[sdiy] rectification/min/max question

Mon Nov 19 19:22:13 CET 2007

Hi Dave,

thanks! This looks very clear. Is the whole second op-amp (U2A+B) being 
used for the voltage reference? Could it be replaced with a simple +5 V 
(or other reference voltage) through the Offset pot to the positive 
input of U1A? Or is more being done down there?

best,
d.

Dave Manley wrote:
> 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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>>
> 
> 
> 

-- 
derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 112:
"Magnify the most difficult details"