I'm redesigning a custom device that must replace a similar one designed and manufactured years ago by another company.
I must maintain compatibility with the older one, and I must use two Light Dependant Resistors coupled with an incandescent lamp as a dual tracking, insulated attenuator.
My problem is that I cannot identify the LDRs used in the old design (the previous manufacturer has disappeared) and all the LDRs I could find show a temperature coefficient that makes them unusable, because the lamp will heat the LDR's.
I have in my junkbox some very old (30 years) LDRs that were made in Europe (maybe by Siemens) that seems to be less temperature dependant, but they are unmarked and I do not have two with the same value for a quick test.
Does anybody has some information about LDR's thermal coefficient? Manufacturers do not give this value.
I've tried with LED couplers : some commercial Vactrols and similars, some built by me with a LED and a LDR, and I've seen that, due to lower heat generated by the LED the temperature effect is negligible, but the transfer function (LED current vs LDR resistance) is not suitable for the job.
I need a lamp, and the lamp and the two LDRs must be encased in a small box to keep them in the dark.
I even thought about a thermostatic control of the box's temperature, but it will increase too much the complexity an the cost of the product.
Thanks to anybody that will post some suggestion and/or some information.
Best regards