[sdiy] rectification/min/max question

[David Moylan]

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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>>
>>
>