[sdiy] FSR-controlled CV/LED

[Tom Wiltshire]

The I-to-V convertor is inverting, so I guess if you want a positive output, you use a negative Vref. The datasheet shows a 0-5V output, so it's probably a -5V Vref that they're using. The value of Rg sets the gain, and they suggest that you should use Rg the same as the *minimum* Rfsr to get unity gain.

Stewart Pye described the LED driver as a *non-inverting* VCCS, but I guess we could put the LED either way around, so perhaps that isn't important. If you were to use 0-5V as the range, you could specify the resistor for the LEDs maximum current at that voltage. Then it'd give you off to maximum brightness over the 0-5V range, and hopefully that's what the FSR output would give you.

My prediction: There'll be some tweaking. There always is!

HTH,
Tom


On 26 Sep 2012, at 18:24, Derek Holzer wrote:

> Hi Tom,
> 
> just for clarity, looking at the FSR Current-to-Voltage Converter on page 23 of the PDF you sent.... VREF going into the FSR should be biased?
> 
> And do I need to similarly bias the VOUT of that circuit before it becomes the VIN of the linear LED driver you also linked to?
> 
> Best!
> Derek
> 
> On 26/9/12 6:38 PM, Tom Wiltshire wrote:
>> 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
>> 
>>  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
> 
> 
> 
> -- 
> derek holzer
> noise.art.technology
> http://macumbista.net
> 
> 
> -- 
> derek holzer
> noise.art.technology
> http://macumbista.net