This question specifically concerns my issues with an ORP12 like:-


I've read the data sheet and I see that resistance changes wrt short time (15 sec). Also note 1 mentions 16 hours. But what happens after? Is it possible that the resistance could double (69k to 136k) over the course of a couple of weeks? I need this question answering in order to rule out other possibilities for my resistance change in an otherwise steady state system.

  • \$\begingroup\$ There not very precise parts and very temperature sensitive. the repeatability improves with light intensity and lags for a long time in darkness (15s) \$\endgroup\$ Mar 7, 2017 at 2:55
  • \$\begingroup\$ Repeatability is something like this obrazki.elektroda.pl/9650040400_1488855740.jpg maybe as bad as 2:1 in low light \$\endgroup\$ Mar 7, 2017 at 3:02
  • \$\begingroup\$ If one wants accurate light detection, one uses a silicon PD. \$\endgroup\$ Mar 7, 2017 at 3:05
  • \$\begingroup\$ See also: electronics.stackexchange.com/questions/295969/… In my case I estimate a factor 5 to 10, but over a span of 10 years, not weeks. \$\endgroup\$
    – jippie
    Mar 31, 2017 at 21:00
  • \$\begingroup\$ LDR's contain Cd which is on the contamination mineral list. \$\endgroup\$ Aug 20, 2018 at 15:31

2 Answers 2


The LDR is not very stable, as has been commented. What does happen over time is that the dark resistance decreases. UV from the sun may be a factor here. You must design your decision circuit to be tolerant of this decreasing dark resistance, otherwise your product will fail at some point in the field. The high sensitivity of the LDR means simple circuits can work. In fact people put them straight into weakly pulled up digital inputs. The decreasing dark resistance issue means that this high sensitivity cannot be utilised in a high reliability commercial product. The old LDRs used cadmium which is not ROHs. New LDRs apparently do not use any cadmium. Both new and old LDRs appear to have this problem.

  • \$\begingroup\$ The ORP12 is only lit by a white LED. What happens to dark resistance without any sunlight whatsoever? It couldn't double could it? \$\endgroup\$
    – Paul Uszak
    Mar 7, 2017 at 11:38
  • 1
    \$\begingroup\$ The dark resistance should be very high ,Say more than 100K and better still 1 meg .The light resistance should be less than 1 K .This huge difference makes setting up a decision circuit easy and allows for degradation in LDR characteristics . \$\endgroup\$
    – Autistic
    Mar 7, 2017 at 19:57

The LDRs in my cheap old solar garden lights got sunburned and failed soon. New solar garden lights don't use LDRs anymore, they use the solar panel to detect light and dark instead.

If sunlight does not destroy an LDR then time will destroy it instead.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.