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I have a photoresistor, R1, and as a voltage divider a fixed resistance, R2. There is a voltage of 5V over these two, and the voltage is measured over the photoresistor.
In my experiment I have a yellow LED on one side, a substance which I add blue color into, and the photoresistor on the other side. How do I decide what R2 to choose so that I can detect small changes in light (when increasing the opacity of the substance)?
Best Answer
This is a voltage divider circuit, sometimes incorrectly called a bridge. The one fixed known resistor in series with the unknown resistance produces a voltage that is a function of the unknown resistance all the way from zero to infinity. In theory, any resistance can be measured.
However, the sensitivity is greatest when the output voltage is in the middle of its range. This is when the unknown resistance is close to the fixed resistance. Therefore, use a fixed resistor (R2) that matches the photoresistor value in the middle of your typical experiment. You can find this by disconnecting the photoresistor from the circuit and measuring it with a ohmmeter. Or, you can pick different fixed resistors for R2 until you get about 2.5 V out for your typical setup.
Is the photoresistor really only 100 Ω when used in your experiment? That's rather low. 2.5 V across this 100 Ω draws 25 mA and causes 63 mW dissipation in the photoresistor. That shouldn't be a problem by itself, but will cause some self-heating, which probably changes the resistance a bit. This means the resistance will change quickly due to light, then more slowly due to different dissipation at the different light level.
Photoresistors come in many different resistance ranges. I would look for something more like 1 kΩ at the intended light level.