Output of light intensity monitoring system

I have completed my first project successfully.
But i have a doubt on the output which i am getting, whether right or wrong according to the connections.
connection - 1 : LDR_ Ao , GND. RESISTOR_Ao,3v3.
Result : light intensity increases, resistance value decreases.
connection - 2 : LDR_ Ao , 3v3. RESISTOR_Ao,GND.
Result : light intensity increases, resistance value increases.
can anyone please clarify it.


That’s a great question. I’ll help you understand the basics in a simple way.

You know that LDR stands for Light Dependent Resistor, so depending on the light(photons), the resistivity decreases. (More light, more conductive)

Now, the circuit we building by making the following connections is also known as a voltage divider circuit. Usually we use 2 resistors to make the circuit and provide our desired amount of voltage and input to the Vout side.
In our case, we are using one of them as the LDR. Refer to the below circuit.


Now, you can imagine replacing Vin with (3v3) and Vo as A0.

In your second case,

You’ll notice that the connection looks exactly the same.

Here, when more photons are detected, the supply voltage(3v3) to the A0 is increased. Hence the value increases. Note: The value of A0 is increased, not resistance. Here, light is the measure we are ready, to know about the resistance, calculation is, resistance = max value - value.

In your first case,

You need to replace the GND and Vin. That is GND is connected with LDR and Vin with a resistor.

In this case, when more photons are detected, current passes through the GND, limiting the 3v3 supply to the A0. And value decreases. Again, its not the Resistance that you are reading, it is the value in terms of amount of light.

Let me know if you have any other queries.


Thank you so much sir for the clear explanation.
now I got clear idea sir.