[sdiy] Performance control - processing a control voltage

[Tom Wiltshire]

Here's a quick sketch of how I'd do it. It doesn't quite meet your specification, since I used a +/-15V supply, which makes it four wires with the output.

	http://www.electricdruid.net/images/LDRto0-5VCV.png

It's probably not quite there yet, and might have errors in it, but the gist is as follows:

Potential divider provides a varying voltage which is more than 0V at the low end and less than +15V at the top end. So the range cannot be more than 3 times what we need.
The next op-amp stage reduces the range to 5V, without worrying about the offset.
The final op-amp stage reinverts the CV, and deals with the offset by adding in a static voltage from a preset. The 470K mixer input resistor reduces the 30V range across the offset divider preset down to 6 and bit volts, so offsets only +/-3V. As currently set up, you'll only need half that range (since the offset can only be -ve, you only need +ve) but I left it since you could easily swap the 3K and the LDR over to get the alternate light/dark response.

It's using the rails as a reference voltage (very dirty trick in many people's eyes, since supply noise goes onto the output). You can adjust to taste with as much supply filtering or voltage reference ICs as you see fit.

HTH,
Tom


On 29 Jan 2012, at 00:10, Matthew Smith wrote:

> I want to use LDRs for performance control, getting a 0-5V output depending on the amount of illumination they receive. (There is a feedback element here too, as there will also be light emitters in the system, but that's another story.)
> 
> If I make the LDR one half of a potential divider which has 5V across it, how exactly do I calibrate the system so that the maximum resistance of the LDR is, say 0V (depending on which end of the leg it is placed in) and a given value of illumination is 5V?
> 
> I realise that I'm going to need at least one op amp here and, if I can get away with it, I'd like to power it/them with just the single 5V supply going to the potential divider - so I can create a simple, three-connection module.
> 
> As far as I can tell, I need to both amplify and offset the voltage coming out of the potential divider, but not entirely sure of the optimum way of doing this.
> 
> Ideas?
> 
> Cheers
> 
> M
> 
> -- 
> Matthew Smith
> 
> Business: http://www.smiffytech.com
> Blog:     http://www.smiffysplace.com
> Linkedin: http://www.linkedin.com/in/smiffy
> Flickr:   http://www.flickr.com/photos/msmiffy
> Twitter:  http://twitter.com/smiffy
> 
> ABN 16 391 203 815
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy