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I am using LDR sensor in my project. The source of light comes from the different colour of images. As we know, the spectrum of each colour is different and it will produce a different intensity of light. I know, the colour sensor able to do this task but it's not listed as options.

Here I attached the schematics and LDR datasheet as the reference (http://kennarar.vma.is/thor/v2011/vgr402/ldr.pdf)

enter image description here

My question is how to increase the sensitivity of this sensor so that it is able to measure the change of resistance within 200ms?

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    \$\begingroup\$ and, your question is too broad and unclear. Are you building an LDR, are you using one? If the latter, which one in which way in which schematic? \$\endgroup\$ – Marcus Müller Nov 2 '17 at 7:08
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    \$\begingroup\$ Please add a schematic diagram to your question and a link to the datasheet for the LDR and someone will help find the response time specifications. \$\endgroup\$ – Transistor Nov 2 '17 at 7:27
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    \$\begingroup\$ 200ms is about all you will get at low light levels. You can use a lens to increase the illumination level for the sensor or take a look at phototransitors [easiest solution] or photodiodes [more difficult to implement but even faster]. Most phototransistors react within a couple of microseconds, carefully designed photodiode detectors can react in the sub nano-second realm. \$\endgroup\$ – Dean Franks Nov 2 '17 at 10:56
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    \$\begingroup\$ If you want speed, an LDR seems an odd choice. \$\endgroup\$ – Dampmaskin Nov 2 '17 at 11:26
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    \$\begingroup\$ LDRs are slow. Period. If you want fast, you have to go with a photodiode or phototransistor. \$\endgroup\$ – JRE Nov 2 '17 at 11:27
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My question is how to increase the sensitivity of this sensor

You can't. The response of the sensor itself is what it is. You would have to modify the sensor to change that.

You can, however, amplify the output of the sensor.

so that it is able to measure the change of resistance within 200ms?

Now you are asking about response time. That is completely independent of sensitivity. Amplifying the output of the sensor will at best not increase the response time in a meaningful way.

You are using a CdS light-dependent resistor. Those have good sensitivity, but are quite slow. Depending on the low and high light levels, it might be possible to get a response in 200 ms. For example, if the change in light is essentially dark to direct sunlight, then it should be possible to detect that in much less than 200 ms. If you are looking for subtle changes, especially with both levels being quite dim, 200 ms may not be achievable. You'll have to experiment to see what you can do.

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