This document describes the measurement and use of a light dependent resistor (LDR). An LDR is a light-sensitive device whose resistivity changes with light intensity. It works on the principle of photoconductivity and is made of semiconductor materials. LDRs are commonly used as light sensors in applications like street lamps, alarms, and light meters to detect the presence or absence of light. The document provides details on connecting an LDR module to a Raspberry Pi along with Python code to measure the LDR's resistance and determine if it is detecting light or dark.

1. MEASUREMENTS
LIGHT DEPENDENT RESISTOR
By: E NANDANA PRIYANKA
ABSTRACT:
A Light Dependent Resistor (LDR) or a photo resistor is a device whose resistivity is a
function of the incident electromagnetic radiation. Hence, they are light sensitive devices.
They are also called as photo conductors, photo conductive cells or simply photocells. They
are made up of semiconductor materials having high resistance. There are many different
symbols used to indicate a LDR, one of the most commonly used symbol is shown in the
figure below. The arrow indicates light falling on it.
WORKING PRINCIPLE OF LDR:
A light dependent resistor works on the principle of photo conductivity.
APPLICATIONS OF LDR:
LDR’s have low cost and simple structure. They are often used as light sensors. They are
used when there is a need to detect absences or presences of light like in a camera light meter.
Used in street lamps, alarm clock, burglar alarm circuits, light intensity meters, for counting
the packages moving on a conveyor belt, etc.
DIAGRAM:

2. 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 to use it.
Required
 Raspberry Pi
 8 GB SD Card or Micro SD Card if you’re using a Raspberry Pi 2 or B+
 Ethernet Cord or Wifi dongle
 Light sensor (LDR Sensor)
 BreadBoard
 Resistor
Optional
 Raspberry Pi Case
 USB Keyboard
 USB Mouse
 GPIO Breakout Kit
 Breadboard
 Breadboard Wire
CONNECT THE MODULE TO THE RASPBERRY PI:
This light sensor module has 3 wires: VCC, GND and Signal. You can use dupont wires to
connect it to the GPIO pins on Raspberry Pi (A/B/B+).
Raspberry Pi Light Sensor Module
3.3v P1 —————————– VCC (V)
GND P6 —————————- GND (G)
GPIO4 P7 —————————– SIGNAL (S)

3. COMPONENTS USED:
BREAD BOARD:
A breadboard is a solderless device for temporary prototype with electronics and test circuit
designs. Most electronic components in electronic circuits can be interconnected by inserting
their leads or terminals into the holes and then making connections through wires where
appropriate. The breadboard has strips of metal underneath the board and connect the holes on
the top of the board. The metal strips are laid out as shown below. Note that the top and bottom
rows of holes are connected horizontally and split in the middle while the remaining holes are
connected vertically.
LDR:
LDRs or Light Dependent Resistors are very useful especially in light/dark sensor
circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000
000 ohms, but when they are illuminated with light resistance drops dramatically.

4. LED:
LEDs are semiconductor diodes, electronic devices that permit current to flow in only one
direction. The diode is formed by bringing two slightly different materials together to form a
PN junction (Figure 2). In a PN junction, the P side contains excess positive charge ("holes,"
indicating the absence of electrons) while the N side contains excess negative charge
(electrons).
RESISTOR:
A resistor is an electrical component that limits or regulates the flow of electrical current in an
electronic circuit. Resistors can also be used to provide a specific voltage for an active device
such as a transistor.

5. RASPBERRY PI:
The Raspberry pi is a single computer board with credit card size, that can be used for many
tasks that your computer does, like games, word processing, spreadsheets and also to play HD
video.
• The raspberry pi comes in two models, they are model A and model B. The main
difference between model A and model B is USB port.
• Model A board will consume less power and that does not include an Ethernet port.
But, the model B board includes an Ethernet port and designed in china..
• Essential hardware specifications of raspberry pi board mainly include SD card
containing Linux OS, US keyboard, monitor, power supply and video cable.
• Optional hardware specifications include USB mouse, powered USB hub, case,
internet connection, the Model A or B: USB WiFi adaptor is used and internet
connection to Model B is LAN cab

6. CONSTRUCTION:
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 able to detect whether it is dark
or light. In the light this sensor will have a resistance of only a few hundred ohms whilst 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 whilst 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.

8. We’re now ready to move onto the Python code,
PHYTHON CODE:
import RPi.GPIO as GPIO
import time
GPIO.setmode(GPIO.BOARD)
#define the pin that goes to the circuit
pin_to_circuit = 7
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)

9. #Count until the pin goes high
while (GPIO.input(pin_to_circuit) == GPIO.LOW):
count += 1
return count
#Catch when script is interupted, cleanup correctly
try:
# Main loop
while True:
print rc_time(pin_to_circuit)
except KeyboardInterrupt:
pass
finally:
GPIO.cleanup()

10. 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
whilst I was writing up this tutorial.
 Light Activated Alarm – We can use the LDR to detect when it starts to get light so you can
sound an alarm to wake up. If your program & sensor is accurate enough, you could have it
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.
CONCLUSION:
Hence, the LIGHT DEPENDENT RESISTOR project is done using
RASPBERRY PI equipment.