Using an Arduino with a Sensor Input

An Arduino can be used to do many varied tasks. In this example we will be setting it up to read an input from a sensor.
We will use a Light Dependent Resistor (LDR) as the sensor but many other types of sensors can be used in this way.
Here we use the Analogue Input side of the Arduino so it is a good idea to identify this part first (see Fig 1) and identify an LDR (Fig 2).

You now need to think about how to connect your LDR to the Arduino and to program it.
Fig 3 shows how to setup a breadboard to allow an LDR to power an LED.

Fig 3

We are going to use a 10k resistor along with the LDR to create a voltage divider circuit. The varying resistance of the LDR is converted to a varying voltage that the analogue pin of the Arduino will then use.
Connect the A0 pin of the Arduino to the same column where the LDR and resistor is connected (Since the LDR gives out an analogue voltage, it is connected to the analogue input pin on the Arduino. The Arduino, with its built-in Analogue to Digital Converter (ADC), converts the analogue voltage from 0-5V to a digital value in the range of 0-1023. - Now connect the other end of the 10K resistor to the negative rail and the second (free) leg of the LDR to the positive rail.

We will now produce a digital program to get the LDR to turn the LED on when it goes dark (Fig 4).We need to remember that we have connected the LDR to Analogue Input A0 and the LED to Digital Output 13. If we change this, we must also change these numbers to suit the input and output channel used.
We first of all set a constant value to read the Input from the circuit as indicated by ‘const int’
We then tell the Arduino which pins are Inputs and which are outputs ‘pinMode’
As we are using an analogue sensor we have to tell the Arduino to read an Analogue signal as the converter inside the Arduino will give the value a number dependent upon the voltage going through the sensor. We will tell the Arduino that if it reads a number <200 to switch the LED on (go HIGH) . If it reads any number above that figure it will tell the LED to switch off (go LOW).

Fig 4

Use the program shown in Fig 4 and shown below in text format, test the circuit by covering the LDR – if the circuit is working the LED should come on when it goes dark.

You may wish to change the sensitivity of the circuit by altering the ldrStatus number from <200.Experiment with the program and see what it does.

Could you add other inputs or outputs to this circuit?

const int ledPin = 13;

const int ldrPin = A0;

void setup() {

pinMode(ledPin, OUTPUT);

pinMode(ldrPin, INPUT);

}

void loop() {

int ldrStatus = analogRead(ldrPin);

if (ldrStatus <= 200) {

digitalWrite(ledPin, HIGH);

} else {

digitalWrite(ledPin, LOW);

}

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