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Author Topic: laser harp plans and code from Steve Hobley  (Read 5692 times)
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looking for somebody who has the plans to the laser harp that Steve Hobley made because he has sold the rights to it to some company and can no longer distribute them.

Anything would help

thanks
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The basics of it seem simple - a powerful laser or lasers, beams split in a fan pattern, with a web camera or wii camera positioned to see the dot of light on the performer's hands; the laser really is only powerful in order to show the fan for the audience, and to an extent the performer as well, so he knows where to "pluck". However, a basic model (with invisible beams, unfortunately) could be made with a simple red or green laser pointer; with the appropriate filter over the camera, even a <5mw dot will be visible on the camera. Later you can upgrade to a more powerful laser.

The difficult part is the software - you need software that can see the dot or dots, locate them in 3-space, and then use that information to play a midi note (which is then sent on to the rest of your midi stack); this requires a PC running some vision software; its like a Sega Activator on steroids. If I were building one of these, or experimenting with the idea, I would first look into doing a single beam, using a cheapo webcam-based laser rangefinder like this:

http://sites.google.com/site/todddanko/home/webcam_laser_ranger

That would be the starting point. Once you had that working, you would need to set it up so there was as wide a field-of-view (FOV) for the camera as possible, then generate multiple beams with a beam-splitter or some other means; you could potentially use a mechanical scanner to scan a beam, and pulse the laser on/off at various points, tracking the angle of the shaft of the scanner so you knew which "string" was being generated, then the dot on the hand, picked up by the camera, could be figured out (distance) using the info in the above link...

Alternatively, one could steady-state the beam fan, then use one or more cameras and some vision software (like OpenCV), plus a ton of custom coding to locate the dot in 3-space, etc.

I know the above isn't what you wanted, but maybe it will give you or someone else some ideas on how such a system can be implemented...

 smiley
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thanks for the info.  I don't want to have to re-engineer this and was hoping to build one for a Worship group before Oct this year.  I will be buying a 100mw green laser with blanking feature so to do this up right.  I need all his plans and code to save time essentially.  
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I will be buying a 100mw green laser with blanking feature so to do this up right.  I need all his plans and code to save time essentially.
I think his laser was more like >200mW as when you split it up the power decreases on each 'string'/line

The webcam could be connected to processing to analyse the light position and then output a MIDI value to some other software. Should be simple.

Mowcius
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[OT]Couldn't you aim the laser beam at LDRs or LEDs and use them as light sensors for detecting if the laser beam is interrupted or not? I guess that would be much more simple than image recognition with camera. Or does the harp need to detect at which point the beam is interrupted?[/OT]
« Last Edit: July 21, 2010, 06:35:42 am by T.J.L. » Logged

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Or does the harp need to detect at which point the beam is interrupted?
Any stringed instrument makes different sounds depending on where the string is plucked. Plucking near the end causes less vibration (i.e. sound) than plucking near the middle.

Since the idea is to replicate the strings with laser beams and the vibrations with computer-generated sound data, it is necessary to know where along the string the "string" was "plucked".
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Steve Hobley published a circuit in Make Magazine Vol 15 page 64. However, this was a different circuit than his original.
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You will need a 300mw laser at least - anything less than that and you are fighting with the noise floor.

There are two functions you need to create in the Arduino for a frameless harp - control of the galvanometer mirror to "split" the beams, and reading the returned pulses.

I used a hardware interrupt line (2) to read the pulses, and connected some light sensors to differential amplifiers to get a "clean" return signal. This is very much like the signal used in TV remote controls.

In fact the Jameco newsletter this month has an article by Forrest Mims on converting the signal from a TV remote to an audio signal - this circuit is very much like the one I use.

Sorry I can't give out the info any more, but the terms of the deal preclude me from doing that.

Steve
www.stephenhobley.com
www.twitter.com/shobley
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thanks for the info Steve, I knew the laser had to be stronger then what most consumers work with I just wasn't sure how strong.  300 mw is a bit more expensive and more dangerous for the eyes.  I use to work on 25-45 watt yag lasers. I will look at that article in the Jameco newsletter.  The hardest part for me will be  the programming to control the gavanometer mirror and blanking of the the laser.  

Thank you again.
« Last Edit: July 21, 2010, 09:46:01 am by jaaronson » Logged

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You will need a 300mw laser at least - anything less than that and you are fighting with the noise floor.

This sounds like it would be true only if you are measuring the brightness of the reflected dot in order to calculate distance. From the link I posted above, using a cheap webcamera and a consumer grade laser, calculating distance using angular measurement requires much less in terms of laser power.

This homebrew system uses a scanning mirror and photodetector for angular measure, and again, a cheap laser pointer:

http://letsmakerobots.com/node/2651

So - in theory - you could scan a laser (or better, project a laser "line" with a diffraction grating or cylindrical lens), and use a web camera or other system to take distance measurements based on the displacement (and angular measure) of the dot or line; without the need for a larger laser.

I do understand the need of a larger laser for aesthetic and performance (by the artist/musician) reasons; it looks better, and it allows the artist a measure of feedback as to where their hands need to be positioned. A larger laser would probably even make the measurements more accurate (but not necessarily; I know that industrial laser distance rangefinder scanners use sub-5mw lasers, and they are highly accurate). However, for a prototype device, I don't think it is strictly necessary.

Then again, I've never built such a device; I am just basing my conjecture on what others have built and my knowledge of how such devices work.

 smiley
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You are correct, I am measuring the reflectance.

It is a frame-less laser harp.

Some have built reflective laser harps using a webcam. But there are problems with scanning frequency and alignment to overcome.

What I ended up doing was "liberating" a Pixart sensor from a Wiimote and using that to do the tracking. No PC needed - just a uController.
« Last Edit: July 22, 2010, 12:44:12 pm by shobley » Logged

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