Does an LDR age?

Question

I have had a twilight switch lying in my basement for 5 years after it was used in my garden for about 5 years. This week I took it apart to convert it for (solar) use from 230VAC to 12VDC.

What is left (for sake of this discussion) is a resistive bridge as follows:

^{simulate this circuit – Schematic created using CircuitLab}

What supprised me is the threshold voltage V(OUT) was about 200mV at a given twilight level, where I would have expected something somewhat close to \$V_{REF}=4\text{V}\times \frac{33\text{k}\Omega}{33\text{k}\Omega+47\text{k}\Omega}=1.65\text{V}\$. The LDR has a resistance of about 660k at intended light level.

I am thinking of changing R1 to 470k. That should give me a more reasonable V(OUT), somewhere half way the supply voltage as it was once intended considering R2/R3. I pulled the datasheet of the OPAMP and I think I may just get away with input bias and input offset currents, but what is wisdom?

1. Indeed increase R1 or will it age again a lot like it did (about factor 3 at the intended region);
2. Replace the LDR with a new one, for which I will have to reconsider R1 anyway;
3. Replace it with a photodiode or a phototransistor, but I would have to research what the circuit would look like.
4. Am I overlooking something?
5. What is the cause that of the huge difference in expected voltage and actual voltage, or differently put: Do LDR's age and if yes, how?

For a more complete circuit diagram refer to Reverse engineering: Relay driver design consideration. Only the high voltage capacitive power supply part is left out there.

Answer 1

You can answer the question yourself. What was the resistance of LDR1 before? If it was about 660K, then it has not aged!

If you are interested in making the circuit work, then 470K seems to be the right value for R1. If you want exactly 1.65V for Vout, then use a 500k pot and adjust it to get the voltage.