Need advice on applying an envelope to an effect

[soggybag]

I am adding an envelope follower to my Ring Frobnicator. The envelope controls an LED/LDR. I'm trying to figure the best way to add this. Here's link to the GEO schematic for the Frobnicator:

http://www.geofex.com/PCB_layouts/Layouts/frobn.pdf

have been experimenting with connecting the LDR in parallel with R14, or connecting it across U2 pin 7 and the wiper of the Freq pot. My results are working but the range of the effect is not as good as it could be.

Suggestions, observations and advice most appreciated.

thanks.

[Jaicen_solo]

If you want the envelope to replace thefrequency control, you'll need an LDR with a light to dark resistance swing of  close to 0 ohms up to around 100K ohms. In practice, this is never going to happen, so you need to be a bit clever with parallel resistance. Looking at the schematic, the 100K frequency pot goes to Vbias via a 4K7 resistor. So what we can look for now, is an LDR/Resistor parallel combination that will give us a swing of 5K-100K. I guess you could be lazy and use a 27-94K LDR. You could then replace R12 with your two LDR's and get a fairly decent swing (60-100K).The Frequency pot would also allow you to adjust the starting frequency.
Failing that, you'll need to arrive at a parallel combination of one or more LDR's or resistors which will give you the correct swing. I can't work one out at the minute though!
The formula you need is this:

(R1 x R2)
------------
(R1 + R2)

Where R1 is the value of your LDR and R2 is the value of a resistor parallel with R1.

[soggybag]

Thanks Jacien, that was a very lucid explanation. Let me see what I can come up with...

I'm not sure what you men by two LDRs? Did you mean to use two LDRs in parallel in plce of R12?

I had originally been thinking the LDR would go across the Freq pot from pin 1 to 2 or from pin 2 to 3.