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Electric Eye

Electric Eye

Introduction: (Initial Observation)

When electric power was first used to light up streets at night, someone in each street became in charge of turning on the light in the evening and turn it back off in the morning. Today electric eyes automatically turn on the lights when it becomes dark. Electric eyes are used for security in the banks, shopping malls, warehouses and industrial buildings. They are used for safety of workers and automatically stop machinery if the worker takes his hand toward sharp blades.

Electric eyes are used to measure the amount of Iron, sodium, Potassium and many other elements in food and blood. It is used to measure thickness, color and heat. And finally what you heard is just a small portion of uses and applications of electric eyes. With countless applications of electric eyes in our lives, we are wondering what is inside an electric eye and how does it work?


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.  

Project advisor

Adults help and supervision is required for this project.

This is a multi-level project for ages 11 to 16. In selecting experiments go as far as you can understand.

Information Gathering:

Find out about electric eyes. Read books, magazines or ask professionals who might know in order to learn about electric eyes and how they work. Keep track of where you got your information from.

How does electric eye work?

Electric eye is a light sensitive electronic device that can produce or modify electrical currents based on the presence or absence of light. In other words an electric eye does not really see any thing; instead in will sense the light and based on the amount of light or presence/absence of light it will create or modify an electric signal. The main part of an electric eye is a photoelectric cell. What is a photoelectric cell?

photoelectric cell or photocell is a device whose electrical characteristics (e.g., current, voltage, or resistance) vary when light is incident upon it. The most common type of photocells are photo-resistors. A battery or other voltage source connected to the electrodes of a photo resistor sets up a current even in the absence of light; when light strikes the light sensitive section of the photocell, the current in the circuit increases by an amount proportional to the intensity of the light.

Since the current from a photocell can easily be used to operate switches or relays, it is often used in light-actuated counters, automatic door openers, and intrusion alarms. Photocells in such devices are popularly known as electric eyes.

Photoelectric devices can be classified according to how they react when they are struck by light. They may be photo emissive, photoconductive, or photovoltaic.

Photo emissive devices such as phototransistors act like a light sensitive switch. The switch will close when it receives a beam of light.

Photoconductive devices such as photo-resistors change their conductivity or resistance based on the amount of light.

Photovoltaic devices also known as solar cells or photo cells produce electricity when exposed to light.

All of the above three methods may be used in electric eyes.


A typical electric eye system includes a light source that produce a straight light beam. The light beam is focused on a light sensor or photoelectric device that is placed in the opposite side. When something or someone cuts the light beam, the electrical changes in the light sensor will activate a switch that in turn can be connected to an alarm or perform any other function.

Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to learn about the principles of an electric eye and perform experiments to see how can we use electric eye for different applications.

After learning about electric eye you can select any of these additional questions for more experiments and extended project.

  1. Effect of light on the resistance of a photocell
  2. How an electric eye is made?
  3. How can we use an electric eye to measure the concentration of a colored solution?
  4. How can we use electric eye to automate a street light?
  5. How can we use electric eye to sound a siren in case of an intrusion?
  6. How different color lights affect an electric eye? (Is electric eye more sensitive to some colors than others?)

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

In an electric eye made of a cadmium sulfide photocell (also called photo resistor) following is how we define variables.

Independent variables (also known as manipulated variable) is the amount of light getting to the photocell.

Dependent variable is the electrical resistance of photocell.

Constants are size and model of photocell and the multi-meter used to measure its resistance.

Controlled variable is temperature (because I think temperature may affect the sensitivity of photocell). Controlling temperature means make sure that you will not have a large temperature change on photocell during your experiments.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

If this is going to be a research and display project, you will not need to propose a hypothesis. If you are focusing your project on any specific question related to electric eye, then write your hypothesis.

Sample hypothesis for question number 1 is:

The resistance of a photocell reduces when it is exposed to light. My hypothesis is based on the information that I gathered about cadmium sulfide photocell.

Sample hypothesis for question number 6 is:

Electric eyes are more sensitive to cold lights with more electrochemical energy such as UV and blue. They are less sensitive to warm lights such as red and infrared. My hypothesis is based on the fact that many photocells are made of light sensitive chemicals such as cadmium sulfide and lights that can accelerate chemical reactions can possibly have more effect on photocells as well.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: Effect of light on the resistance of a photocell

Introduction: Cadmium sulfide photocells are selected for electric eyes because their electrical resistance changes and such changes can be detected by a variety of electronic circuits. The simplest electronic instrument that can detect and show changes in resistance is a multi-meter, set to work as an Ohm meter. Ohm is the unit of electrical resistance. Ohm sign is like a horseshoe, so if your multi-meter does not show the world Ohms, look for the Ohm sign. In this experiment we will measure and record the electrical resistance of a photocell in the dark and in different light intensities.


  1. Get a multimeter and set it to measure resistance.
  2. Connect the probes of the multimeter to the legs of a cadmium sulfide photocell
  3. Move your hand before the photocell and see the changes in resistance.
  4. Try different setting of resistance in your meter to find the one that is more sensitive to the changes of light on your photocell. In such setting, needle of multimeter moves more when you change the light on photocell. Use this setting for your display.
  5. If you are doing this project as an experimental project and require results table and graph, continue the following steps. Otherwise stop here.


Cover the photocell with a dark cloth to make sure it gets no light. Measure and record the resistance of photocell in the dark.

6.Uncover the photocell and measure the resistance of photocell in room light.

7.Take the photocell and meter outside and measure the resistance in daylight.

8.Move the photocell to the sunlight and record the resistance again.

Record your results in a table like this:

Light Condition Resistance
Room light

10.Use the above results table to make a bar graph. Each bar represents one light condition. The height of each bar represents the resistance. Below each bar write the light condition. Above each bar write the actual resistance of photocell for that light condition.

Experiment 2: Make a Chemical Photoelectric Cell

A photoelectric cell is a device that generates electric current when it is exposed to light. These devices find application in cameras, security systems and televisions. The latest televisions use photoelectric cells to adjust their brightness and contrast automatically so that the viewer gets the best picture quality. Automatic cameras use light cells to determine the amount of light available and bring the camera flash into use automatically when the light is low.
Let us now try and make a chemical photoelectric cell at home.

1. A small beaker of 250 ml
2. A small copper and a small lead plate of equal sizes.
3. A sensitive digital multimeter or galvanometer
4. Nitric Acid
5. Lead Nitrate
6. Table Lamp
7. Wire
8. Bunsen Burner

1. Heat the copper plate over the burner using a pair of tongs till the copper plate becomes coated with copper oxide and turns black in color.
2. Cool this plate and then insert it into the nitric acid. After some time you will notice that a red layer of cuprous oxide is formed.
3. Make a solution of lead nitrate in water in the beaker
4. Insert the lead and copper plates into this beaker
5. Connect these two plates to the multimeter or galvanometer using the connecting wire.
6. Put the table lamp on and focus the light on the copper plate, the galvanometer will show a deflection. When the light is switched off the galvanometer needle comes back to normal. This indicates that electric current is generated when light is incident on cuprous oxide.


Experiment 3:

In this experiment you will use a cadmium sulfide to make a simple light meter. Your light meter can be used to compare light in two different places.


  1. Set your multi-meter to Ohms so it can measure the resistance.
  2. Connect the probes to the pins of a cadmium sulfide photocell and read the resistance.
  3. Now you have a light meter. Use it to measure and record the resistance of your photocell with different amounts of light on photocell.
  4. Record the lowest resistance and the highest resistance of your photocell. Usually you will have high resistance in the dark and low resistance under the sunlight.
  5. Measure the light of your room and compare it to outside or other rooms.

For example the resistance in a dark room is usually a very large number. we estimate it to be 20,000 ohms for our photocell. The resistance under the sun however is a very small number (about 50 Ohms).

Experiment 4: (How much light is there?)

In this experiment you will build a device that turns on a light bulb if the photocell is exposed to the light. This system has many applications. You can use it to see if there is any light inside a device such as a furnace. Flame creates light so your device can detect flame. In many heating devices such system is used to automatically disconnect the fuel if for some reason the flame goes off. Such a mechanism contributes to the safety of heating devices.

You may think that such device can simply be built by connecting a light bulb in sequence with the battery and photocell. But this will not work because photocell limits the current in the circuit, so the electric current will not be enough to turn on the light bulb. You will need to use an amplifier or transistor for this device.

Transistors are kind of switches. Emitter will get connected to Collector if we apply a small electric charge on the base.



  1. Get a photocell and a general purpose NPN transistor such as BC337-25 or PN2222A.
  2. Identify the 3 pins of your transistor (emitter, base, collector)
  3. Connect the emitter pin of the transistor to the negative pole of a 6 volt battery.
  4. Connect the collector pin of the transistor to a light bulb and then to the positive pole of the same battery.
  5. Connect another wire from the base of the transistor to the photocell and then to the positive pole of the battery.

Picture on the right shows this setup. Red wire is connected to the positive pole of the 6 volts battery.

White wire goes to the negative pole of the 6 volts battery.

3 pins of the transistor are identified by letters E=emitter, B=base and C=collector.

Here we cover the photocell by a black plastic cap, so the light goes off.



In the dark electrons from the negative pole of the battery get to the emitter (E), but they can not pass through the transistor to get to collector (C). So the circuit will remain open and the light stays off.

In the light, a small positive charge goes through the photocell and gets to the base (B) and that small charge will make the transistor to pass electrons from E to C to build a closed circuit and the light goes on.


As you see the transistor is like a switch. E will be connected to C if a small charge is applied on the base.

We used a board and some screws to connect wires and components.




Experiment 5: (Remote Control)

In this experiment you will upgrade the light sensor that you made in previous experiment to make a device that can remotely get activated and turn on the light bulb.

You will use a laser pointer to aim a laser beam to the photocell mounted in a black tube.

Use black tube to reduce the interference of other lights.


Move the photocell so it will be faced to one side. Make sure that the two wires of the photocell are not touching each other. Use a black plastic tube to cover the photocell so only a focused straight light beam can get to the photocell.

As shown in the picture we will use the plastic cap of a pen for this purpose. Plastic cap is closed in one end, but we used a cutting tool and opened the closed end.




Insert the tube and make sure that the light is off. In our experiment the light bulb did not turn off until we closed the back end of the tube using some black tape.




Now place your device about 20 feet away and use a laser pointer to aim the black tube of the light sensor. If your laser beam hits the tube, the light will go on.




Experiment 6:

In this experiment you measure the concentration of a solution such as copper sulfate solution. You will be given a sample of copper sulfate solution and you need to find out it’s concentration (percentage of copper sulfate in water). The device or setup that you assemble for this activity is called a colorimeter or spectrophotometer. The idea of using a colorimeter to measure the concentration of solutions is based on a simple fact. When the amount of a colored substance in water increases, more light will be absorbed by the solution and less light will get to the electric eye. Variations on the amount of light that gets to the electric eye can be detected by a multi-meter.


For this activity, the colorimeter measures the absorbance of light through solutions of known concentration to determine the relationship between the absorbance and the concentration. Then, the colorimeter measures the absorbance of light through a solution of unknown concentration. Records absorbance and concentration data in a table and possibly draw a graph. Analysis of the plot of absorbance versus concentration shows the concentration of the unknown solution.


You can use the light meter or electric eye that you made in the previous experiment as a colorimeter. As you see the colored substance must be placed between the light source and the electric eye.

In the above setup we first used a regular light bulb to see if the variation in the color of solution will affect the light. But it did not work well, so we also placed an ammeter (Ampere meter) in the circuit. Some of the problems with this setup are as follows:

  1. A round glass jar is not suitable as a container for color solution because it can cause lots of refraction. A rectangular prism container can be a good choice for this purpose because a light beam can pass right through it without refraction.
  2. White color is not a good choice for light source because only part of it’s spectrum will be absorbed and the rest pass through. Depending on the color of the solution we must find a color light that will be absorbed the most.
  3. Flash light can not produce a parallel light beam. A laser light may be a good choice.
  4. Outside light will reduce the accuracy of our colorimeter device. Whole device must be covered by a black box to stop the interference of outside light.

How to setup a colorimeter?

Place your solution in a clear container and place it between a light source and the light meter. Place an ammeter in the circuit. construct your setup in a way that outside light does not get to your photocell.





Materials and Equipment:

List of material can be extracted from the experiment section.

Usually you can buy an assortment of cadmium sulfide photocells in one pack. Prices are about 10 cents up to 25 cents per photocell that comes in a pack.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


If you do any calculation write the results at this section.

Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


List of References

How Does Transistor Work?