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Many analog transducers convert the physical quantity they are measuring into resistance.  For example, a Light Dependent Resistor (LDR), as the name implies, is a resistor whose resistance is a function of the amount of light falling on it. There are other transducers too, for example, a thermistor whose resistance is a function of temperature.

We usually use a voltage divider to convert the resistance into a voltage, which can then be read in by Arduino.

Voltage Divider

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Image Added 'out' should be connected to one of the Analog input pins (A0-A5). Here, we use A0.	Image Added where  Vcc = 5V for Arduino To Do : Derive the above. You can assume that the current Iout is negligible.

Connecting LDR

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You have two options for choosing R2. This has an implication on the threshold you set in your program for Exercise 3 Solution.

Option 1 : Use a 10K resistor.

Option 2 : Cover the LDR completely. Measure the resistance (Rdark) using multimeter.

Expose the LDR to light. Measure the resistance (Rbright) using multimeter

Choose R2 = (Rdark + Rbright)/2

analogRead()

Arduino uses a 10-bit A/D converter. The input voltage has to be given to one of the pins A0-A5.

A 10-bit converter means that you get input values from 0 to 1023.

0 V → 0

5 V → 1023

Ex:

C++	Blocks
Code Block int sensorValue = analogRead(A0);	Image Added In Tinkercad, we just need Start Simulation, click on the LDR and move the slider to change the amount of light falling on the LDR.  Image Added