[sdiy] FSR-controlled CV/LED

[Derek Holzer]

Hi Tom,

I'm taking this back to the list so I can thank you in public. Your 
suggestions made perfect sense, and not only that, but they actually 
worked ;-)

Here's the final circuit, in case someone else ever needs to control a 
LED using an FSR *without* an Arduino:

http://macumbista.net/wp-content/uploads/2012/09/fsr-circuit.jpg

Thanks and best wishes!
Derek


On 27/9/12 11:20 AM, Tom Wiltshire wrote:

>
> Hi Derek,
>
> It looks to me like this is getting a bit tricky because of the 
> conversion to single supply.
>
> The positive input is at half-supply, eg +4.5V. The FSR is supposed 
> to have its far end at a Vref of -5V, which we don't have. But 
> connecting the far end to the battery ground would give -4.5V w.r.t. to 
> the virtual ground, which is close enough. The output should be 0-4.5V 
> w.r.t. that virtual ground, which is actually 4.5V to 9V output.
>
> Similarly, the feedback components on the second op-amp are going to 
> be going to a virtual ground rather than the battery ground, which 
> should make the 4.5V offset irrelevant.
>
> But for now, let's make sure we know what's happening with that first 
> op-amp. Good luck!
>
> Regards,
> Tom
>
>

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