Raspberry Pi Light Sensor
In this Raspberry Pi light sensor tutorial, I show you how to connect the photoresistor sensor up to the GPIO pins correctly. Lastly, I show you how it can be used in a simple python script, so you’re able to gather and use the data from it.

This photoresistor is yet another sensor that I will be looking at incorporating into future projects such as a light activated alarm clock.

I explain a bit further down each of the parts that I will be using in this circuit. Be sure to read up on it if you need more information on these. It is important to note that for this tutorial I am just using a simple photocell sensor while these are perfect for some tasks they might not be as accurate as you would like.

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If you want to see visually step by step on how to set up the light sensor circuit & code, then be sure to check out the video below. It contains almost everything the text version of this tutorial does. You can find text instructions & information right underneath the video.

Equipment:

You will need the following equipment to be able to complete this Raspberry Pi light sensor tutorial. You can do this without any breadboard gear, but I would highly recommend investing in some if you plan on doing a lot of circuitry work.

Recommended:

Raspberry Pi

Power Supply

SD Card or Micro SD Card if you’re using a Raspberry Pi 2, 3 or B+ (8gb+ is recommended)

Ethernet Cord or Wifi dongle

Light sensor (LDR Sensor)

1uF Capacitor

Optional:

Raspberry Pi Case

USB Keyboard

USB Mouse

GPIO Breakout Kit

Breadboard

Breadboard Wire

The Raspberry Pi Light Sensor Circuit

The circuit we are going to make for this tutorial is super simple and is great for anyone who is just starting out with circuitry.

The light dependent resistor or also known as the LDR sensor is the most important piece of equipment in our circuit (obviously). Without it, we wouldn’t be able to detect whether it is dark or light. In the light, this sensor will have a resistance of only a few hundred ohms while in the dark, it can have a resistance of several megohms.

The capacitor in our circuit is there, so we’re able to measure the resistance of the LDR sensor. A capacitor essentially acts like a battery charging up while receiving power and then discharging when no longer receiving power. Using this in series with the LDR, we can work out how much resistance the LDR is giving out thus whether it is light or dark.

To get the light sensor circuit built correctly follow the steps below or check out the circuit diagram right underneath the steps. In the following steps, I am referring to the physical numbers of the pins (Logical order).

1. First, connect pin #1 (3v3) to the positive rail on the breadboard.

2. Next, connect pin #6 (ground) to the ground rail on the breadboard.

3. Now place the LDR sensor onto the board and have a wire go from one end to the positive rail.

4. On the other side of the LDR sensor place a wire leading back to the Raspberry Pi. Hook this to pin #7.

5. Finally, place the capacitor from the wire to the negative rail on the breadboard. Make sure you have the negative pin of the capacitor in the negative rail.

We’re now ready to move onto the Python code. If you have any trouble with the circuit, refer to the diagram below.

Light Sensor Circuit

The Code

The code for this project is pretty simple and will tell us roughly whether it is light, shady or completely dark.

The biggest problem we face with this circuit is the fact that the Pi doesn’t have any analogue pins. They’re all digital, so we can’t accurately measure the variance in resistance on our input. This lack of analogue pins wasn’t a problem in the motion sensor tutorial since the output from it was either high or low (Digital). Instead, we will measure the time it takes for the capacitor to charge and send the pin high. This method is an easy but inaccurate way of telling whether it is light or dark.

I will briefly explain the Raspberry Pi light sensor code, and what it does, if you want the code you can copy and paste it below or download it from my GitHub.

To begin, we import the GPIO package that we will need so that we can communicate with the GPIO pins. We also import the time package, so we’re able to put the script to sleep for when we need to.

#!/usr/local/bin/python

import RPi.GPIO as GPIO
import time

We then set the GPIO mode to GPIO.BOARD this means all the numbering we use in this script will refer to the physical numbering of the pins. Since we only have one input/output pin we only need to set one variable. Set this variable to the number of the pin you have acting as the input/output pin.

GPIO.setmode(GPIO.BOARD)

#define the pin that goes to the circuit
pin_to_circuit = 7

Next, we have a function called rc_time that requires one parameter which is the pin number to the circuit. In this function we initialize a variable called count, we will return this value once the pin goes to high.

We then set our pin to act as an output and then set it to low, we then have the script sleep for 10ms.

After this, we then set the pin to become input and then we enter a while loop. We stay in this loop until the pin goes to high, this is when the capacitor charges to about 3/4. Once it goes high, we return the count value to the main function.  You can use this value to turn on and off an LED, activate something else or log the data and keep statistics on any variance in light.

def rc_time (pin_to_circuit):
    count = 0
  
    #Output on the pin for 
    GPIO.setup(pin_to_circuit, GPIO.OUT)
    GPIO.output(pin_to_circuit, GPIO.LOW)
    time.sleep(0.1)

    #Change the pin back to input
    GPIO.setup(pin_to_circuit, GPIO.IN)
  
    #Count until the pin goes high
    while (GPIO.input(pin_to_circuit) == GPIO.LOW):
        count += 1

    return count

#Catch when script is interrupted, cleanup correctly
try:
    # Main loop
    while True:
        print rc_time(pin_to_circuit)
except KeyboardInterrupt:
    pass
finally:
    GPIO.cleanup()

Deploying & Running the Code on your Raspberry Pi

This step is incredibly easy, but I will quickly go through the steps so you can have it up and running on your Pi as quickly and smoothly as possible. Like all the tutorials on this website, I am using Raspbian if you need help on getting Raspbian installed then check out my guide to installing Raspbian.

While all the software packages should already be installed in some cases, it may not be. If you want to learn more about the GPIO pins and how to update and install the software, then be sure to check out my tutorial on setting up the GPIO pins on the Raspberry Pi.

You can download the code by using git clone. The following command will do exactly just that:

git clone https://github.com/pimylifeup/Light_Sensor/
cd ./Light_Sensor

Alternatively, you can copy and paste the code just make sure the file is a python script.

sudo nano light_sensor.py

Once you’re done in the file, simply use ctrl x then y to save and exit.

Finally, run the code by using the following command:

sudo python light_sensor.py

I hope you now have the script working and you’re receiving data that correctly reflects the changes in light on the sensor. If you’re having trouble, please don’t hesitate to leave a comment below.

Improving Accuracy & Possible Uses

There are countless uses for a light sensor in a circuit. I will just name a few that I thought of while I was writing up this tutorial.

  • Light Activated Alarm – I mentioned this one earlier, but you can use the LDR to detect when it starts to get light so you can sound an alarm to wake up. If the program and sensor are accurate, then you can the alarm slowly get louder as it gets lighter.
  • Garden monitor – A light sensor could be used in a garden to check how much sun a certain area of the garden is getting. This could be useful information if you’re planting something that needs lots of sun or vice versa.
  • Room Monitor – Want to make sure lights are always turned off in a certain room? You could use this to alert you whenever light is detected where it shouldn’t be.

There is so much you can do with this cool little sensor but also remember if you require something a little more accurate than the photocell, then look at something like the Adafruit high dynamic range digital light sensor.

I hope you have been able to set up this Raspberry Pi light sensor without any issues. If you do come across a problem, have feedback, I have missed something or anything else you would like to say then feel free to drop a comment below.

Credit: This tutorial is based on Adafruit’s Resistor Sensor tutorial.

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