H
J
I
Fig. H: Measuring total current drawn by the harp lasers.
Fig. I: Wooden plug power connection to the laser pointer.
Fig. J: Author Stephen Hobley plays his full-sized
laser harp, which was a huge hit at the Bay Area
Maker Faire in May (see page 86).
2. 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.
5. 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 (Figure D,
previous page). The rock diffuses the light, which
helps the sensor see it.
6. Next, attach the range sensor to the laser. I found
that it worked better mounted vertically (Figure E) —
3. Take one of your laser pointers apart and measure when it was horizontal, I think the IR beam was
the battery voltage. Adjust the variable regulator on reflecting off the laser pointer’s barrel and causing
your power supply board until its output matches misreadings. Having a rangefinder too close to
this voltage. This lets you run the laser from your a wall can also diminish accuracy.
power supply board. One neat way to connect it is
with alligator clips.
4. 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.
7. 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 (Figure G). 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
68 Make: Volume 15
