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.

Step 1: 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 DigiKey.com. All quantities are single unless specified otherwise.

Parts:

• 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

Tools:

• Variable Resistor Trimmer Pen (or a small screwdriver)
• Needle-nose Pliers (not essential but useful for breadboarding)

Comments

[ssteet]

nnn

[rstark3]

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]

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.

[rstark3]

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]

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.

[rstark3]

ok, so the out put to the led would go to the relay to make it switch on and off. right?

[WolfKodi]

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.

[althepal12]

Can send full spec for the parts? Especially fro the LDR?

[WolfKodi]

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.

[Owen and Garrett]

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]

Yes, you can use transistors but if your clock really needs more direct power, use a relay switch instead.

[Diagram]

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]

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.

[zarinazats]

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]

Yes, you can use the single LM311N Comparator with 8 pins.

[powerman666]

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. http://electroschematics.com/5899/diy-ldr-switch-circuits/

[TobaTobias]

Doesn't it have 17 pins?
http://www.steren.com.mx/catalogo/prod.asp?p=2170

[zarinazats]

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]

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.

[muted0irony]

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]

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.

[Bednarz]

Hi there, sorry to be late to the questions asking party but...is 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]

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 jan]

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. ....

[Roger Bradfield]

hi - what changes would need to be made to make this work with 12v?

[WolfKodi]

No changes need.

[Roger Bradfield]

thanks

[zarinazats]

can I use 3362P-102-ND 10K Ohms Variable Resistor instead of 3362P-103-ND 10K Ohms Variable Resistor??

[WolfKodi]

Yes you can.

[monty324]

i made one at school with a trasistor, 3 resistors, an led, a ldr and a variable resistor.

[elq3qa3]

thanks, you can too use the transistor (PNP or NPN) whit small change , but your way is the best.

[Amar143institute]

cool Guy.thnks..////////////////////////

[matt.e.jenkins]

Very Cool, thanks for this.