# Troubleshooting of plant monitoring

just collected data values …was getting the same issue of having constant 1thousand value .the issue was resolved by replacing 10k resistor with 300 ohm resistor but i am not able to understand that why this happened?please explain in depth what happens internally when we replace 10k with 300 ohm resistor?

Hi,
For this,some of the light dependent resistor they have INTERNAL RESISTOR for this we must not use high value resistor …this is the reason for your issue

Hi,
The output voltage of an ldr is usually calculated as follows:
Rldr=500/Lux, or
Lux=500/Rldr (in kOhm)
// Note: Light is measured in Lux, Rldr- inbuilt resistance inside ldr
and
Vout=5 * Rldr/(10+Rldr), for a 10k Ohm resistor
Vout=5*Rldr/(0.3+Rldr), for a 300 Ohm resistor

//5 is the input voltage

As you can see, the Vout for a 10k Ohm resistor does not show high variations to changes in light intensity while a 300 Ohm resistor can show significant changes in Vout for slight changes in Light Intensity. That’s why the ldr was showing constant 1k values for a 10k Ohm resistor.

Hope this helps you

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@harshinisivanathbabu answer is true, but there is something that I would like to add to it.

Basically, the A0 pin takes the 10-bit voltage input at the junction, meaning, the binary voltage data is taken in the range of 2^10 i.e. 1024 values. Hence, the voltage data shown in the readings are in milli-Volts with a range of 0 - 1024 mV.

The calculations below will make it easier to understand

Here, for the 10k resistor, you will need a minimum of about 33000-10000 = 23000 ohm of internal resistance by the LDR to get proper readings i.e. below 1.024 V, but it will surely be above 2V, because of which you will see the readings as 1.024 only, and also it will not be sensitive to changes in resistance of the LDR.
While, for the 330 ohm resistor, a minimum of about 1089-330 = 759 ohm internal resistance is sufficient to show proper readings. The LDR maybe has resistance in this range, hence this setup works.

Hope this helps.

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Thanks for the insight.