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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:

schematic

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.

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jippie
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    Rules of life... Everything ages! Anything exposed to the sun ages a lot faster. – Trevor_G Mar 31 '17 at 19:52
  • Also resistance goes UP with less light. So darker should go closer to zero volts. – Trevor_G Mar 31 '17 at 19:55
  • @Trevor agreed, but in full light V(OUT) was about 1.2V, 400mV too low for the OPAMP comparator to turn off ... – jippie Mar 31 '17 at 20:35
  • Ya it has probably aged. I saw a graph somewhere a week or so ago...but of course I cant find it again. – Trevor_G Mar 31 '17 at 20:36
  • So did it get more stable over time or did it get worse? I guess that is the question. Will I be redesigning the thresholds this weekend and be fine for years or will I be redesigning them every so many months from now on. In the later case I should replace the LDR. @Trevor – jippie Mar 31 '17 at 20:38
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    Is this a duplicate of http://electronics.stackexchange.com/questions/290817/are-light-dependent-resistors-stable-wrt-time ? – Paul Uszak Mar 31 '17 at 20:50
  • I was just going to add that @PaulUszak :) – Trevor_G Mar 31 '17 at 20:50
  • Partial duplicate @PaulUszak. See #1..5 which are not so much the topic of the other question. Good find by the way. – jippie Mar 31 '17 at 21:03
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    I'd replace the LDR, or go with a photo transistor instead. LDRs are pretty old school. – Trevor_G Mar 31 '17 at 21:22
  • @jippie .Interesting question +1 .Does this mean that an LDR can age without UV exposure?I know that LDRs are not accurate .I could find some 25+ year old LDRs from my junkbox that I think were in an Attic so they did not get light but were subject to temp cycling .I have not used them on any commercial designs because of their terrible spreads .I will find and measure these .The LDR has a nice linear resistance characteristic that is unique so that is why I saved them . – Autistic Mar 31 '17 at 22:00
  • Or you try to understand how and why things actually work and design it first time right instead of fixing it 20 times for a dollar. @SDsolar – jippie May 22 '17 at 18:05
  • I ended up replacing the LDR as it failed completely about half year after modifying the circuit. As the circuit is in daily there was no real possibility any more to redesign the circuit utilizing a phototransistor or diode. At least I did learn couple things from this :-) – jippie Aug 19 '17 at 12:34

1 Answers1

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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.

Guill
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