What does the LDR response time mean? how can it be compared to Ohms/second? - Page 1
Forum section: electronics / beginners

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 24, 2023, 03:38:04 pm
content: Does the response time stand for how long it takes for the LDR to sweep it's entire lit-resistance to dark-resistance range? Surely not right?

author: wasedadoc
author_rank: Super Contributor
timestamp: July 24, 2023, 04:14:22 pm
content: Maybe not "entire". Data sheet should be more specific.

author: Zero999
author_rank: Super Contributor
timestamp: July 24, 2023, 09:22:11 pm
content: Yes, it should be specified.Note LDRs are very slow, compared to photodiodes and transistors.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 24, 2023, 10:15:41 pm
content: Not sure how fast do you need it to react, but definitely not good for more than 50Hz or so.You could see this by yourself, have a powerful led blinking right over ther LDR, try different frequencies between 10Hz and 500Hz, as you increase the frequency it'll start looking like a triangular shape.But for regular ambient light detection it's fine!Some waveforms:https://jeelabs.org/2012/03/18/detecting-a-blinking-led/index.htmlhttps://hackaday.io/project/12552-seelablet-instrument-cluster-for-laboratories/log/41850-response-time-of-an-ldr

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2023, 06:54:09 pm
content: Quote from: ELS122 on July 25, 2023, 06:45:34 pmCan you link the datasheet showing this response time? Perhaps there might be some clues how to interpret it. If one is given, it won't be arbitrary but I think the point is that the full characteristics of the photocell can't be boiled down to a single number, so if a response time is given it isn't going to be a very precise way of determining how it behaves in your application.

author: DavidAlfa
author_rank: Super Contributor
timestamp: July 25, 2023, 07:34:32 pm
content: Quote from: ELS122 on July 25, 2023, 06:45:34 pmWhat? Of course it means something. Not sure how it's measured, but imagine it like moving a potentiometer, it won't change the reistance instantly, it need several ms.

author: helius
author_rank: Super Contributor
timestamp: July 26, 2023, 01:25:55 pm
content: On page 3, it says that the dark resistance is measured 100ms after "closing 10 lux" (in other words, after light is removed). On pages 6-7, it lists reponse time for "Rise" and "Decay"; but you would have to guess as to what parameter is "rising". My guess is that resistance is rising in the dark condition. But since the two response time figures are close to equal it doesn't matter much.CdS resistors are slow as has been stated. The response time may be the time between a change from light-to-dark to registering 1sigma of resistance change.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2023, 02:14:16 pm
content: Quote from: ELS122 on July 26, 2023, 01:47:20 pmOne one hand those are the most carefully specified CdS cells I've seen, on the other there do seem to be some language issues that leave us guessing. "10th seconds" is ambiguous to me--it could mean 100ms or 10s. As for rise/decay, I'd guess that maybe those are the time constants of a quasi-exponential curve for each. I'd think 'rise' would be lights on and 'decay' would be dark given that the decay times are all longer. I don't think ohms per second is a valid measure unless you consider the overall resistances of the cells. You could have two cells with the exact same characteristics except one has a trace that is twice as long and your Ω/s would double. Ditto for two cells in series.

author: MrAl
author_rank: Super Contributor
timestamp: July 26, 2023, 02:41:21 pm
content: Quote from: ELS122 on July 24, 2023, 03:38:04 pmHello there,I took a look at the datasheet you posted and saw the rise and decay times listed.The usual way to interpret rise and fall times is to assume it is measured from 10 percent of the response to 90 percent of the response. You'd have to determine what the min and max is for your device.These are usually used for stuff like audio compressors and light controlled interior/exterior lighting. Those applications don't require extremely fast response, probably 100ms is good enough for the audio compressor, and even 1 second is ok for the light control device.Sometimes you just have to do some of the testing yourself, or if you care to, get in touch with the manufacturer as sometimes you just have to call to get a decent answer.

author: MrAl
author_rank: Super Contributor
timestamp: July 26, 2023, 02:43:11 pm
content: Quote from: ELS122 on July 26, 2023, 02:39:01 pmHi,Where did you find that info?I ask because that sounds more like the "time constant" not a rise or fall specification.It could be right, but it doesn't sound right.

author: bdunham7
author_rank: Super Contributor
timestamp: July 26, 2023, 02:46:48 pm
content: Quote from: ELS122 on July 26, 2023, 02:39:01 pme = 2.7182818.....https://en.wikipedia.org/wiki/E_(mathematical_constant)

author: Zero999
author_rank: Super Contributor
timestamp: July 26, 2023, 07:05:47 pm
content: Quote from: ELS122 on July 26, 2023, 01:47:20 pmIt's slow. Phototransistors are an order of magnitude faster and photodiodes faster still.

author: golden_labels
author_rank: Super Contributor
timestamp: July 27, 2023, 04:23:09 pm
content: These values are delays before the photoresistor’s resistance starts changing in reaction to light. The bandwidth limit is not specified at all.