I am using an LDR + LED in a light encapsulated enclosure. It generates changes in resistance as the LED change intensity. The question is, how do I make the changes to be faster? I think that the LDR takes a while to change its resistance value.

Thanks,

 

A photo-transistor is much faster than an LDR.
A photo-diode is faster than a photo-transistor.

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Can I generate changes in resistance with any of those devices?

Going back to the LDR+LED, can I make the resistance changes go faster?

Thanks,

 

Photo-diodes and photo-transistors vary the current as the light intensity changes and photo-resistors just change the the current.

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No, an LDR is a slow device - Audioguru has suggested faster methods.

How fast do you want anyway?.

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Even though LDRs are slower than photodiodes, LDRs are quite fast in human terms.

 

Oh no they're not, the change in resistance is perceivable - it takes a second or two for a CdS cell to go from its dar resistacne to light resistance.

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Hi prprog,

Perhaps if you tell us what the device is being used for someone can offer some more insight into what might a good solution.

As Nigel asked, how fast do you want it to be? You said you want it to be "faster" but how fast is fast enough, and what are your criteria?


Torben

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How do you know this? It takes about that time for a multimeter to register the change alright but it certainly does not take that long.

 

I've used a/an LDR for audio ranges (modulating a laser and using an LDR as the receiver) and it works OK, but that's about the limit. For smaller changes in resistance, the LDR is OK at audio. If you are talking about extreme changes in resistance, it is slow. The LDR is interesting in that it does have one of the largest dynamic ranges of any electronic component, having a range from a few tens of ohms in intense light to several hundred megohms in total darkness. It's difficult to construct a light-tight box with experimenter-type breadboard access where you can use an LDR in its most-high-resistance mode. It can detect the tiniest light leak ....

Dean

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Thanks to all.

How fast? Well I cannot give a fixed or determine time. This is for an electronic musical instrument, so it more about that for "slow musical passage" it works fine but for fast musical sections it is left behind.

Sorry I cannot be more specific. I will read more about phototransistors and photodiode for faster devices.

Thanks again.

 

OK. One thing you might want to look into is using a JFET as a voltage-controlled resistor: http://freespace.virgin.net/ljmayes.mal/comp/vcr.htm (there are lots of other pages on the net on that topic).

Do you need the resistance of the device to be fairly linear?


Torben

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Thanks for the web site reading...but to dense for me. Still on the basics. Sorry.

No it does not have to be linear...Few month ago I try and failed to used a MPF102 (this is the FET I have ) it need a negative voltage to operate which I did not undertood how to generate - (I am still undertanding the term "less positive"). Can you suggest a circuit that I can re-try with the MPF102?

Thanks,

 

Sorry, I'm not really that experienced with FETs so I wouldn't be able to easily suggest anything for you.

How are you sensing the resistance change in the LDR?

You said you don't need a linear response--so it's OK if the thing just turns on or off in response to a certain light level? If so then I'd recommend going with the earlier suggestions of a phototransistor or photodiode. If you search the forum for "beam break" or "dark alarm" you should find lots of examples of how to do that.


Torben

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Comparators are much faster than DVMs.

One of the first projects I built was a comparator that switched a light on when it got dark. It used a CdS cell and there was a noticeable delay between the cell being covered and the light turning on.

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