Hard coding ldr values?

[msi]

There was a lot of talk about how to convert the aout into variable resistors.  Why would it not be possible to hard code ldr values into the mbhp core?  So what if I were to test the ldr to see how much voltage is requred to get a value.  So lets assume I have a LDR that has a on resistance of 200 ohm, and an off resistance of 300meg.  After testing I find the ldr gives me a value of 1meg at a voltage of 2v.  I can then divide 2v by what ever resolution I have.  Again assuming a resolution of 128 steps.  If I'm trying to replace a 1meg pot thats wired as a variable resistor I can predict what the voltage will be at any given point in theory.  So given 128 steps, at mid point, we can estimate 1 volt would give  a resistance of 500k. 

The way to verify this is to have two ldrs set up.  One that is attached to an aout and in the circuit to be controlled. The other is attached to the same aout and is used to verify the voltages and attached to an ain of the core.  The core would then sample the ain to ensure a proper voltage is kept.  This assumes that the two ldrs are of a high quality and don't differ to much. 

This wouldn't be great for a box where the core has lots of things to do such as a sequencer, but as a dedicated box that is used to store and retreive settings it should work decent.  The other concern I have is I don't know what accuracy the ain modual has.  Assuming the scenario I described before, the voltage change between steps would only be 0.02 volts.  I know some people would want a higher resolution, but for a box where you set a value then forget it, I think it should be fine.  Similar devices such as the mesa boogie triaxis and the ada mp-1 have a resolution of about 16, maybe.  Further more a dout could be used to control relays and switch, and encoders could be used to manipulate the knob values.

So am I just blowing wind into the air?  I've ordered a set of H11F3's for another project and plan to test them to see how sensitive they are and how accurate two diffrent ldr's are between the two.  Thanks.

[TheAncientOne]

This looks interesting - I've never used the Fairchild opto's - I'll get some myself.

I can see one initial problem on page 4 of the data sheet, Fig. 4. The device seems quite temperature sensitive, so some kind of servo loop. such as you describe, might be needed. I used an analogue version of this for the outputs of an ECG amplifier, sadly it was a long time ago and I don't think I've kept the notes. No: I am not a relative of Fermat......

Best wishes

Mike