volume attenuator LDR

[arivel]

HI .
I thought I'd ask for an opinion on something.
it is a volume attenuator that uses 2 pairs of LDRs as you can see in the photo.
the two opamp chains for each LDR pair are used to ensure that as the input signal varies, when the value of a photoresistor increases, the other connected one decreases in synchrony. same thing for the other branch being 2 channels.
the curves of the LDRs are not equal therefore the task of U7 is to vary the output voltage of the opamp chain of the group below to vary the voltage that controls the led of the photocouplers and obtain the same attenuation value of the 2 channels.
it is true that the u7 opamp inputs are on the audio path but having very high input impedance it is irrelevant.
it is a draft that I tried in the simulator but if I don't connect the U7 output to create the feedback of the other opamps it works but not anymore if I connect .
of course everything that comes before is missing but it is not necessary to know it.
what do you think about it ? Can it work if done right?

 

[danadak]

Why are the non-inverting inputs left floating of the OpAmps ?


Regards, Dana.

 

[arivel]

Excuse me .
there is an error. this is the correct pattern

 

[rjenkinsgb]

I'd have the opto LED driver opamps in current feedback, rather than "open ended" as you have them.

eg. Connect the opamp output to LED anode and put the load resistor from the cathode to 0V, with the cathode-resistor junction connecting to the inverting input. The non-inverting input voltage will then directly control LED current.

It should give a rather more linear response, as the LED voltage drop is not involved (as long as the supply is adequate and you select the load resistor appropriately.

Use one input opamp to feed the upper driver, then the other as a unity gain inverter with an appropriate bias on the noninverting input, to feed the lower driver.

I do not see how the "matching" feedback can work, unless you add a DC bias at the audio inputs to provide it with voltages to compare, and a lot! of low pass filtering on the comparison amp circuit, so it does not respond at all to anything within the audio range.

 

[arivel]

It's not entirely clear to me what you wrote. Unfortunately I don't master the English language well.
Maybe I'd better clarify one thing.
u1-u5-u8-u9 are commanded by a logarithmic scale . I don't know if this is what you need to know