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gilid 5th June 2002 02:26 PM

Stepping motor volume control
Has anyone tried to control a stepped attenuator via a stepping motor? I have a DIY preamp which uses a two Shallco stepped attenuators (32 positions), and I want to implement an IR remote control. Problem is that this calls for accurate, repeatable 11.25 deg rotational increments, at a decent torque.

It seems this may get very complex - I have visions of gears, clutches (to allow knob control via the front panel), etc. Anyone have any simple ideas?

Brett 5th June 2002 07:37 PM

I thought about this a while ago when I was looking at the nice 47 position Shallcos. It never got past a thought experiment, but I think you will need a gearbox and a 200 step/rev motor. Tamiya who make all the plastic models and RC cars have gearbox sets available reasonably cheaply, and the motors are esy to get cheap surplus. I wanted a box and fine stepping for the motor so I could have each switch position be a number of steps, so if a couple were lost in the startup, it would still hit the desired position each time. The box also gives the motor a nice torque advantage.

In the end I did mine with 18 relays in a shunt, like the Pass pre's do.

FBJ 5th June 2002 07:55 PM

Hey, you guys should do a search cause I remember a German web site that a member posted that builds and designs motor controls for audio step attenuators. I have the site bookmarked at home but I'm at work now and whenever I go to that web site I got an MISUSE of Company Equipment screen so I don't have it bookmark here on my work computer.

Hey Brett- my computer been misbehaving at home. I will get to those pictures of the Dynaco transformers this weekend hopefully.

Brett 5th June 2002 08:17 PM

Hi Freddie
<b>I remember a German web site that a member posted that builds and designs motor controls for audio step attenuators.</b>

Thanks for reminding me. Would this be the one you're thinking of?

<b>Hey Brett- my computer been misbehaving at home. I will get to those pictures of the Dynaco transformers this weekend hopefully.</b>

Thank you. Whenever you get a moment'll be fine. I'm looking forward to getting them, and building the amps:)

jan.didden 5th June 2002 08:39 PM

motor control
I once designed a stepper control for pots, published in Audio Amateur 2/91
The idea was to use 2 single pots driven by stepper motors and some logic to implement both volume and balance functions.
(Actually they were dual pots but one section was used for position feedback).
This can easily be used with stepping attenuators IF you choose the rotational steps of the stepper motors (in degrees) equal or an integer ratio of the step size of the stepper attenuator. You then remove the indent mechanism and let the stepper motors do the discrete steps.
(If I had to build it again I would use a microcontroller like a PIC, of course, but this was 1991...)

Cheers, Jan Didden

FBJ 5th June 2002 10:30 PM

Yes, Jan Didden. I had read that article in TAA many times but never had the time to build anything. Since I am at work I didn't mention it cause I couldn't remember the issuse it's in. I have been designing with the Holtek IR chips to control motorized volume and balance pots. And I have a PIC programer and basic software but no time to use any of the stuff yet with stepper motors which doesn't look to hard.

MRehorst 6th June 2002 12:14 AM

use solenoids?
Maybe you could use two solenoids- one that pulls clockwise one step at a time and the other pulling counterclockwise. You could put a simple ratchet gear on the shaft of the attenuator. The length of stroke of the solenoid would determine the angle the shaft rotates with each pull.

It seems like it would be pretty easy to make, especially compared to a motor setup where you're going to have problems knowing when to stop the motor for each position of the attenuator shaft.

Whether you use motors or solenoids, you're going to need some feedback into the controller to know when to stop turning the attenuator shaft.


jan.didden 6th June 2002 07:17 AM

motor control

Not difficult at all! Remember, a stepper motor rotates a well defined number of degrees per pulse. If that corresponds with the angle of each stepped attenuator step (sorry for the skewed language) you don't need anything like ratches, mechanical stuff etc. Just pulse the motor once, and voila, the attenuator steps exactly one position.

Other implementations are as easy: take a 24 position attenuator (that's 15 degrees right) and 7.5degree stepper motor which are widely available and you just step it 2 pulses at the time.

Alternate pulses (or as I did pairs of pulses, 1 CW and 1 CCW) give you the balance function.

If one uses a PIC microcontroller, it's easy to keep track of the positions. The beauty of this mech stuff is the inherent memory, so it comes up exactly as you turn it off.

No solenoids, for pete's sake! This is not a wharf!

Cheers, Jan Didden

jan.didden 6th June 2002 11:40 AM

motor control

As an add-on to my last post, you can detect end-of-rotation on the stepper attenuator in several ways (need to do that, don't want to step CCW from min level to max level in one step, of course). With a stepped att I would add an extra deck with the end positions wired to an input of the microcontroller to detect max and min volume.

And of course, there are actually two pulse lines going to each stepper motor's chip, and their relative order determines the step direction.

NickC 6th June 2002 02:41 PM

Hey do you guys have the schematic for ir control stepped . looking to built one

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