Picture of Dark Sensor with LDR, transistor and a LED
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.
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Step 1: Connecting parts in the breadboard

Picture of 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.

Step 2: Connect everything

Picture of Connect everything
Just do the circuit like this and all should be working fine.

Remember, the LDR resistance depends on the light you have on the room or where you are constructing that. If you have light, the LDR resistance is low so, the LED should be turned off. When you obstruct the light that focus on the LDR, the LED should light up. 

If for any reason your LED is always lighted up, try to chance the transistor. Switch the Emissor and the Collector. If the problem persist, increase the resistor you have in the base.

Step 3: All should be working

Picture of All should be working
Connect the power source you have to the breadboard. The positive pole in the red line and the negative in the blue line. 

Try your circuit and see what happens.

Here's the video of my circuit working:

If you have any questions feel free to ask in the coments. If you think I should improve something, let me know. It's my first guide so it's normal I made some mistakes.

Kind regards,
Ricardo Silva
SiddharthK10 days ago

Can we use 2 resistors of resistance 1 Kilo ohms ???

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?

WalidM28 days ago

I wnt to use dc motor instead of led . wht hv I need to do ?

NanasoS1 month ago

In case of Breadboard we used General purpose board.

What is difference between

1 Resistor of 1KOhm;
1 Resistor of 680KOhm;

Imetomi NanasoS29 days ago
1st resistor is 1 ohm and 1k resistor is 1000 ohm. k means kilo, (1000)
NanasoS1 month ago

In case of Breadboard we used General purpose board.

What is difference between

1 Resistor of 1KOhm;
1 Resistor of 680KOhm;

What's the purpose of the transistor on the circuit and how does it function

Can i use BC 548 instead of BC 547

LED blow in dark and light both'....please help me out

LED blow in dark and light both'....please help me out

RiyaJ3 months ago

can you give an explanation for this as i need to present this in my project

thank you

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.

is it working on transducer principle? m confused.. plz help as soon as possible

bjoshi3 made it!4 months ago

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

normelb4 months ago

if i use 12v led or relay what resistor value i need to use??? Thanks in advance

itz a nice project
Rohan_Xperiment made it!5 months ago

Thanx...its working great. made a security project with ur system :D :)

Guys, I only have a 3.7v battery. what are the resistor values that i need to change with??
HariniS7 months ago

how does this circuit work? can u explain pls?

also can i use this for my cbse project?

davkjw7 months ago

Working absolutely fine. Thank you! :)

aarjav jain9 months ago

i have make dark sensor circuit but when i connect the battery only light blows dark sensor is not working

aarjav jain9 months ago

in my crcuit only light is blowing dark sensor is not working

fuegoarctis12 months ago

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

Would I put a sensitivity trimpot on the LDR or the R2 ?

In series with R2.

aaamir21 year ago

i have done everything but its working as light sensor rather then a dark sensor any idea why ???

You've got R2 and the LDR swapped in your circuit.

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?
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?