ±30°,
which is enough to produce an interesting pattern
The 3-terminal regulator and its associated parts (including the heatsink) are
mounted on the rear panel.
Where To Buy The Parts
All parts for this project are available from Oatley Electronics who own the design
copyright. Their address is PO Box 89, Oatley, NSW 2223. Phone (02) 9584 3563;
fax (02) 9584 3561. The prices are as follows:
PC board plus on-board parts, motors, mirrors, electret microphone....... $44.00
5mW 650nm laser module........................................................................ $25.00
13.8V 1A DC plugpack.............................................................................. $12.00
Complete kit, including all above parts, 3-terminal regulator & case........ $85.00
86 Silicon Chip
Before you can mount the motors,
you need to attach the mirrors to the
shafts and fit them with tethers. The
two small mirrors supplied have aluminium metallisation on one side
and this side must be used to reflect
the laser beam. If the glass side of the
mirror is used to deflect the beam, the
effect will be to defocus it.
You can glue the mirrors to the
mirror shafts using 5-minute epoxy
adhesive. Make sure you don’t get any
adhesive on the aluminium side of the
mirrors. Once the mirrors are glued in
place, you can attach the tethers between the mirrors and the motor cases.
The tethers are strips of polycarbonate
film and should be long enough to let
the mirrors be deflected by a maximum
of ±30°. This is more than enough to
give good deflection of the laser beam
and will not unduly load the motors.
Again, the polycarbonate tethers can
be glued in place with 5-minute epoxy
adhesive.
Our prototype had small metal
brackets soldered to the motors and
these were then screwed to the base
of the case. The laser module was
mounted by holding its lens assembly
with a circular clamp attached to a
vertical bracket.
In practice, you could mount the
laser as shown in the photos but with
the baseplate screw not tightened.
Then you could position and angle
the motors so that the laser can be
aimed and deflected as required. Once
you are satisfied with the laser beam
deflection, the motor positions can be
marked, holes drilled in the baseplate
and then the motors can be secured.
Do not mount the electret microphone inside the case. If this is done, it
will inevitably pick up the vibration of
the motors and the whole system will
then oscillate at a low frequency. This
is the reason for connecting the electret
microphone via a length of shielded
cable. That way, it can pick up sound
from your music system rather than
from the motors.
The electret microphone insert used
in our prototype has the shielded cable
attached directly to its rear lugs and
then it was neatly shrouded with heatshrink tubing to anchor and provide
stress relief for the cable.
Before you can put the Laser Light
Show to use, you will need to adjust
trimpot VR1 for adequate gain from the
electret and then set VR2 and VR3 for
SC
optimum mirror deflection.
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