Laser Harp Page

Dec 2022 thru June 2024

While visiting Meow Wolf in Santa Fe I saw my first laser musical instrument and had some fun playing around with it. It only had 3 lasers but it was still fun. When I returned from my vacation, the idea of building a more full on laser harp planted itself in my brain. I decided to build one with 12 laser strings and which would have no physical controls but be controlled entirely by a web application. It turns out I built the harp twice which will be described below.

As usual the plan for my laser harp kept expanding and I wanted all of the following features/functions:

Feature	Function
MIDI Operation	I wanted the harp to be a MIDI instrument that could be plugged into any MIDI supporting equipment. The original harp didn't produce any sound on its own, it just generated MIDI codes which cause MIDI sound sources to which it would be connected to produce sound. The new harp does generate sounds.
Controlled via Web Page	I didn't want to build physical controls into the harp. I wanted the harp to be completely controlled via a web app.
Key Change	The ability of the harp to play in any and every key.
Octave Shift	The ability of the harp to play over a wide octave range
Scale	The ability of the harp to play in many different scales from the basic chromatic scale to more obscure east Asian scales.
Program/Voice	The ability of the harp to use the full 128 programs/voices supported by General MIDI
MIDI Channel	The ability of the harp to send MIDI messages over any MIDI channel (1 .. 16). Note, if you select channel 10 the harp plays drums.
String Order	The ability of the harp to reorder the strings so that the lowest note is the left most string or the right most string. In other words, the string order should be able to be set Left to Right or Right to Left.
Volume	The ability of the harp to control its volume.
Modulation	The ability of the harp to control modulation level.
Reverb	The ability of the harp to control reverb level.
Chorus	The ability of the harp to control chorus level.
Portamento Time	The ability of the harp to control glide time between sequential notes (portamento time).
Onboard MIDI synthesizer	The original triangular harp could only output to an external synthesizer via MIDI to create sounds. The new harp, however, has a built in synthesizer in the form of an M5 Synthesizer Unit based on the SAM 2695 chip. Details are here. The new harp can still output external MIDI like the original. Additionally, there is a toggle switch now built in with can turn off the internal synth for when external MIDI is being used.

This project is made up of three parts: First, the structure that houses the lasers and the embedded electronics which I made out of laser cut Baltic Birch. Second, the electronics that runs the harp software. I wanted the electronics to be as simple as possible and I think I succeeded in that it consists of only a WEMOS D1 Mini ESP32 module, 12 lasers, 12 light dependent resistors (LDR) and miscellaneous resistors. Finally, the software that runs the harp and powers the web app.

Here are some pictures taken during the laser harp's construction.

Picture	Description
	The electronics that drives the laser harp. On the left is the external power jack. The ESP32 module is in the middle. The MIDI DIN connector is on the right. This module will be mounted inside the laser harp. The harp will be externally powered via a 5 volt wall wort.
	Rear view of the electronics. There is a 1.8K resistor for each of the 12 strings and 2 resistors for the MIDI output. Each string will also have a Light Dependent Resistor (LDR) for sensing laser light.
	Point to point wiring was used as can be seen here.
	In preparation for wiring up the harp I attached all of the required wires to the breadboard because once it is installed in the harp I wouldn't be able to solder them easily.
	Initially I wanted to build the laser harp into a triangle structure so I built what you see here. Unfortunately, in this configuration it was impossible to accurately aim the lasers onto the light dependent resistors housed in the horizontal part of the triangle. So I had to give up this idea and opt instead for a more rectangular arrangement.
	I decided to use my newly purchased laser cutter to make a top box which would contain the lasers and a bottom box that would form and base and house the electronics. I decided to engrave the top box as well.
	I created a plate as shown that the lasers in their machined housing would be mounted to via zip ties. I machined the laser housings out of an aluminum rod I had.
	Here you see the laser plate mated to the top, laser cut box.
	This is a bottom view of the connected top and bottom boxes and the effort I had to go through to make the LDR's position adjustable in two dimensions to allow for variations in the lasers themselves. The LDRs are mounted on a laser cut gear like piece of Baltic Birch which is in turn glued to a very strong magnetic ring. The black rectangular items you see are stainless steel so that the LDR modules attract and stay in position. Turns out this didn't even work so I ended up having to hot glue the LDR modules in place.
	Here is the operational laser harp in the new, rectangular configuration. As you can see the top and bottom laser cut boxes are held together with two PVC pipe sections with a threaded rod through each and nuts to tighten things down. There is a toggle switch and an M5 Synth module mounted in the bottom box. The switch turns the synth module off and on. So the harp will produce sounds when the module is on and will be silent when the harp is connected to an external synthesizer.
	Here is a schematic of the laser harp. As you can see there isn't much to it. 12 lasers, 12 light dependent resistors (LDRs), miscellaneous resistors, an M5 Synth module, a MIDI DIN connector and an ESP32 module. The laser harp magic is in the software which is available here.
	Here is the web app web page used to control the laser harp. The harp provides its own WiFi access point called "Laser Harp". To control the laser harp, connect your device to the access point and point your browser to: 192.168.4.1 and you should see this UI. Any change made via this UI is reflected immediately in the laser harp's operation. I know this is a crude UI but I am not a web developer as you can tell from this website. The important thing is that it works.

The laser harp now works but my only regret is that I didn't come up with a better method for aiming the lasers at the LDRs. Perhaps if I had placed the stainless steel plates on the top of the bottom box (instead of underneath) the magnets might have had enough strength to keep the LDRs in place and still allow them to be re-aimed easily. Gluing the LDRs in place will only work until the lasers are disturbed again and then they will have to be removed and re-aimed.

Questions and comments to me Craig at: calhjh@gmail.com