[sdiy] FSR-controlled CV/LED

[Tom Wiltshire]

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
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