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Laser harp - how to do an open design?

Maciejas

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Hi all :)

I'm planning to build a laser harp - a small one, for indoors use. And I have two questions:

1. https://dx.com/p/30mw-532nm-green-laser-module-3v-11-9mm-26888 - does anybody have any experience with those modules? Do they give off a visible beam in a darkened room? Are they up to spec?

2. I'm planning for my harp to have an "open" design - i.e. the beams are supposed not to stop at the upper frame. How is it usually done? From what I've found, it seems like I'd need a piece of a translucent material which will brighten up while a beam is passing through it, and a photoresistor next to it. If it's so, are there any certain properties that I should be looking at, or will any piece of glass do?
 

camvo

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Like this?





I think that 30mW has way too low power.

I used a 532nm, 375mW laser in this video.
 

Maciejas

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Like this?





I think that 30mW has way too low power.

I used a 532nm, 375mW laser in this video.
Great sound :) For a minute there I've expected Third Rendez-Vous to play :)

I understand that it's a single 375mW laser which is then scanned to form nine beams? Doesn't this mean that a single beam would be equivalent to about 40mW then (since they're only on for 1/9 of a time)? If we used separate 30 or 50mW modules for each beam, wouldn't it even out?

If not, could you point me to a reasonably priced laser of that power?

Also, in your design, how far off from the photocells can you be on the beam to still detect the reflection? Do you need to break the beam at the very specific height? I'm asking, since we're also planning to attach some rangefinders to it to widen the range, and possibly modulate the sound Theremin-style.
 

Things

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Unfortunately being able to move your hand up and down in the beams to modulate pitch is extremely difficult to do (at least, on more than 1 beam at once), not possible with just photosensors. I'm working on something at the moment that will allow it, but it's not really DIY-able at this stage.

The open harp design relies on the fact that there is only ever 1 beam at any point in time, and the microcontroller knows which one it's currently "drawing". So basically you have the photosensor pick up the reflected light, then the microcontroller checks which beam was active at the time it picked up the light. I've just used 3 bare photosensors in my design, which seems to work pretty well at a couple metres from the actual harp. Basically you just have to shield the sensors from reflections on the roof and you're fine.
 
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camvo

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Thanks Maciejas!

I could not agree more with Things.

The distance from the photo sensor depends also on the amount of ambient light.
But as Things said, with good shielding, you can improve the harp's sensitivity a lot.
 
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Maciejas

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Unfortunately being able to move your hand up and down in the beams to modulate pitch is extremely difficult to do (at least, on more than 1 beam at once), not possible with just photosensors. I'm working on something at the moment that will allow it, but it's not really DIY-able at this stage.

The open harp design relies on the fact that there is only ever 1 beam at any point in time, and the microcontroller knows which one it's currently "drawing". So basically you have the photosensor pick up the reflected light, then the microcontroller checks which beam was active at the time it picked up the light. I've just used 3 bare photosensors in my design, which seems to work pretty well at a couple metres from the actual harp. Basically you just have to shield the sensors from reflections on the roof and you're fine.
Won't a simple HC-SR04 module (about $3 on DX) pointed at each beam (actual or scanned) cut it? Guess it'll be a little harder to position with non-parallel beams, but still...

I understand the idea, it's really nice, but requires a single powerful laser. I'm way more comfortable with separate modules of less power - I'm a laser rookie, and I don't want to poke my or my lab partner's eye ;) Besides, I can't find a powerful enough module - those little ones only go up to 200mW.

What servo was used in this project to allow fast enough movement? Also, how long is the laser kept in place in proportion to movement time?

I've been thinking more along the lines of this: Screenshot by Lightshot - so it's technically a framed design, but still allowing the beam to pass through to the ceiling (or space :) ). The problem is, I don't know what kind of glass to use so that the non-reflected beam will be powerful enough, but still allow some to reflect to the photocell. Or whether it's even possible for that matter ;)
 




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