LDR Recovery time - Page 1
Forum section: electronics / projects

author: reflowman
author_rank: Contributor
timestamp: May 14, 2018, 11:20:17 am
content: I'm using an LDR (G5528) in series with a 100k pot to detect my electricity meter's led flashes. (1 flash per Wh)It works fine but I wanted to roughly calculate the max. flash detection rate / power consumption limit.At 1kW consumption the calculated period is 3.6S. <-- oodles of recovery time, assuming flash length is small (100mS - guess?)At 10kW consumption the calculated period is 360mS. <-- abs. max. for my house. 260mS recovery time. Probably OK?. Some LDRs are hideously slow to recover though?At 24kW consumption the calculated period is 150mS. <-- abs. max. for meter. 50mS recovery time. Maybe OK?.I couldn't find much info. on this G5528 to guesstimate response times, lux levels, bleed currents etc. and I can't get test kit to the device in actual operation. (I also don't know the all important length of a flash.)All gut reactions and informed opinions gratefully received.

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2018, 11:28:54 am
content: LDRs tend to be slow. An oscilloscope, signal generator and LED can be used to determine how slow.If you need more speed use a photodiode, which can be an ordinary LED.

author: Benta
author_rank: Super Contributor
timestamp: May 14, 2018, 03:11:16 pm
content: I'd suggest a phototransistor. Much easier.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 14, 2018, 04:36:14 pm
content: LDRs are slow. However the response is not just a single time constant like in an RC decay. It is a mixture of fast and slow processes. So the "full" recovery to 0.1% might take minutes, but one could still see ms time scale flashes with a reasonable amplitude.The actual length of the LED flash would not make a big difference for the LDR.A phototransistor is likely the better choice, especially if the LED is read, as LDRs are not that sensitive to red - there sensitivity is best with green.

author: reflowman
author_rank: Contributor
timestamp: May 14, 2018, 10:13:31 pm
content: Thanks for the replies everyone. Appreciated.Quote from: Kleinstein on May 14, 2018, 04:36:14 pmThanks for that, I had no idea. (The LED is red. Some time ago I tried using the same LDR for detecting the green LED on my boiler. Itseemedless sensitive but I was comparing apples with oranges as there is a plastic window in front of the LED which probably cut the light down considerably.)

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2018, 07:49:10 am
content: Green LEDs, especially the old yellowish-green variety, tend to be less efficient than red LEDs. LDRs have a fairly matched spectral response to the human eye, so it it appears brighter to you, then the LDR will also register it as brighter.How well is it isolated from ambient light? Since it's only the change in brightness, i.e. the rising and falling edges, you're interested in, then a high pass filter may be a good idea.

author: nfmax
author_rank: Super Contributor
timestamp: May 15, 2018, 08:04:43 am
content: I made a similar optical pickup to read my meter LED flashes a while back. I used an SFH300 phototransistor, with a 1M load resistance to a +24 supply, coupled via a 4n7 capacitor to an OPAMP Schmidt trigger, designed for about 0.2V hysteresis. It's been working happily for almost 6 years now. The phototransistor is mounted in a plastic 'joining block', rescued from an old kitchen unit, which happened to have a hole of just the right size, held onto the meter with double-sided sticky tape. The rest of the circuit is in a plastic box 30cm or so away. The phototransistor is right in front of the LED. It still sees some ambient light when the meter cupboard is opened, but the AC coupling + Schmidt combination renders this harmless (provided it's not so bright as to saturate the phototransistor). I tested it to up to 20Hz, which is more than enough for my needs.