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I have to explain limitations for a photoresistor/LDR . I've read the book, and searched online but find nothing.

Can I get help with this ? Are things that the sensor can NOT do, which other tools can do better? What are the limitations? How do the sensors' features set restrictions?

LDR datasheet

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  • \$\begingroup\$ Could you edit your question to provide a little more clarity? You mention an LDR, and then ask about a transistor, which is confusing. Also, ALL CAPS are not appreciated here, please use proper capitalization. \$\endgroup\$ – uint128_t May 28 '16 at 17:03
  • \$\begingroup\$ Also link to a data sheet of a typical device. \$\endgroup\$ – Andy aka May 28 '16 at 17:03
  • \$\begingroup\$ done! How sensors' features sets restrictions? \$\endgroup\$ – Sabaayaz May 28 '16 at 17:06
  • \$\begingroup\$ Well, it won't detect sound too well. Sounds like the sort of question where the answer was explained in class - its not a particularly good engineering question... \$\endgroup\$ – Sean Houlihane May 28 '16 at 17:51
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Good bits.

Their resistance varies over a wide range, many decades, from light to dark.

Their resistance is in a 'nice' range for detecting with meters, and the analogue inputs of Arduinos and PICs, so are very easy for a noob to use.

They are good linear resistors, so are useful in audio volume controls and for controlling the frequency of RC filters, you can't use photo diodes for audio.

Bad bits.

They are slow (at least compared to photo diodes), so they are OK for indicating light levels, but not OK for receiving optical communication.

They are strongly temperature dependent, so they are not OK for stable repeatable precision light level measurement.

Having said they are slow, they have a very, very slow 'tail' when increasing their resistance in the dark. Note the data sheet specifies the dark resistance after 10 seconds!

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  • \$\begingroup\$ I did not understand what u meant with: so they are OK for indicating light levels, but not OK for receiving optical communication. \$\endgroup\$ – Sabaayaz May 28 '16 at 18:03
  • \$\begingroup\$ with a response time of 10s of mS, you would have to signal verrrrrry verrrrrry slowly, there are no standard modems that run that slow. \$\endgroup\$ – Neil_UK May 28 '16 at 18:54
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While it's a little bit tricky to compare the specs of an LDR to those of another light-sensing device, such as a photodiode, phototransistor, or solar cell (because they are very different devices), I'll try and summarize the main differences here:

  • Response time: from the LDR datasheet, it has response times in the 10's of ms (20-40ms). Photodiodes and similar devices are sub 1us, often in the 100ns or faster range (much faster).

  • Spectral response: both LDRs and photodiodes (and similar) can be manufactured to have a wide variety of spectral responses. Some are very selective (a small band), and others measure wider bands (such as "ambient light sensors").

  • To measure an LDR, you need to measure it's resistance, which requires passing current through it to measure the voltage across it. It behaves linearly, which can be nice. Photodiodes can be used in two different modes, photovoltaic and photoconductive, and these have their own characteristic advantages and disadvantages. In photoconductive mode, which is commonly used, a photodiode is non-linear, which may be nice (with digital signals) or may be not nice (with analog signals).

This is a very broad question, and for more in depth study of the differences, Wikipedia would be a good place to start.

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Limitations of LDR: They require a few milliseconds or more to respond fully to the changes in light intensity. They will take few seconds to return to their normal dark resistance once light is removed. The sensitivity and resistance range of the LDRs will vary from one device to another.

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