[sdiy] Circle Machine prototype

[cheater cheater]

On Mon, Aug 16, 2010 at 19:01, David Brown <davebr at earthlink.net> wrote:
> At 01:15 AM 8/16/2010, cheater cheater wrote:
>>
>> This is absolutely extremely cool. Can't wait to hear some sound demos.
>>
>> Several comments and questions:
>>
>> 1. you mention interference between the lamps. Try adding small
>> matte-painted black walls between the separate lamps. Even better if
>> you are able to connect them all together and move them in and out -
>> then you can change how separate the lamps are.
>>
>> 2. different solution: take small plastic or sheet metal 'pipes' into
>> which you insert the lamps. Make sure those pipes are at the top
>> separated from the sensor by maybe 5 mm, and make sure the light
>> source in the lamp is maybe 1-2 cm below the top of the pipe. Then you
>> should get fairly good separation. Additionally you can move the lamps
>> in and out to change the separation
>
> All those are good suggestions.  I think the light is reflecting off the
> bottom vane.  I really don't need that vane as it just was a convenient
> place to mount the photo resistor.  I should be able to remove it and just
> let the photo resistor hang in free space.

Hmm... I'm somehow having trouble picturing that. I thought the vane
was supporting the lamps, not the photoresistor?

> It's light enough in mass that
> it won't be a problem.  The interference is pretty minimal but I won't mess
> with it until I decide if I'm going to put a photo transistor there.  I will
> have to remove the vane as there isn't physical clearance.
>
>> 3. you don't want too much separation, or the sensor will go dim
>> between notes i guess :)
>
> The sensor does go dim between notes.  You can see those as the vertical
> spikes on the last scope image.  The CdS sensor turns off faster than it
> turns on.

This sort of behavior can be fixed with an asymmetric low pass filter
(i.e. one that has a different time constant for "rise" and "fall")

On the other hand, those spikes could be part of "the sound" :)

> This was actually one of my concerns with the original design.
>  At slow speeds, there must have been a bunch of dead time if there was good
> separation between notes.  I don't see how you could get a good blending.
>  It was one of the reasons that I chose to use a stepper motor to step
> between lamps.  I figured I wouldn't have to experiment with the best light
> positioning to get a good transition.  Note that these spikes are only about
> 60 mS wide so they are not that objectionable.  A bit of lag eliminated them
> when I processed the output via analog.
>
>> 4. have you tried it with audio? I think that could sound like a
>> really cool low-pass filter. But again, I'm not really sure what the
>> settling time of such a sensor is :)
>
> I'm not really sure what you mean with audio?  All I'm doing is generating
> an output of lamp brightess.  Certainly if I used LEDs I could modulate them
> at audio rates.

Yeah. Basically, get LEDs in there, and send the same audio to each
led, or maybe two channels to alternating leds, and then just spin the
photo sensor around.

>
>> 5. How have you made the arm? did you weld it yourself?
>
> The arm and the slip-disc are part of the Hammond vibrato scanner.  That's
> why I chose to use it as a base.  It's a great slip ring.

Where do you get one of those?

>> 7. Do you think diodes could work just as well?
>
> Diodes - you mean LEDs?  I thought of using those but I didn't want to build
> the circuitry for dimming them.  Incandescent worked fine with rheostats.
>
>> 8. How fast does it go?
>
> I'm using half-steps so there are 200 per revolution.  I am clocking them at
> 4 mS each, so I can do 1.25 revolutions per second.  I can clock at 2 mS but
> there are some resonances so at specific frequencies I miss some steps.  I
> haven't tried between 2 and 4 mS since I decided this was too fast for a
> sequencer anyway.

That's just about 192 BPM if you want quarter notes. Not so bad.

>> 9. Can it go backwards? Have you tried a 'random mode'? :)
>
> I have a direction pin so I can reverse the motor anytime.  That's another
> reason I chose to use a stepper motor and a ComputerVoltageSource as the
> controller.  I will most likely add a reverse, forward-back, and random
> mode.  The problem with random mode is keeping a constant rate for the
> sequence.  A 15 lamp rotation takes .71 seconds.  A single lamp rotation
> takes 0.048 seconds.  You would want to incorporate the rotational time into
> the per-step delay to hold a constant note rate.  This is another reason why
> I chose to sample the output as there is a lot of dead time with multiple
> lamp rotations.

Sample the output?

>> 10. it's a bummer you can't get pitch perfect notes from it, but then
>> again, maybe that makes it that much better for other uses :) That's
>> not a reason to give up, much rather a reason to go on and extract
>> that feature imo :) you've got something really cool on your hands :)
>
> I can get perfect notes from it.  I just can't tune it statically and then
> rotate it.  I have to tune while rotating.  It's easy since I display the
> note on the LCD.  At the faster rates where it detunes, it's too fast to use
> for a sequencer as the notes just all mash together (to my ears).  Again, it
> is repeatable so you just have to tune it at the rotational rate.  I think a
> photo transistor would work much better.

Would love to hear what the photo transistor works out like for you

Good luck with your project :)
D.

>
>
>> Cheers,
>> D.
>>
>> On Mon, Aug 16, 2010 at 07:13, David Brown <davebr at earthlink.net> wrote:
>> > I'm sure it did.  I wasn't trying to build a faithful recreation - just
>> > one
>> > using lights and a spinner. It seemed like a stepper motor would be
>> > easier
>> > than a slow variable speed.  Besides, it gave me a chance to play with a
>> > stepper motor.  - Dave
>> >
>> > At 09:54 PM 8/15/2010, David Griffith wrote:
>> >>
>> >> On Sun, 15 Aug 2010, David Brown wrote:
>> >>
>> >>> I've been working on building a working model of Raymond Scott's
>> >>> circle
>> >>> machine.  I thought it would be fun to build something with motors and
>> >>> lamps.
>> >>
>> >> [snip]
>> >>
>> >> I was under the impression that the Circle Machine didn't use a
>> >> stepper,
>> >> but instead a more mundane variable-speed motor and didn't have a skip
>> >> feature.
>> >> --
>> >> David Griffith
>> >> dgriffi at cs.csubak.edu
>