Cheap Diy easy Galvos based on the Tangent law of Magnetisim?

The tangent law of magnetism states (i think,not sure of the science)
that the angle of the compass in two perpendicular fields is proportional to the relative strengths of those fields.
what i i'm suggesting is building essentially one of these:A model galvanometer | Practical Physics
using an electromagnet or strong permanent magnet instead of the earths field and having a mirror on the compass.

would it work? seems like there should be less resonance and moving parts to fail then a rubberband or similar mechanical open loop stuff.

Galvos of that sort have no feedback system, there were a few people that have successfully built a closed loop feedback system from scratch...

ELM - Home Built Laser Projector

With a scanpro20 selling for $120, I'm not sure why anyone would homebuild a galvo system...

i'm not trying to build closed loop. this is just another closed loop.
and the reason is that 120$ is not cheap, especially when you add in control electronics and software. i am thinking to use a CD player instead of a dac(yes i know about how it might not like dc, my pager motor post describes a fix for that.) and maybe bring the whole system down to below the cost of just those scanners. what is with everyone thinking that everyone interested in lasers has 500$
to spend on a casual idea? not everyone is trying to be a full time laser person.

YEAH said:

i'm not trying to build closed loop. this is just another closed loop.
and the reason is that 120$ is not cheap, especially when you add in control electronics and software. i am thinking to use a CD player instead of a dac(yes i know about how it might not like dc, my pager motor post describes a fix for that.) and maybe bring the whole system down to below the cost of just those scanners. what is with everyone thinking that everyone interested in lasers has 500$
to spend on a casual idea? not everyone is trying to be a full time laser person.

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Dude, you need to chill; I'm not trying to trash your idea, I'm simply pointing out some flaws.

First off, the system you're shooting for is simply "random". The principle of the galvo hasn't changed but you gotta realize that simply modulating the galvos with "a CD player" probably won't achieve what you want. If you're planning to use only 1 galvo, you're simply gonna end up with a variable width liquid sky(hardly interesting). A galvo of the design you're proposing will probably "spin" about π/2 Rad(a lot of sweep). Seeing that you're planning to use a compass, you may also want to consider the time it takes to "reset" itself to it's original position...

i was going for graphics here. i see no reason why full y scanning/animation
should be completely out of reach for most of us. and it should'nt take too long to reset as it is being returned by a electromagnet or NIB magnet. also, it CAN sweep that much but it doesn't have to. that depends on the input signal. as for the CD player not beight good enough, i was going to record a modulated constant 24khz tone,burn it to a cd, then rectify it to drive the galvos.
right channel full loudness=beam all the way right. right channel quiet=beam all the way left. and same principle with up down.

blanking shouldn't be needed if you control the speed. when you want to make a bright line, scan it slowly. if you want a line that barely shows up, scan full speed.
if you want a completely dark screen steer the beam to a corner, where a piece of metal is in the way, so it gets blocked. only hard part is the software to make the cds, and the galvos themselves. you could also substitute a soundcard for the cd player.

i am wondering why they don't make a MEMS based vector scanner commercially yet.

I am not sure what you are proposing with that "model galvonameter". You would never get any performance out of it. You will be able to slowly direct a compass needle but I doubt you would be able to control a mirror that has inertia to draw anything other than a snake.

Your CD player idea is basically the same as using a sound card which is already being done. You can't just hook up the output of a CD player. The voltages and offsets are all wrong so you would have to build a correction amp for it.

If you want to go cheap, you can get a 15K set of scanners for $80 and they will perform a lot better than that compass thing. You can build a sound card DAC for next to nothing if you design your own board. Not sure why you would want to reinvent the wheel but go for it if you think you have something.

umm i think that rectifier thing deals with the "cant output dc" thing.
this only outputs a constant 24khz tone with changing volume.
pure sine. what kind of cd/soundcard can't output a pure sine through a rectifier and an amp?

umm You are way off base because the output of a CD player is already DC. Rectifying it would serve no purpose. What you want for a galvo is AC so that you can deflect it in both directions. Otherwise, you are just varying the deflection in one direction and not getting the full potential out of it. The purpose of a correction amp is to remove the bias and amplify it to a signal level that is cabable of doing something. A rectifier does none of these functions. Even if the signal from the CD player WAS AC, a rectifier would just distort it by chopping off the negative portion, which is definitely no desireable.

The 24Khz sinewave makes no sense. What kind of laser scanner only displays a 24Khz signal. Why not just build a variable frequency oscillator with a few components from radio shack and put a couple knobs on it? Or better yet, just go buy one of those $10 laser spirograph things from Spensers gifts. It would be better than what you are proposing.

I would have to agree with Gary^. What you are proposing probably will not work, but even if it did, the system would not perform well. To get a clean image out of a set of galvos takes engineering skills and expertise that will cost you WAY more in time and effort than just paying the $120-150 for a cheap set of actual galvos. I don't consider galvos to be a candidate for DIY work in the first place. Not like lasers (which you can make yourself in many cases) or drivers or cases, etc.. If you want to own a useful laser scanner that does more than 4-5 basic shapes you'll need to save up for while and get yourself a set of commercal galvos, which will include the galvo amps.. You can build a sound card DAC for under $100 and buy Spaghetti for $50 or use LFI laser player which is free (Spaghetti has the best all around performance)..

I think you could have more fun with a microcontroller and some stepper motors

IIRC, you can do scanning with 2 variable speed motors and individual speed controllers. Attach a mirror to the shaft and mount them in a similar configuration to a real set of galvos; ensure that the mirrors do not hit each other when they are spinning. Theory is, as you increase or reduce the speed of each motor, it "sweeps" faster and slower(respectively). This should achieve the effect you want...

the 24khz is Amplitude modulated. the loudness of the signal controls the galvo.
also a half wave rectifier chops off the negative. a Fullwave reverses the polarity with a diode bridge when it is negative.

Lasers aren't a cheap hobby, specifically laser shows. If you want it to work well, you gotta spend the big money ...

Sure, you can build some galvo's and draw a few scribbles on your wall (Not much else you can do without feedback), but you get tired of that really quick, and just end up buying some real galvo's anyway. If you can't afford $120 for some galvo's (Thats very, very cheap), then I don't think laser show systems are for you ...

Sure, DIY it, it may work, but you'll get bored of it.

By DIY I mean galvo's etc, things like the sound card DAC are a great thing to DIY

yeah, that idea might not work. galvo design might be better left to the pros

Things said:

Lasers aren't a cheap hobby, specifically laser shows. If you want it to work well, you gotta spend the big money ...

Sure, you can build some galvo's and draw a few scribbles on your wall (Not much else you can do without feedback), but you get tired of that really quick, and just end up buring some real galvo's anyway. If you can't afford $120 for some galvo's (Thats very, very cheap), then I don't think laser show systems are for you ...

Sure, DIY it, it may work, but you'll get bored of it.

By DIY I mean galvo's etc, things like the sound card DAC are a great thing to DIY

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Where can you get scanpro20s for $120?
I've been looking for ages

when you say closed loop do you mean just back and forth because if you do if you just flip flop polarity back and forth that's what would give you a closed loop and if you had an adjustable speed you could get results with that. i built a circuit like this just waiting to assemble some disassembled hard drives. this could also be applied to any electro mechanical set up you build.