[sdiy] rectification/min/max question

[Dave Manley]

Take a look at this, it has adjustable gain and offset:

http://www.oldcrows.net/~patchell/synthmodulesII/200-1018.pdf

The circuit could be simplified, as there is a voltage reference for the 
offset pot you probably don't need.

-Dave

Derek Holzer wrote:
> Hi David,
> 
> I could if you could show me an example circuit ;-) I'm not sure how to 
> set it up to handle offset and amplification at the same time.
> 
> best,
> d.
> 
> David Moylan wrote:
>> Couldn't you just use a single opamp stage to provide offset and 
>> amplification?  You could add a trimmer to adjust the offset to bring 
>> black to 0 volts.
>>
>> Dave
>>
>> Derek Holzer wrote:
>>> Hi Ian,
>>>
>>> yes, I'm thinking absolute simplicity here. This sounds workable.
>>>
>>> The diodes are in series, with normal polarity, correct? Not grounded 
>>> or reversed? So I'm just exploiting the diode voltage drop? Yes, very 
>>> simple, I will try it. What is the formula to calculate the diode 
>>> drop, in case I need more or less?
>>>
>>> thx + best,
>>> d.
>>>
>>> Ian Fritz wrote:
>>>> 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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>>>>
>>>>
>>>
>>
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