Simple Light Detector with Sensitivity Control

Picture of Simple Light Detector with Sensitivity Control
Light detectors are one of the most popular sensor and they are commonly found in many real-world applications. They are widely used by electronic hobbyists and projects because they are practical and intriguing yet surprising easy to construct. This instructable will guide and show you how easy it is to breadboard your own light activated Light Emitting Diode (LED) with minimal tools and materials. The whole project is simple enough for beginners and should take at most 10 minutes to construct. This implementation can be used for an educational demo or applied directly to the practical world.
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Step 1: Gather Parts and Tools

Picture of Gather Parts and Tools
The following are the list of materials and tools required for this project. These are widely available and can be easily obtained from sites like All quantities are single unless specified otherwise.

  • Breadboard
  • A few Jumper Wires
  • 9 Volts Battery
  • 9 Volts Battery Clip
  • Light Dependent Resistor (LDR)
  • Light Emitting Diode (LED) with any color of choice
  • TLC3704 Quad Comparator (only one of its four comparators will be used) (Alternatively, you can use the single LM311N Comparator with 8pin)
  • 3362P-103-ND 10K Ohms Variable Resistor
  • 1K Ohms (Brown-Black-Red) Resistor X2
  • 330 Ohms (Orange-Orange-Brown) Resistor
  • Variable Resistor Trimmer Pen (or a small screwdriver)
  • Needle-nose Pliers (not essential but useful for breadboarding)

Step 2: Understand How It Works

Picture of Understand How It Works
The schematic diagram for the circuit is given in the picture above.

Like its name suggests, a comparator compares two given voltages. The pair of 1K ohms resistors create a voltage divider and provide a 4.5 volts reference for the comparator. The variable resistor and LDR both form another pair for a second voltage divider. When light falls on the LDR, its resistance lowers and that voltage divider provides a voltage lower than 4.5 volts. The comparator produces no output (0 volts). When light is absent, the resistance of the LDR and the voltage increases. When the voltage increases over 4.5, the comparator activates its output and supplies 9 volts to power the LED.

An interactive flash animation of how the circuit works can be accessed by clicking here or on the file below.
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danial24943 months ago

what is that conclusion?

aayushsoni0074 months ago

any way to make it more simple ??

anishshah097 months ago

Hi, I want to purchase a light detector which is capable of detecting lights while the detector is in motion at 120km/hr.

Any comments?

anishshah097 months ago


I am planning to install a light detector on fast moving cars at approximately 120km/hr. The signal from the detector will than be fed into a PLC.

Could you please recommend any pre-made sensors?

seedling20009 months ago

can we use 3296X-103 10K Ohm variable resistor instead of one mentione? if yes, please tell me the way it should be attached on the breadboard because all of its three pins are in a single line.... thnx

vomm9 months ago

HI There the circuit worked very well for me but the Diode is not completely off during bright light.

The negative output voltage from the comparator is probably giving that small light

vomm9 months ago
jbates1311 months ago
I can not get this thing to work! I followed the instructions no dice,I am using a Radio Shack quadcomparator LM339 could that make a difference! Thank You
ssteet2 years ago
mouse00792 years ago
hi there, I would like to make this but frist I need to know what all to use for 120V as I would have it come on at sun rise and of at sun set and it would turn on the grow lamps (fluorescent bulbs) in my green house, this way I can take out the timer. any and all info would be nice. Thanks
WolfKodi (author)  mouse00792 years ago
You'll need a relay rated for switching 120 voltages. And you might want to hook the power supply to an adapter instead of using a 9V battery to save the trouble of replacing them when they run out.
Also I'm lost on this part " simply just exchange the positions of the Variable Resistor and the LDR " <--- in making the circuit operate in reverse . Sorry but this new to me and I would like to get it right the first time so I need to know what pin gose where being the Variable Resistor has 3 and the ldr has 2.
WolfKodi (author)  mouse00792 years ago
We are only using two pins on the Variable Resistor. The center pin is a common terminal and connecting the other terminal to either of the two side pins create the same variable resistor. The only difference is in the direction you rotate the screw to achieve similar resistance. Thus, you should swipe the same two pins we are using for the Variable Resistor with the 2 pins of the LDR. Both devices have no polarity so it does not matter which side you connect them.
ok, so the out put to the led would go to the relay to make it switch on and off. right?
WolfKodi (author)  mouse00792 years ago
Yup, replace the LED with the relay. You do not need the resistor just before the LED too depending on the input voltage rating of your relay.
althepal122 years ago
Can send full spec for the parts? Especially fro the LDR?
WolfKodi (author)  althepal122 years ago
These parts and pretty standard and generally interchangeable between manufacturers. The picture showing the parts should be a good indicator. As long as the LDR has two pins and has that orangy-red snake-shaped pattern on top, it should be fine.
Hello - This looks like a great circuit. I've built something similar but without the IC Comparator. I am trying to figure out how to use the output signal (LED) voltage to turn on a clock. Most small AA battery operated clocks I have found need exposure to the actual battery voltage to operate properly. So I would like to use the output signal of my circuit to essentially switch the battery voltage on and off to the clock in the presence of light. Is there a way this can be done using transistors?
WolfKodi (author)  Owen and Garrett2 years ago
Yes, you can use transistors but if your clock really needs more direct power, use a relay switch instead.
Diagram2 years ago
I have a LM311P comparator that looks just the same as the N version, but it won't turn on even though I followed the same step for the LM311N. Anything else I'm missing, or is the P version incompatible or needs to be adjusted?

Thanks for the ible though, it's great for beginners like me!
WolfKodi (author)  Diagram2 years ago
I have not actually tested it with LM311P but looking at the specs, they are indeed the same. P supports slightly higher switching speeds and lower operating voltages.
zarinazats2 years ago
I have another question, I couldn't find the other part for this project, which is TLC3704 Quad Comparator...where did you find it? I searched everywhere, and unfortunately I couldn't find it, is it possible if there's a replacement for that? or is it okay if I will not include it?? please reply asap
WolfKodi (author)  zarinazats2 years ago
Yes, you can use the single LM311N Comparator with 8 pins.
I did this with a LM311N but for some reason I could only get the LED to light up by using a 2N222 Transistor and reversed the LED. I looked at this schematic to help me figure out how to hook it up.
Doesn't it have 17 pins?
I have another question, if I will use the LM311N Comparator, should I place it on the 6th to 9th column?? I'm sorry if I ask too much, because I will do this on my Physics project and I'm still in high school so I really don't know how to work these things
WolfKodi (author)  zarinazats2 years ago
Yes, place it on the 6th to 9th column. But you've got to rewire the circuit. Look at the last step. I've added an extra image. Compare both of them for changes. Note the LM311N's Notch location.
muted0irony2 years ago
Hi, I have absolutely no background in this - but I would like to ask: Would this device be able to detect a change in light over a large surface area - for example window glass? (Change in light meaning - when the window is dirty, it lets in less light; will this device be able to detect relatively minute changes in light?) I do hope you'll reply! Thanks!
WolfKodi (author)  muted0irony2 years ago
I cannot see how this is feasible for your "window-dirt-detection" application. The changes in cloud cover and day light intensity varies too much.
Bednarz2 years ago
Hi there, sorry to be late to the questions asking party there a reason you went with the 9V battery? I'm a little rusty on my electronics, but I'm pretty sure that the LED only needs a couple volts. Does the comparator need a higher voltage for activation? I'm thinking of building something like this, but was planning to use a couple AA's rather than a 9V.
WolfKodi (author)  Bednarz2 years ago
Nope, you can you at least 2 AA batteries as the minimum supply voltage required by the comparator is 3V. I simply find using a single 9V batteries handier compared to a 2 AA batteries with the bulky holder. If you're using only 2 AA batteries, lose the resistor in series with the LED.
musaib jan2 years ago
hey buddy.. i really do appreciate your work.. this is going to be my first year project... i could not use TLC3704 but of its unavailing... so em using LM311N... but em really confuse with the last picture you post on ur instructable with LM311N... i will really appreciate you if you explain this to me. ....
hi - what changes would need to be made to make this work with 12v?
WolfKodi (author)  Roger Bradfield2 years ago
No changes need.
zarinazats2 years ago
can I use 3362P-102-ND 10K Ohms Variable Resistor instead of 3362P-103-ND 10K Ohms Variable Resistor??
WolfKodi (author)  zarinazats2 years ago
Yes you can.
monty3243 years ago
i made one at school with a trasistor, 3 resistors, an led, a ldr and a variable resistor.
elq3qa33 years ago
thanks, you can too use the transistor (PNP or NPN) whit small change , but your way is the best.
cool Guy.thnks..////////////////////////
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