You wan't do a dark sensor with a LDR, one transistor and one LED and you don't know how to do it? In this small tutorial I will explain to you how you can do that even if you know only the basics of electronic.

Parts you need:

1 Breadboard;
1 Power source of 12v;
1 Transistor (in this case we use the BC547);
1 Resistor of 1KOhm;
1 Resistor of 680KOhm;
1 LED (in this case blue);
1 LDR:
Some condutor cables to connect the parts.

Step 1: Connecting parts in the breadboard

In the above image we can see the circuit we will construct. It's a simple electronic circuit and if you know how to do something on a breadboard, you should be able to do that dark sensor.
<p>I wnt to use dc motor instead of led . wht hv I need to do ?</p>
<p>My motor is YAMAHA not a SUZUKI </p>
Good work... can you please give its working...means how it works
<p>In case of Breadboard we used General purpose board.</p><p>What is difference between </p><p>1 Resistor of 1KOhm;<br> 1 Resistor of 680KOhm;</p>
680 ohm is less than 1k ohm. R2 with ldr are in series so they gather and form total rezistanc eof roughly 680 ohm. The thing is that the less the rezistance the more current will flow. So when light is up on ldr its rezistance is low so R2 + R of Ldr will be 680 ohm , less than 1 k phm which means that current will flow to R2 and not R1
<p>hey that's not 680, its 680k</p>
I have a solar powered waterfall pump that I want to shut off when no power is coming from the solar panel. How could I hook this up to make that work?
<p>But it will automatically turn off if there is no current from solar panel right ?</p>
<p>What are the applications of this device ?</p><p>Please answer my question , I am making this for my project so I want the applications.</p>
<p>can we use laser light instead of normal light</p>
<p>Can i use BC 548 instead of BC 547</p>
<p>Bc 548 and BC547 are almost same</p>
<p>Can we use 2 resistors of resistance 1 Kilo ohms ???</p>
<p>i made a very very basic circuit with just a led, a battery and e ldr but the leds turns on when ther's light and turns off in the dark? what could i possibly done wrong?</p>
<p>In case of Breadboard we used General purpose board.</p><p>What is difference between </p><p>1 Resistor of 1KOhm;<br> 1 Resistor of 680KOhm;</p>
1st resistor is 1 ohm and 1k resistor is 1000 ohm. k means kilo, (1000)
Hi <br>What's the purpose of the transistor on the circuit and how does it function
<p>LED blow in dark and light both'....please help me out</p>
<p>LED blow in dark and light both'....please help me out</p>
<p>can you give an explanation for this as i need to present this in my project</p><p>thank you</p>
<p>Hi, how can I make a relay with the above circuit which can power a LED strip of 12v and 1A ? Please let me know if its possible. Thank you.</p>
<p>is it working on transducer principle? m confused.. plz help as soon as possible<br>thanks</p>
<p>ricardo silva u r a great and truth man , i am Bj from India , i was searching for this projects from a month and didn't got the perfect results and i lost hope that how will i pass in my school exams without the project . i was in depression i left the project thinking that i can't do this , my teachers refused to help me ......... but today just before a day of examinations i tried with Instructables and got success ............. thank ya man ......... obliged really .......... i did this with a 224ohm resistor with a 9v power supply</p>
<p>if i use 12v led or relay what resistor value i need to use??? Thanks in advance</p>
itz a nice project
<p>Thanx...its working great. made a security project with ur system :D :)</p>
Guys, I only have a 3.7v battery. what are the resistor values that i need to change with??
<p>how does this circuit work? can u explain pls?</p><p>also can i use this for my cbse project?</p>
<p>Working absolutely fine. Thank you! :)</p>
<p>i have make dark sensor circuit but when i connect the battery only light blows dark sensor is not working</p>
<p>in my crcuit only light is blowing dark sensor is not working</p>
<p>your video isn't available , or maybe it isn't loading correctly could you please help me out I really need to understand the circuit working</p>
Would I put a sensitivity trimpot on the LDR or the R2 ?
<p>In series with R2.</p>
<p>i have done everything but its working as light sensor rather then a dark sensor any idea why ???</p>
<p>You've got R2 and the LDR swapped in your circuit.</p>
why he use Resistor of 680KOhm; ? tell me plz if we use 330ohm Resistor rhen circui will work?
As is usually the case, I forgot an important part of my question until after I walked away from my computer. It should be: I have a solar powered waterfall pump that is hooked up to a battery and solar panel. I want the pump to shut off when there is no power coming from the solar panel to charge the battery. How could I hook this up to make that work?

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Bio: 21 years old electronics student who loves all the things about electricity.
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