Musical Instruments

Laser Harp

Makej Projects

Laser Harp

Written By: Stephen Hobley

TOOLS:

PARTS:

Alligator test leads (1)

Arduino microcontroller (1)

Computer (1)

Laser pointer (6)

Drill (1)

DC power source, switched, 8V - 12V, 2

Hack saw M)

- 3 amps (1)

Insulated wire (1)

Adjustable voltaae requlator (1)

MIDI utility software (1)

7805 Voltaae Reaulator (1)

Multimeter (1)

LM324 quad op-amp chips (2)

Software synthesizer (1)

LED. Red (6)

Solderinq Iron and rosin core solder. (1)

Resistors. 1/4-watt: 220Q. 1.5kQ. 3.9kQ.

• USB-MIDI interfaced)
Vise and clamps (1)
Wire cutters and strippers M)

68kQ. 1MQ(6)
Capacitors: 0.1 F (3)
Capacitors: 300 F tantalum (6)
Photocell. 100mW(6)

• SharpGP2D12orGP2D120IRranqe

sensors (6)

* Tumbled rocks, translucent (6)

Potentiometer. 100kQ (1)

5-pin DIN (MIDI) connector (1)

Blank circuit boards, dual mini and

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Laser Harp

medium (1)

24-gauge hookup wire various colors (1)

Heat-shrink tubing (1)

8-pin headers (optional) (5)

Aluminum tubes. 1/2" x 36" (2)

Wood and screws 1/2" fiberboard (1)

Black paint (1)

SUMMARY

One of my most vivid concert memories is seeing Jean Michel Jarre perform in 1986 at the
city of Houston's 150th birthday celebration. He played music by breaking laser beams with
his hands. The beams came out of the stage and went off into space, and for a long time I
thought it was a fake — I couldn't understand how this instrument could work without any
sensors above. That started me researching and tinkering, and 22 years later, I figured it all
out and built my own.

Now I have several versions of the laser harp. The one I perform with uses a powerful laser
and a scanning mirror system, designed for professional lighting effects, that splits one
beam into multiple beams that can fan out and move dramatically. This article describes a
simpler harp I designed more recently, which uses inexpensive laser pointers and doesn't
need the scanner.

The harp works as a MIDI controller, so it doesn't make sound itself, but generates a stream
of MIDI data to drive an audio synthesizer. Each beam strikes a photocell, and when the
player's hand interrupts it, the sensor prompts an Arduino microcontroller to send a MIDI
"Note On" message. Additionally, a range sensor reads the position of the hand, which
spawns MIDI controller messages that change the sound's qualities.

First I'll show how to make a single-beam laser theremin, which changes pitch with the
position of your hand. Then we'll replicate the circuit and reprogram the Arduino to produce a
multi-string harp, with each beam corresponding to a different note. The Arduino has 6
analog inputs, so this harp is limited to 6 beams, but at the end of the article I'll suggest
ways to expand it.

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Laser Harp

Step 1 — Build the Pieces

• We'll build the main electronic
components first and then put them
together. We'll start with the
common power supply, light
detector circuit board, and MIDI
output jack. Then we'll add
photocells, range sensors, and
lasers — 1 of each for the
theremin, and 6 of each for the
harp.

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Laser Harp

Step 2 — Power Supply

• I built the power supply on a mini
circuit board following the
schematic .

• A 7805 regulator steps down the
8.5V from my camcorder charger
to 5V for the range sensors. An
adjustable voltage regulator lets
you tune the power to the lasers to
just above the detection threshold.

• The Arduino gets 8.5V directly,
since it has its own onboard
voltage regulator. To suck up any
power spikes, I added a 0.1 F
"bypass cap" across each of the 3
output voltages: 5V, variable, and
8.5V. These capacitors are
optional.

• Finally, to neaten the connections
out, I used two 8-pin headers for
the outputs to the lasers and range
sensors.

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Laser Harp

Step 3 — Detector Circuits

• The photocells (aka light-dependent
resistors or LDRs) are on their own
little boards, across from the
lasers. But I assembled the rest of
the detector circuitry onto the
larger circuit board.

• The board needs to have 1 circuit
for the theremin, or 6 circuits in
parallel for the harp. Each of the
two LM324 op-amp chips supports
4 detector circuits, and I went
ahead and created 8 circuits, even
though the harp only needs 6. The
68K and 100K resistors create a
shared reference voltage, so we
only need 1 of each. See the
schematic online.

Step 4— MIDI Jack

• Wire the MIDI output jack by
connecting pin 5 to the TX pin on
the Arduino, pin 2 to circuit ground,
and pin 4 through a 220Q resistor
to +5V. (MIDI jack pins are
numbered 3, 5, 2, 4, 1 , from left to
right, facing the pins.)

• The outermost pins, 1 and 3, are
not used for MIDI.

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Laser Harp

Step 5 — Lasers and Photocells

• The lasers connect in parallel to
the variable voltage on one side
and to ground on the other. I
soldered the photocells to small
pieces of pert board for easier
mounting.

• They connect in parallel to +5V on
one side and to the + input pins of
the op-amps on the other.

Step 6 — Range Sensors

• Anyone who has played with a touchless D-Beam control on a Roland synthesizer will
recognize these sensors immediately. The GP2D12/GP2D120 range sensors fire a pulse
of IR light and measure distance by triangulating on the reflection.

• For musical applications, I've found that the output from these sensors can be noisy, due
to the constantly flashing IR drawing a lot of current every 40ms. You can smooth the
output by connecting a capacitor between voltage (pin 3) and ground (pin 2); I used some
300 F tantalum caps.

• You can also filter the signal with a dedicated filter circuit (see schematic online), or in the
software, by averaging consecutive readings and using the average value.

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Laser Harp

Step 7

• Before connecting the range sensors, I removed them from their plastic housings. They
connect in parallel to +5V power, ground, and the Arduino's analog input pins 0-5.

• The output from the range sensors is nonlinear, so the software converts output voltage
into centimeters of distance using a simple equation, courtesy of Acroname Robotics .

• For the GP2D12 sensor: Range [cm] = (6787 / (Voltage - 3)) - 4

• And for the GP2D120: R = (2914 / (V + 5)) - 1

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Laser Harp

Step 8 — Laser Theremin

• Here's an optical version of a
theremin, with 1 laser beam
controlling both Note On/Off and
pitch.

• Download the Arduino programming
software from

http://www.arduino.ee . Upload the
program "MAKE_MIDI_TEST.pde"
to your Arduino. (Included in this
zip file .) This program lets the
Arduino generate test MIDI
messages. Set the baud rate of the
Arduino to 31250.

• Connect the MIDI jack to your
computer with a USB-MIDI
interface. Launch MIDI-OX (or
equivalent software) and open that
port. You should see Note On and
Note Off messages in the MIDI
inspector. If not, then test the +5V,
ground, and TX pins for
connectivity.

• If the MIDI test is OK, upload the
laser theremin program
MAKE_THEREMIN.pde to the
Arduino (also included in the zip file
above).

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• Take one of your laser pointers
apart and measure the battery
voltage. Adjust the variable
regulator on your power supply
board until its output matches this
voltage. This lets you run the laser
from your power supply board. One
neat way to connect it is with
alligator clips.

• Now make the physical frame. I cut
a long piece of scrap wood into 3
pieces: to make a base, a laser
holder, and a detector holder. Drill
the laser holder piece for the laser
to fit through horizontally, and drill
a smaller perpendicular hole for a
screw to hold down its power
button.

• Drill a hole through the detector
holder where the laser will shine,
tape the photosensor board to the
outside with the sensor facing in,
and glue a translucent tumbled
rock over the hole in front. The
rock diffuses the light, which helps
the sensor see it.

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Laser Harp

Step 10

"s^X * A. 1 1

• Next, attach the range sensor to the laser. I found that it worked better mounted vertically
than when it was horizontal. I think the IR beam was reflecting off the laser pointer's barrel
and causing misreadings. Having a rangefinder too close to a wall can also diminish
accuracy.

• Connect the photosensor circuit's output from the op-amp to pin 2 on the Arduino and
connect the rangefinder's output to Analog In 0. Connect your computer back to the MIDI
out and run MIDI-OX. Switch everything on.

• Adjust the pot on the detector board so that the LED just comes on. At this point, breaking
the laser beam with your hand should switch it off, and MIDI-OX should show you Note
On, Note Off, and Pitch Bend messages as you move your hand in the beam.

• That's it. Swap MIDI-OX for a soft-synth, or plug the MIDI jack into a hardware
synthesizer, and you're playing!

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Laser Harp

Step 11 — Laser Harp

• Now we'll expand on the theremin
idea and create a 6-beam laser
harp.

• Build the frame. I made a wooden
base to hold the lasers,
rangefinders, and circuitry. Two
metal tubes at either end support a
top tube, which has 6 holes drilled
through its underside to expose the
photosensors. Space the lasers at
least 4" apart, or else cross-talk
between the range sensors can
throw off their readings.

• Wire the other lasers and
photosensor/detector/ rangef i nder
loops in parallel with the first ones;
see the schematic online. For
neatness on the detector board, I
used 8-pin headers for the
photosensor and Arduino
connections. Also check that the
variable regulator can handle the
current drawn by the lasers:
multiply the lasers' amperage by 6,
and confirm that it's below the
voltage regulator's rated max
current. To make sure, you can
also measure the current that
comes into the regulator.

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Laser Harp
Step 12

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Laser Harp

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• Instead of messing with alligator
clips, I made connector plugs for
the laser pointers. I cut a slot in the
back of each with a Dremel tool,
made a wood plug to fit into the
barrel, and thumbtacked a wire to
each end. Insert the plug, pass the
wires through the slot, and screw
on the back. The case contact in
back will be ground. To keep the
lasers switched on if the screws
slip, wrap the barrels with electrical
tape.

• Connect the detector outputs from
the op-amps to Arduino pins 2-7
and connect the range sensor
outputs to the Arduino's analog
input pins 0-5. Adjust the
potentiometer until all 6 LEDs come
on. You should now be able to turn
them off individually by breaking
the 6 beams. If ambient light
becomes a problem, cut rings of
narrow PVC pipe, paint them black,
and attach one around each
detector. If the lasers just miss the
photosensor holes, glue on tumbled
rocks as diff users.

• Upload the program
MAKE_HARP1_CTRL.PDE to the
Arduino, and start playing (program
available in zip file download in
Step 8). The software assigns the
MIDI note numbers 60, 62, 64, 65,
67, and 69 to the beams, but you
can change this by editing the

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Laser Harp

notearray[ ] structure. The
controller messages from the range
sensors are sent as note 74.

• With my synthesizer, this changes
the filter sweep and creates a
funky, retro synth sound.

• You can also try
MAKE_HARP1_VELPDE, a
modified version of the code that
maps your hand position to MIDI
velocity, to mimic how hard you
would strike a key on a keyboard
(program available in zip file
download in Step 8).

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Laser Harp

Step 13

• You're not limited to just playing notes. Ableton Live software allows you to MIDI-trigger
drum loops, sequences, and other musical events. It's not free, but you can download a
demo version at http://www.ableton.com .

• If you want to go crazy and add more beams, you'll need to expand the digital and analog
inputs of the Arduino using a multiplexer. There are a couple of neat, off-the-shelf ways to
do this. I'll mention two here, and you can find more at the Arduino Playground .

• One approach is to use an analog multiplexer like the R4 AIN MIDIbox module, which is
based on the 4051 chip (kit available from AVI Showtech, avishowtech.com). This will
support 32 inputs, for 32 harp strings.

• With some clever programming, you should be able to bypass the detector board and read
the laser harp through the analog multiplexer. To do this, connect the photocell array's
outputs to the multiplexer's inputs, feed the multiplexed output to the Arduino, and detect
which beams are broken in your software.

• You can also use a digital multiplexer like the R5 DIN Module, another MIDIbox kit from
AVI Show- tech, which is based on the 74HC165 chip. With these, you can chain modules
together to support an unlimited number of inputs.

• If you want to tackle a fullsized scanning laser harp, visit my website,
http://www.stephenhobley.com .

This project first appeared in MAKE Volume 15 . page 64.

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