[sdiy] rectification/min/max question

[Derek Holzer]

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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> 
> 

-- 
derek holzer ::: http://www.umatic.nl ::: http://blog.myspace.com/macumbista
---Oblique Strategy # 157:
"Think
- inside the work
- outside the work"