Laser-synthitar From a Guitar-hero-like Toy Guitar

I was very inspired by all the youtube videos of laser harps but i found them all too big to bring along for a jam session or they needed a complicated setup and a pc etc. I thought of a guitar with lasers instead of strings.
Then I found a broken toy guitar similar to a guitar hero controller at a flea market. I'm already gathering too much electro-junk but i had to make something out of this. With some cheap laserpointers I was ready to make a

Laserguitar Synth
or a laser-triggered analog synthitar

No computer interfacing, no microcontroller, just 3 oscillators (its a first prototype, I wanted to keep it simple and started with 3) that are triggered by 3 photodiodes and 3 lasers. The buttons on the neck are used to change the pitch.
I'm a beginner in electronics but i had made some 555-Circuits so far, so i used 555s as the oscillators, the square waves sound similar to e-guitars. To mix the 3 oscillators i used an opamp as an audio mixer.

It looks messy, with exposed electronics and duct tape and hot glue and it sounds as ugly as expected but im glad that it actually works. Its a first prototype and proof of concept. Now that I know it works, I will probably make a next version.

This is my first instructable and english is not my native language, so plz be nice :)

Video below: too bad that i suck at playing it, but you can see it works.

Please vote for this instructable at the art of sound contest...

Step 1: Materials You Need

- a toy guitar or some other guitar-shaped casing
- ne555 ICs - as many as you want "strings" on your instrument
- an opamp
- an inverter chip for between the photodiodes and the 555s, i used a 74ac14, with that 6 "strings" would be possible
- 100K pots for tuning of the fret buttons, this guitar has 5 of them
- 3 x 10K pots
- 0.1 uF capacitor
- audio jack
- perfboard

for each oscillator or "string" you need
- cheap laserpointer
- IR photodiode
- 100K pot
- 500K pot
- 4,7K resistor
- 1M resistor
- 2 x 10K resistor
- 0.1uF capacitor
- 0.2 uF capacitor

- solder
- hot glue

Step 2: Schematics

The schematics should be correct but I made them after i built it and that was the first time i used eagle. Normally I put part of the circuit on a breadboard, test it and if it works i solder it on the perfboard. I recommend you to test it first too.
Its simpler than it looks. The 555 is wired as an astable multivibrator. The tone can be tuned with the 100K pot that are connected to the 555. The Wires from the fret buttons are going to the control voltage pin. The pin is pulled up to V+ by the 10K Resistor and the fret button pulls it towards GND, raising the pitch. The amount is controlled by the 100K pot connected to the fret button. So every fret can be tuned individually.

UPDATE
added schematics for parts of the circuit and simplified overview for better understanding

Step 3: Prepare the Toy Guitar

First i took out the original pcb and removed the buttons on the body. The fret buttons needed to be rewired. There were less wires than buttons coming out of the neck and some diodes were soldered in. I took these out and soldered one wire to each button and one to the common ground, sry no pics of this step but that's easy and may be different on another toy guitar.

Step 4: Build the Main Pcb

Now build the main circuit, i tested every oscillator with a small speaker before soldering the opamp part. I also used 3 buttons for testing before starting the laser part. I also put them on the outside of the casing for testing if it can be played. The audio signal will be too weak for headphones but it works well on my guitar amp.

Step 5: Build the Laser Part

Then solder the photodiode circuit on a small piece of perfboard, test if outputs are high without light and low when you shine a laser on it. You can adjust this with the pots.
Modify the 3 Laserpointers so they can be hooked on the 9V. I put 300 Ohm Resistors on them, they run on 2x 3V Button cells normally, now they are not quite as bright as before but enough to function. I had no other resistors around and was too lazy to calculate it. That will vary with the type of laserpointer you use.
Glue the pcb in place. The laserpointers must hit the photodiodes exactly at the right spot. I used hot glue for them and held it in place until it cooled down (with power on and always aiming at the photodiode). Maybe there is a better method that also looks better. I had to adjust it later by bending the leads of the photodiode which looks even worse.

Step 6: Put It Together and Play

Now you can hook the laser pcb to the main pcb. Drill a hole for the audio jack and put it in. Solder the wires from the fret buttons to the according pots if you havent done this before for testing. Make a hole in the back of the body into the place where the batteries used to be in the toy and put the battery clip through. I used the same velcro that was already in place to hold the 9V battery.

Now it should work. If it produces tones when you break the laser beams you can start to tune it.
I tuned the 3rd string one octave above the 1st and the 2nd in between, so its a chord when all strings are played at the same time, tune it the way it works best for you. It just has to be a chord because of the fret buttons that raise the pitch of all strings together which is a bit limiting.

Now try what you can play on it, simple basslines dont sound that bad and chords really sound quite metal-like, only the square waves dont seem to mix very well when playing lower pitched chords. Its a bit unusual that you dont feel the "strings" so you need some other position reference if you dont want to look down all the time.

As i mentioned before, its the very first version and can still be improved. A crazy effect on the tremolo bar could make up for the limited tone range. I will try a low-pass filter to simulate dampened strings to make it more metal-like. Maybe forget the fret buttons, make a new casing and make an optical solution for the frets... It would be much cooler if the lasers went all the length of the guitar anyway but they can only be interrupted once and i dont know how to use them as position sensors - maybe someone else knows? Let me know if you have done a better version.

The idea of a laserguitar is too fascinating so im sure to see more of that in the future. Lots of inventions have been made by several people in parallel at the same time.

After writing this i found it... it has been done before, also with buttons on the neck but with midi...
http://pages.cs.wisc.edu/~handiono/techpage/laserguitar/
And this one is on youtube:
http://www.youtube.com/watch?v=1sgdlaRv2hk