LDR sensor 5

berlin subway metro at night

Inputs and outputs: LDR sensor

Turn a led on when ambient light is low

What are we going to do?

In this activity you'll write some code that turn a led on o off when ambient light is low. We will achieve this thanks to a LDR sensor brick or LDR circuit.

You can use the code from Arduino IDE -> File -> Examples -> Digital -> Button but you'll need to change some parts.

UNO board

You can use any Arduino UNO original or compatible board

USB cable

Depending on your board, you'll need a microUSB cable or a miniUSB cable


You need a computer with Arduino IDE software. If you don't have it, follow this link with instructions to download and install it.

Download Arduino IDE

LDR sensor brick

A simple LDR sensor brick with SVG connections

LED brick

A monocolor LED brick with integrated resistors and SVG connections

Let's get started

Follow these instructions carefully. You can click on some of the images to zoom in.

Always connect the components BEFORE powering the UNO board or YOU MAY DAMAGE YOUR BOARD OR COMPONENTS.


1. Connect your components

Zoom in

2. Open Arduino IDE

Once you've opened the editor, open the button example from File->Examples->Digital->Button

3. Connect your board

Connect your board with USB cable and wait a bit until you computer recognises it. Now select the correct serial port from Tools->Serial port->(choose correct port) COM1, 2, 3...

Click to zoom

3. Change code

We'll need to modify the button example to make it more suitable to our circuit.

  • Change buttonPin variable name for ldrSensorPin on all the variable references on your code
  • Change ledPin = 13 variable value (which is 13 on the example) to suit your connections
  • Change buttonState variable name for sensorState on all the variable references on your code
4. Upload to the board

Use the upload button to transfer the program to your board. Once done, you should read Upload completed. That means the transfer was succesful. If it's not succesful, try disconnecting your board and connecting it again and repeating the upload process.

5. Final result

After uploading, everything should be working as planned. Otherwise, double check connections and code.

Congratulations, you've done it! You're on the right path to be a maker! 🙂

Now, what else?

Now is your turn:

  • Think about how would you modify the code so the sensor acts differently depending on the ambient light.
  • Do you have any real life application for this kind of circuit?

 Continue your journey with the next activity from below


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