This is a project the two of us had always wanted to build and tried several several simpler designs that just never looked good enough. So we embarked on this project to create something that worked well and looked decent. Here is how we did it.
Using an arduino microprocessor to drive the harp, we wrote the software to control the electronics that makes the harp function. Each beam in the harp is a separate note and when broken with output a MIDI note to a synthesizer so you can hear it. Although it appears like there are 7 beams, there is in fact only one single laser. It is being rapidly turned on, off re-positioned, and turned on and off again. This is repeated multiple times per second and because of the human eye's persistence of vision, it looks like 7 separate beams.
By having the code manage the on/off and position of the beam, we can tell which beam is being broken at any point in time. Light sensors on the harp pick up the reflected light from a broken beam and signal a not to be played. A separate sensor monitors ambient light and the difference between the two allow the harp to be operated under different light conditions.
So how does it work? The laser beam from a 200mW green laser is directed onto a face silvered mirror attached to a galvonometer. The galvo can oscillate at 20,000 steps per secs. So we can move the position of the mirror extremely quickly, all through the software. At position 1, the laser is turned on for a a fraction of a second and then off again. While off, the mirror moves to position 2 and the laser turned on again. This is repeated 7 times, and then stepped back down again and so forth. Each cycle is completed in about half a second.
If the laser is not turned off between mirror adjustments, the harp will look like a solid green fan, and not individual beams. There are of course all sorts of electronic controls to drive the laser and the galvo, and conversions between and analog and digital signals, that is too technical to cover here. The result is a cool musical instrument that we have no idea how to play. It is a strange sensation playing a musical instrument with no tactile interaction.
