I made one of those and the light came on instantly. Their response time is a matter of milliseconds.

 

Doesn't sound like you made an opto-coupler using them (like we're talking about) - CDS cells are SLOW - it's commonly known, and always has been.

You weren't using it for anything where speed is an issue.

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that doesn't change the fact that their repsonse time is milliseconds.
You're talking a couple hundred milliseconds at the very most.

 

Exactly - incredibly slow - electronics works in picoseconds or nanoseconds, 100's of milliseconds is so slow it's not moving at all!

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Sure, slow in electronics terms, but not in human terms but
not perceivable in human terms.

 

Not perceivable? - 100's of milliseconds are EASILY perceivable - 500 milliseconds is half a second, I could punch you faster than that!

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I made a counter with a LDR and laser and uC. No matter how fast I waved my hand back and forth through the laser, no count was missed. That's about 4 counts a second at least.

 

hi Colin,
If you want to measure the 'fastest' rate, switch the laser ON/OFF with a pulse generator. Thats if its not a 'soft start' laser module.

PS: watch out, Nigel is a very fast puncher.

Its all relative anyway.!

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Four counts a second is 250 milliseconds each, like I keep telling you, this is INCREDIBLY slow in electronics - you're also only switching, you're not doing a linear transfer. If you were talking four MILLION counts per second, that's still not considered fast.

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We've established that. It's nothing to do with the point I'm making.
LDRs changing resistance is not perceivable.

 

So what is the point you're making? - we've all said LDR's are slow devices - you've then seemed to have suggested that they aren't, but quoted an exceptionally slow speed for them (disproving your own point).

What are we missing?.

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We're arguing whether the reponse time of an LDR is perceivable. You then spun it into an argument of whether LDRs are fast or slow compared to other electronic devices.

 

I think an LDR is fast enough to count great-grandmothers crawling along.

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The speed of components varies quite widely and there are other factors like the current flowing through the cell and the threashold of the comparator, i.e, it takes much longer for the CdS cell to change from 10M to 100R than it does for it to change from 100k to 10k. The response is probably exponential, like a low pass filter.

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It's clearly perceivable, or this thread wouldn't be here!

It's slowness is 'usually' an advantage, the "opto-coupler" type device in question is usually used for audio AGC or modulation effects in audio systems. The slow response of the LDR makes it perfect for doing this, and a method using a phototransistor requires special consideration to make it operate at a similar slow speed.

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