Back to your original question, how about a 'frequency to voltage ic'? Feed the output from that into one side of a comparator and then use the output to drive the led. One like this - http://www.digikey.com/product-search/en?mpart=ADVFC32KNZ&vendor=505

Or, a color organ with different led's driven from the outputs of it. they are designed to flash at different frequency bands. One web page/design, many more online. http://www.craigandheather.net/celepgco.html

Another thought, how about a small speaker with a mirror glued to the cone. And the led shining at the mirror. By focusing the led to a portion of the mirror each frequency would bounce off of the mirror for a different length of time.

 

OK, Y'all, here's a video of a crude setup doing what I want. Keep in mind I'm driving the laser directly from the speaker output, which means I had to drive it fairly loud just to get the laser to laise. (not sure of spelling) The motor/mirror was driven from two AA batteries, so the speed was not controlled. Neither was the base / treble tones, so the base tended to over power the drive while the higher frequencies were unrecognized. Also keep in mind I shot this on my cell phone, not the best camera for shooting video's but at least it allowed me to shoot in a very dark room. And a lot was visible that the camera just didn't pick up.

Notice how a long dash (base note) appears to circle one direction but when a slightly higher note is played it circles in the opposite direction. Also notice that when a full octave higher base note was played there were two dashes, smaller and diametrically opposed to each other. It didn't come out bad despite the lack of control and balance. Also had to stick magnets from a Sonic Toothbrush into styrofoam just to hold the motor and the laser. (nifty) Also note that the laser was about 24 inches from the wall. A longer distance would have produced a larger circle and more definition.

For those who don't know what you're looking at - it's a laser being driven by the amplitude and frequency of Suite Judy Blue Eyes by the Moody Blues. It can be likened to a Z axes oscilloscope that has its tracer dot looped back in a continuous circle.

Here's a link to my DropBox. I don't see where AAC allows uploading of MOV files (Movies)

https://www.dropbox.com/s/i2t5lml0uqk2liq/Laser Moody Blues Suite Judy Blue Eyes.mp4?dl=0

 

how about a 'frequency to voltage ic'?

Thanks, but that'd defeat the goal.

 

Thanks, but that'd defeat the goal.

I guess I still don't understand what your trying to do. The frequency to voltage would give signals to leds from each freq range, it would need more than one convertor. And would take more than one led, one for each frequency range. Have you looked at how others have approached this? https://www.google.com/search?q=diy+laser+show&ie=utf-8&oe=utf-8

 

OK, Y'all, here's a video of a crude setup doing what I want. Keep in mind I'm driving the laser directly from the speaker output, which means I had to drive it fairly loud just to get the laser to laise. (not sure of spelling) The motor/mirror was driven from two AA batteries, so the speed was not controlled. Neither was the base / treble tones, so the base tended to over power the drive while the higher frequencies were unrecognized. Also keep in mind I shot this on my cell phone, not the best camera for shooting video's but at least it allowed me to shoot in a very dark room. And a lot was visible that the camera just didn't pick up.

Notice how a long dash (base note) appears to circle one direction but when a slightly higher note is played it circles in the opposite direction. Also notice that when a full octave higher base note was played there were two dashes, smaller and diametrically opposed to each other. It didn't come out bad despite the lack of control and balance. Also had to stick magnets from a Sonic Toothbrush into styrofoam just to hold the motor and the laser. (nifty) Also note that the laser was about 24 inches from the wall. A longer distance would have produced a larger circle and more definition.

For those who don't know what you're looking at - it's a laser being driven by the amplitude and frequency of Suite Judy Blue Eyes by the Moody Blues. It can be likened to a Z axes oscilloscope that has its tracer dot looped back in a continuous circle.

Here's a link to my DropBox. I don't see where AAC allows uploading of MOV files (Movies)

https://www.dropbox.com/s/i2t5lml0uqk2liq/Laser Moody Blues Suite Judy Blue Eyes.mp4?dl=0

Did you run this simulation?
http://forum.allaboutcircuits.com/t...d-pulse-width-modulation.127468/#post-1040206
It simply powers up your speaker so any note drives the laser.
You may need to adjust the current. I don't think you ever stated it.

 

@Threeneurons Thanks, but this doesn't seem to work. At least not in the sim.

I don't understand why you have two voltage dividers on the first Op-Amp. Nevertheless, thanks for the effort. All I get is a square wave that doesn't vary with amplitude.

I've used this circuit many times. For one, its not an op amp. The LM393 is a dual voltage comparator. The difference between a voltage comparator, and an op amp, is that the output is open collector, and there is no frequency compensation, so it can oscillate at much higher frequencies. That's the simplistic version of the differences. Also why the 4.7K resisistors (4K7) are needed.

The top half of the circuit is an oscillator. To make your simulation work, add a resistor, say 150K between pin 2 and V+. This shouldn't be needed in the real circuit, but simulators are picky. The divider formed by the three 47K resistors make a node that bounces between 1/3V+ to 2/3V+. The other 47K connected to the 0.01uf cap, and pin 2, rise and fall at a delayed rate determined by their values. If the output is high, then pin 2 slowly rises to 2/3V+. At this point it causes the output to go zero. Pin 3 now has 1/3V+, and the pin 2 slowly falls until it just drops below 1/3V+, when the output, now goes high, and pin 3 steps up to 2/3V+, and the cycle repeats.

The lower portion, is a basic comparator. Depending on where the pot is set, determines where the sawtooth crosses its set voltage. That determines if the output is high or low. If the pot is set low, the sawtooth only sips below it for a short duration, and you get a narrow output pulse. In the middle, and you get a near 50% duty cycle pulse. Cranked full up, and the pot voltage is always greater than the sawtooth, and the output is always high.

There is way too much dependence on simulations. I use LTSpice from time to time, but there's no real substitute to just wiring the thing up.

Been selling the kit, below, since 2011. It uses that LM393 oscillator for its boost converter, generating 450V !

A dual op amp will fail in this circuit on the two points, I indicated for the differences. A open collector output is required, and most op amps are too slow.