Tired of biting into raw fruit?

I will measure how a photoresistor's resistance changes when I illuminate different color surfaces with a red LED, and then use that information to build a circuit that can differentiate between red and green surfaces
Harshita Chokkalingam
Grade 8


If the colour of the object matches the colour of the light shined at it, then the object will reflect the light into the photoresistor, reducing resistance which triggers the second LED light to turn on thus differentiating the coloured object.



Fruit can come in all shapes and sizes, making it hard to tell if it is ripe or not. Colours are one way you can tell if a fruit or vegetable is ready to be eaten. When we go to stores to buy fruits we usually pick out fruits that are almost ripe. Stores usually reject raw fruits because customers don't buy unripe fruit. Have you ever thought about how farmers predict exactly when a fruit is ready to be harvested yet? It takes a lot of time just to handpick all these fruits, let alone having to pick all the ripened fruits among the many fruits on the farm. Farmers face many challenges throughout the growing season including not being able to tell if a fruit or vegetable is ripe or not. Over the years different technologies and methods using different fields of science have been made in order to identify if a fruit is ripe or not. The food industry today uses many methods to test ripeness. One is to measure firmness using penetrometers and sclerometers. Both devices rely on measuring the force required to insert a probe into the fruit. And another test in use today called the starch test measures ripeness from the fruit’s starch content. To do so, the fruit is cut open and sprayed with iodine which then reacts with the starch content in the fruit, making the fruit turn black (a visual indication of ripeness). Unfortunately, all these methods are indecisive as they spoil the fruit in the process. 

Recently a device called a spectrometer was made that measures if a fruit is ripe or not. This device can evaluate how ripe a fruit is by measuring the glow of chlorophyll in the fruit’s skin under UV light. Unfortunately, this device is very expensive ranging from around $150-$250.  I will be making a cheaper circuit ranging around $30 similar to this, except my circuit will be measuring the amount of light being reflected off of the fruit’s skin when visible light is shined on it.

What is light and visible light? Light is what makes vision possible. It is a form of energy that enables us to see our surroundings. Light is both a particle and a wave and it can also be described as a stream of photons, massless packets of energy, each travelling at the speed of  299,792,458 m per second. Visible light is a small part of the electromagnetic spectrum that human eyes are sensitive to and can detect. They are the only electromagnetic waves we can see. We see these waves as the colours of the rainbow. Each colour has a different wavelength. Red has the longest wavelength, and violet has the shortest wavelength. When all the waves are seen together, they make white light.

Visible light can be reflected or absorbed by different surfaces. Objects appear in different colours because they absorb some colours and reflect other colours. The colours we see depend on what colours of light are being reflected back into our eyes. When we look at a surface that is illuminated by white light, some colours are absorbed by the surface, and we see the colour or colours that are bounced back. For example, objects that we see as red reflect mostly red light, and objects we see as black absorb all light. Let's say instead of illuminating the red object with white light we illuminated it with red light, what would happen then? That same red light would be reflected back! But what if you illuminated the red light on a green surface? What would happen then? The green surface would absorb the red light and our eyes would see it as black! I will be incorporating this discovery into my project. Another LED light should turn on when I shine a red light onto a red fruit (red pepper), and when I shine the red light onto a green fruit (green pepper) the other LED light shouldn't on.


Manipulated Variable:

-the colour of fruit(red and green)

Responding Variable:

-whether or not the second LED light will light on or not (differentiating red and green)

Controlled Variable:

-distance from LED light to the fruit 

-amount of light shown towards the fruit

-the same circuit used for both fruits

-amount of light in the surrounding environment 

     -Color of first LED light


1. Gather all materials

2.Assembling circuit

  • 1.Get solderless breadboard

2.Connect the 120Ω resistor from a (+) bus to F13

3.Connect clear red LED long leg to J13 and the short leg to J15 (direction matters)

4.Connect photoresistor to J16 and J18

5.Connect a jumper wire from a (-) bus to F15

6.Connect another jumper wire to F16 and leave the other side free

7.Connect another jumper wire to F18 and leave the other side free

8.Now set the multimeter to measure resistance in ohms

9.Connect the black probe to COM port and red probe to INPUT port. 

          10.Connect multimeter probes to jumper wires with alligator clips (red to red, and black to black)

          11.Connect battery pack to the solderless breadboard

3.Start testing to see if the resistance changes when different colours are put in front of LED light

  • 1.Make sure everything is working-battery pack is on, LED is on, and the multimeter is connected

2.Get a coloured construction paper and put it up against a cardboard box

3.Read and record resistance from the multimeter 

4.Repeat steps 2 and 3 for all colours

4. Adding on to the circuit so a light turns on when red produce is put in front of it

  • 1.disassemble all materials from breadboard except for the resistor [(+) bus to F13], clear LED light [J13 to J15], Photoresistor [J16 to J18], and jumper wire [(-) bus to F15]

2.Connect a jumper wire from a (+) bus to F16

3.Connect a jumper wire from a (-) bus to A20

4.Connect a jumper wire from E19 to F18

5.Connect a jumper wire from G18 to G25

6.Connect a jumper wire from a (-) bus to F27

7. Connect transistor pins in H25, H26, and H27 with the large metal tab facing to the left(direction matters)

8.Connect diffused yellow LED light long leg to E26 and short leg to F26(direction matters)

9.Connect second 120Ω resistor from a (+) bus to A26

10. Connect potentiometer pins in D18,D19,and D20

11.Lastly connect battery pack- black lead to a (-) bus and red lead to a (+) bus 

5.Now you will make some adjustments to get your circuit working properly

  • Adjust the circuit to get the second LED light to on only when red produce is in front of it

  • The resistance of the potentiometer: You can adjust the circuit's sensitivity to light by turning the knob clockwise or counterclockwise (The farther clockwise you turn it, the more light is required to turn on the diffused LED).

6. Testing 

  • 1.Get a red and green pepper and put it 1cm away from the LED light 

  • 2.Test to see if the LED light turns on only when the red pepper is shown to the circuit 3.Observe and record

  • 4. Do steps 2 and 3 for 2 more trials



Table 1: How much light each coloured contruction paper reflected back into the photoresistor when red light is passed through it {resistance~Ω}



Paper colour

Trial 1

Trial 2

Trial 3
















































Table 2: Whether or not the second LED light turned on

Colour of fruit

Trial 1

Trial 2

Trial 3




Table 1 shows the resistance measured by the multimeter when red light was passed through all coloured construction paper. Green had a higher resistance than red did. Green had an average resistance of 1.038Ω and red had an average resistance of 0.717Ω. When a photoresistor is exposed to less light, it results in a higher resistance whereas when a photoresistor is exposed to more light it has a lower resistance. In my case when the red light was illuminated on green construction paper, the surface mostly absorbed the red light, which resulted in a higher reading in resistance. When red light was illuminated onto red construction paper, the light was reflected off the surface, which resulted in a lower reading in resistance.

Table 2 shows whether or not the second LED light turned on when both red and green coloured peppers were put in front of the red LED light. It shows that when I put the red-coloured pepper in front of the red-coloured LED light the second LED light turned on for all 3 trials. When I put the green-coloured pepper in front of the red-coloured LED light the second LED light did not turn on for all three trials. The second LED light couldn't light up when the green pepper was put in front of the red LED light because the resistance was high, The red LED light will only light up with a certain amount of resistance. This is only possible with the potentiometer, which controlled the sensitivity to light in my circuit.  If there is a higher resistance the potentiometer won't allow the second LED light to turn on, whereas if there is a low resistance the potentiometer will allow the second LED light to turn on. 


Did my circuit that used light resistance and visible light identify the difference between red produce and green produce? I predicted that it would because if I shined a red light on a red object, the light would reflect off the object onto the photoresistor, then causing the resistance to decrease(triggering another LED light to turn on). I based my hypothesis on the idea that light can be absorbed or reflected depending on the surface, and the results indicate that my hypothesis is supported and successful! Because of the results of this experiment, I wonder if this same experiment would work if I altered the colour of light and colour of fruit so my circuit works on a variety of fruits.


My results can be used to quickly identify whether a piece of produce is ripe just by showing it in front of the LED. Such a machine can be useful in the daily life of a farmer to select, harvest, and sort the ripened fruit or vegetable quickly and efficiently.

Sources Of Error

-When testing, I kept the amount of ambient light as controlled as possible by doing the experiment at the same time of the day, but weather and season change could have influenced changes in my results.

-there are a number of shades in 1 colour, so the resistance of a shade could have been different for another shade of the same colour

-construction paper is usually rough so the light could have reflected off to different directions rather than directly into the photoresistor. (Light reflects from a smooth surface at the same angle as it hits the surface. For a rough surface, reflected light rays scatter in all directions.)

Physics for Kids: Behavior of Light as a Wave







She gave me a good understanding about light and the concepts of it

"Light reflects off certain surfaces differently" 

"the angle of incidence and  angle of reflection are not always the same for every surface"

"rough surfaces scatter light in all directions for example, scrumpled up tinfoil "


she gave me the idea of the project

"I could'nt tell which apples were ripe"

"mabye there are electronic devices farmers use to identify ripened fruits

"If its hard for us to pick out ripened fruits, imagine how hard it would be for farmers to pick them"


He looked around Calgary, bought and helped me get all the materials I needed for my experiment