Framed Laser Harp: Need help picking diodes

[sharksystems]

Firstly, I know almost nothing about lasers, so I need a bit of help. I'm working on a laser harp project, using a framed assembly. This means I'm not using galvanometers, instead using a separate diode for each beam.

I'd like the beam to be 532nm. I don't know how powerful the diode needs to be. I've created a dedicated 3.3V supply for the diodes, so I'd like to stay within that voltage range.
What I don't know is:
How powerful do the diodes need to be? I've specced 100mw diodes, but I'm not sure if those are necessary. Would 100mw diodes be overkill? If so, what range would make an effective beam?
Thanks!
Ross

[White-Light]

532 aren't available as diodes, well they are just emerging but I'm not sure they're available to the hobbyist market to purchase so far as I'm aware.

[Carl B]

Hi Ross

Welcome to PL.
You will find a number of Laser harp fans here, my self included.
I have actually built my self a framed harp that you can see in action in the link below.
http://www.youtube.com/watch?v=mAX781r65QE
It's a bit late tonight to go in to details as to how it works but you may be able to tell that the harp is scanned and I think it would be your best way to go as well.

A few Questions if you don't mind.
1. How many beams are you planning to have?
2. Are you planning to use LDR,s with in the top of the frame to detect the beams?
3. How much can you spend on this project?

Anyway it's good to have another Laser Harp fan on the forum and i'm here if you need any pointers on building one.

Carl

[sharksystems]

6-8 beams, likely 8. Yes, LDR's for detection, with IR rangefinders sending CC messages to softsynths, i.e for filter cutoff. I don't want to spend a ton, but I don't want to create an unreliable product.

I've always wanted to try a scanner, but I fear it might be a lot more work than simply running a couple of modules off a 3.3v supply. I understand the concept of a galvanometer/scanner, but what I don't understand is the control aspect. Plus, the initial investment would obviously be higher. However, with discrete modules, I would in most cases have to deal with cheap fleabay modules.

If there's an easy way to do a scanner, count me in!

[Stoney3K]

If you use static beams, 100mW (1 diode for each beam) is way too much. The harp will be too bright and you may have some safety hazards when someone is operating the harp with a reflective surface (e.g. wearing a ring or a watch).

For a static projection, 5-10mW should be more than sufficient. These modules are cheap to come by and should also have no trouble when you use LDRs. 100mW may damage LDRs in the long run.

[Things]

I'd go with 8 beams, then you can fit a whole octave on your harp. I've built myself a frameless harp, however I'm also going to build a framed version soon.

What you would need are small green DPSS modules. However, 100mW would probably be a bit overkill, and could cause burns on your hand if you keep them in a beam too long. Even 5mW of green would be easily visible in a bit of fog, and the modules are much cheaper and safer than 100mW.

[dnar]

Forgive my ignorance (42^69) but doesn't Jarre wear safety goggles and asbestos gloves with his harp?

[Stoney3K]

Forgive my ignorance (42^69) but doesn't Jarre wear safety goggles and asbestos gloves with his harp?

He did when doing his massive outdoor concerts, and those were done with multi-watt Spectra-Physics ion tubes. He used a grating for The Concerts in China, and subsequently switched to a scanning version.

His newer systems are DPSS-based and are scanning fast enough that the instantaneous power hitting his hand is not enough to cause any damage. He still wears safety goggles (more to prevent any injury due to an inadvertent direct beam impact).

The white gloves are more to help a diffuse reflection from the beam appear more bright to the sensors underneath. For his indoors systems he uses just that -- plain white cotton gloves, which are easy to get. For a framed harp with detection on the stage top, you will not need them.

I understand the concept of a galvanometer/scanner, but what I don't understand is the control aspect. Plus, the initial investment would obviously be higher.

A scanning harp with galvanometers does give you more flexibility and the option to animate it. It can still be a framed harp, but you will also be able to select how many beams you want and which pattern they should display.

The control, simplified, boils down to this: You use a microprocessor to generate a stepped voltage waveform which is fed into the galvanometers. These, in turn, will stop at each point where the voltage stays constant for a certain amount of time, making the beam more visible in a 'still' position than when the mirror is moving.

At the same time, a light sensor at the bottom of the harp picks up the reflected light from the player's hand. The sensor will only trigger on certain points in time, which will be synchronized with the step waveform generated by the microprocesssor. The processor is therefore able to compare the timing from the light sensor with the timing from the signal generator, and determine which beam (or multiples) have been triggered.

If you use a framed harp exclusively, you can opt to use a half-silvered mirror or a plain diffuser at the top as a beam block. This provides more light for the sensor to detect, the only difference will be that the detector's signal will be inverted and it will trigger on the absence of light.

The most challenging parts of the build are building the analog electronics for the detectors (which should be extremely sensitive to your laser but completely oblivious to surrounding stage lighting) and gluing all of your control electronics together. Arduino is a very good starting point if you're new to programming.

[Carl B]

As mentioned in my post above my framed harp uses a scanner to create the beam's.
At the time of filming the video linked above, the harp had a 500mW laserwave head in it. At the moment I am using Pangolin Livequick to scan the 9 beams scanning 4 points per beam at about 10kpps. I intend to have a circuit built to take over from the software some time in the near future.

Starting with the laser box. This is basically an ILDA projector with only one axis scanner that sends the beam up and out of a slot aperture. A standard ILDA projector could also be used.



The sensor bar has 9 x 5mm LDR's with pass red bounce green dicro's fitted on to a short pieces of plastic tube cut to 45 degrees and a tissue paper light diffuser on the back of the dicro. Around 95 % of the beam is reflect by the dicro's to send beams over the heads of the audience. The small % of light that passes through the dicro is enough for the LDR to react to. The dicro's seem to filter out all unwanted ambient light. I got these nice, large, cheap dicros from Coloured GU10 halogen spot lamps from CPC.


The so called brain's for the harp is the box below.

To the top right is the 5 pin MIDI socket and the 15 pin D socket for the sensor bar.
To the bottom are the black and white phono jack sockets that are laid out like the keys of a keyboard. The nine phono plug patch leads go to the N/O contacts of the relays on each of the nine circuits illustrated below.

You have to build one of these circuits for each beam.
The black and white phono sockets are connected to a MIDI controller board i managed to buy off the web about 8 - 9 years ago but I have no idea if they are still available. I have found the web page for the manufacturer but the midi board I have no longer seems to be listed. The Board model No is "MIDI 4A". http://www.midi-hardware.com/

A bit of a breef discription but I hope it helps.

Carl

[sharksystems]

In the case of using discrete modules, what would be the best choice in the range of 50mW? There are obviously the cheap eBay specials, but those would probably last about 10 seconds.
I don't know where to start with regards to a scanner system, as all the plans are either in French, or taken off the market (Steven Hobley's). If there are any other tutorials for building a scanning harp, that would be much appreciated. I'm more of a musician than an engineer, but I want to learn.

What it looks like now is:
8 50mw 532nm modules (on custom PSU circuit)
These drive LDR's (I like Carl's idea with the outward beams, that's something I've seen Jarre do), which will be picked up by Steven Hobley's beam detector circuit (using LM324s), with an Arduino handling the A/D.
If there are low-cost scanning systems I could retrofit, I would go straight to a frameless harp, as I don't exactly understand the magic behind the frameless harp, and the framed harp is simple.