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.

  • 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)
<p>That should have been &quot;in parallel&quot;!! So that when the reed closes it shunts the +ve pin of the comparator (pin 7) to earth. But does it need a resistor in series with the reed switch to limit any current (I assume the value would be about the same as the dark resistance of the LDR)?</p>
<p>I would like to modify the circuit with a proximity reed switch so that the LED does NOT come on (or goes OFF) when a magnet gets near the proximity switch. I think I can just put the reed switch in series with the LDR. Would that work?</p>
<p>I built the circuit, but used a 12v power supply (2 6 volt lithium batteries in series) and a LM311N instead of the TLC3704, which should work the same.</p><p>I also used 6 LEDS and a 100ohm resistor in series for the &quot;light&quot;.</p><p>I wired the LM311N correctly and used all the other specified components.</p><p>When I apply power, the output pin of the LM311N produces ~1.5 volts, no matter what the var.resistor is turned to, or with the lights off or on (LDR).</p><p>It should produce no voltage or the supply voltage, correct?</p><p>I tried another LM311N (I bought 5 of them), but same thing, about 1.5 volts.</p><p>I read on another site that pin 4 (Vee) should be grounded, but when I do that, I get no voltage from the output pin of the LM311N, adjusting var.resistor and lights on and off doesn't produce any voltage.</p><p>the LDR is a photocell with 16-33k ohm range.</p><p>The var. resistor is a 10K.</p><p>I tried adding a resistor to the Var. resistor side, but no difference.</p><p>I'm missing something, but can't figure it out.</p>
<p>great project</p><p>can i place your project on my website.</p><p>i'm working on a website which is related to electrical projects.</p><p>i also mention your name, link and other info.</p><p>plz reply</p>
<p>and how can i put more than 1 led light .</p>
<p>i have a project.. and this ones suits for it.., but i need to make a prototype., i will build a little house so i want the led to be the light bulb of the house so the wire of the led must be long., is there a way i can make it?. and i want to put the sensor in the side of the led.., and i want to just put the breadboard hidden somewhere inside the prototype house... how will i do that?</p>
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?<br><br>Thanks for the ible though, it's great for beginners like me!
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.
please tell me this projects short discription.
please tell me this prijects factionality
<p>Do you share the code?</p>
<p>Built this successfully in the light-on-&gt;LED on version. Would like to trigger a timer such as in <a href="http://electronicsclub.inf" rel="nofollow">http://electronicsclub.info/p_timer.htm</a> which then sets off an alarm to remind me I turned on my kitchen stove before whatever I cook becomes coal. Do not quite understand how to trigger the timer. Have built that circuit and it stays on timing, doesn't switch to bleeper. Can anybody help, please?</p>
Hey admin I'm not able to find the above said quad comparator but can I use LM311P comparator instead.
<p>Nice but i'd like to try something and here comes my train of questions. Can the LDR be replaced by any sensor? let us say a piezo disc? And can the led be replaced by a &quot;9v Power Off Time Delay Relay Circuit&quot;? If yes, the last thing is like to know is can you use this system in someway on a domestic 220v, 40w light? The purpose of this is making vibration-sensitive tiles to trigger the light to go on and shut back of after a few seconds.</p>
<p>Hi.. my led light is on but the ldr doesn't seems to work, it doesn't detect the sensor but the light just on all the time.. how to solve this problem?</p>
<p>now i have solved the problem of on and off.. but mine was when my hands get closer it will off but when my hands further it will on.. how can i do it the other way round?</p>
<p>is there any way to make this without the TLC3704 ?</p>
<p>is there any way to make this without the TLC3704 ?</p>
<p>what is that conclusion?</p>
<p>any way to make it more simple ??</p>
<p>Hi, I want to purchase a light detector which is capable of detecting lights while the detector is in motion at 120km/hr.</p><p>Any comments?</p>
<p>Hi,</p><p>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.</p><p>Could you please recommend any pre-made sensors?</p>
<p>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</p>
<p>HI There the circuit worked very well for me but the Diode is not completely off during bright light.</p><p>The negative output voltage from the comparator is probably giving that small light</p>
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
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
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 &quot; simply just exchange the positions of the Variable Resistor and the LDR &quot; &lt;--- 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.
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?
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.
Can send full spec for the parts? Especially fro the LDR?
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?
Yes, you can use transistors but if your clock really needs more direct power, use a relay switch instead.
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
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. http://electroschematics.com/5899/diy-ldr-switch-circuits/
Doesn't it have 17 pins? <br>http://www.steren.com.mx/catalogo/prod.asp?p=2170
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
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.
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!
I cannot see how this is feasible for your &quot;window-dirt-detection&quot; application. The changes in cloud cover and day light intensity varies too much.
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.
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.
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?
No changes need.

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