[sdiy] FSR-controlled CV/LED

[Derek Holzer]

Exactly the info I was looking for, Tom... for a start at least while I 
work on this opto-electronic project.

When I get to the TKB-type CV controller, I'll take your VCA idea into 
consideration David.

Thanks to you both!
Derek


On 26/9/12 6:38 PM, Tom Wiltshire wrote:
> Hi Derek,
>
> The FSR "integration guide" suggests using the FSR from a voltage to 
> an I-to-V convertor for better linearity. E.g. with the FSR as the 
> input resistor of an inverting op-amp arrangement (pg 23 onwards).
>
> http://www.sparkfun.com/datasheets/Sensors/Pressure/fsrguide.pdf
>
> (I've been looking into these for a ribbon controller too).
>
>   If you're running an op-amp off +9V, you'll need to bias the +ve 
> input to half-supply with a couple of resistors and a cap. This can be 
> copied from lots of guitar effects schematics.
>
> You could then try following that with one of the 'more linear' LED 
> drivers that Justin was asking about the other day. One of the 
> favourites was this circuit:
>
> 
> http://webphysics.davidson.edu/Course_Material/Py310/WK13_circuit3.jpg
>
> Replace the 'load' with the LED. The whole discussion was entitled 
> "Improving LED response when driven from half-wave rectifier".
>
> It's two op-amps just to drive an LED from an FSR, which seems like a 
> lot, but I guess it's only one 8-pin chip that costs pennies, so maybe 
> it's not so bad.
>
> HTH,
> Tom
>
>
>
>
> On 26 Sep 2012, at 17:01, derek wrote:
>
>> Hi folks,
>>
>> I'm experimenting with Force Sensing Resistors for two specific 
>> purposes right now. One is to control the brightness of a LED. The 
>> light from this LED will be interrupted by a spinning pattern printed 
>> on a transparent disk, which creates a tone when it falls on a 
>> photodiode. An optical organ so to speak:
>>
>> http://macumbista.net/?page_id=539
>>
>> The brightness of the LED would affect the loudness of the output 
>> signal. Obviously, a PWM solution would not work: you would hear the 
>> PWM frequency in the audio output. So I'm looking for direct, analog 
>> control.
>>
>> I am using Ken Stone's LED driver circuit:
>>
>> http://www.cgs.synth.net/modules/cgsld.html
>>
>> and playing around with resistor values. I place the FSR in series 
>> before the 100K input resistor, with +9V being sent through it:
>>
>> +9V------FSR----100K----base of transistor
>>
>> The FSR is an FSR-406. I can get the circuit to respond, however the 
>> slightest pressure lights the LED to full brightness. So my question: 
>> how can I change the response curve here?
>>
>> Circuit will be single supply, 9V battery powered, if that makes any 
>> difference.
>>
>> I am also looking at using FSRs in place of the capacitive sensors 
>> in a Serge TKB-like arrangement. And my question would be the same: 
>> how do I affect the response curve to get more variation from the 
>> pressure? Again, digital PWM options won't work here... the slew 
>> needed to get rid of the PWM frequency in the output CV would start to 
>> affect the response time of the FSR.
>>
>> I apologize for the fairly basic level of this question, as I'm 
>> primarily an artist and not an EE.
>>
>> Thanks for any and all suggestions!
>> Derek
>> --
>> derek holzer
>> noise.art.technology
>> http://macumbista.net
>>
>> --
>> derek holzer
>> noise.art.technology
>> http://macumbista.net
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>

-- 
derek holzer
noise.art.technology
http://macumbista.net