[sdiy] rectification/min/max question

[Ian Fritz]

It sounds to me like you could use something really simple here.  So think 
about this:

First, run your raw signal through a string of 4 diodes followed by a 1.8k 
resistor to ground.  This will clip about 2.5V off the bottom of the range.

Then amplify the signal across the resistor by 2x with a noninverting opamp 
amplifier.  For example:
1) top of 1.8k resistor to (+) input, 2) (-) input to two 33k resistors, 
one to ground the other to the opamp output.

   Ian


At 05:20 AM 11/19/2007, Derek Holzer wrote:
>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"
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