[sdiy] rectification/min/max question

[Derek Holzer]

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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-- 
derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 13:
"Are there sections?  Consider transitions"