[sdiy] rectification/min/max question

[Derek Holzer]

Dear list,

sorry to repost, but I didn't get any working replies on this yet...

Perhaps it might help if I clarified what I'm doing:

I am using phototransistors to read the light level passing through a 
spinning transparent disc with patterns printed on it. The voltage which 
comes out can be CV voltage when the discs spin slowly (which is then 
used to control VCA or filter CV), or audio when the disks spin quickly 
(in which case they are something like the VCOs in the system). Images 
and info here:

http://www.umatic.nl/tonewheels.html

The problem is that the contrast of the disks is not perfect, so black 
does not equal 0V. So I'm looking for a way to "gate" the voltage, so 
that voltage below a certain threshold is read as 0v, and voltage above 
that threshold is passed. Simply attenuating the voltage isn't quite 
enough, because the gain of the voltage after that often has a very 
small range/strength.

So I imagine something like this:

Vin -> voltage "gate" -> amplify remaining voltage if needed -> Vout (to 
VCA, etc)
                   *
                   |
Vref -----------*

Would negative voltage applied at the Vref input, and then half-wave 
rectifying solve this problem? I'm open to other suggestions (so long as 
they can be done on the overhead projector as shown on the website..i.e. 
no "black boxes")!

thx + best,
d.

Derek Holzer wrote:
> A while back, I posted on the topic "Is this a comparator?". My problem 
> is to only pass control voltages above a certain threshold. Several 
> posters recommended either rectifying the signal, or using a Min or Max 
> construction. This answers half the question, but not the other half.
> 
> When using a Max function, for example, with a threshold voltage of 2.5 
> V, anything above 2.5 V will go through, but otherwise the output 
> voltage will remain at 2.5 V. A rectifier would do the same, if I 
> understand properly.
> 
> What if I knew that my signal had a dynamic range of 0-5 V, but I wanted 
> anything under 2.5 V to equal zero, and anything over 2.5 V to scale 
> from 0 - 2.5 V? Or ideally, it would scale from 0 - 5 V? I don't know 
> that would be easier or more difficult...
> 
> So I guess the question is, how to remove the offset voltage? Invert the 
> threshold voltage and add it to output?
> 
> And after that, how to regain the full 0 - 5 V range in the signal that 
> is passed?
> 
> Thx + best,
> Derek
> 

-- 
derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 106:
"Look at a very small object, look at its centre"