Instructables

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
aarjav jain12 days ago

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

aarjav jain12 days ago

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

fuegoarctis2 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

oliverkellow11 months ago
Would I put a sensitivity trimpot on the LDR or the R2 ?

In series with R2.

aaamir23 months 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.

flightdream126 months ago
why he use Resistor of 680KOhm; ? tell me plz if we use 330ohm Resistor rhen circui will work?
lilypadhoper9 months ago
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?
lilypadhoper9 months ago
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?
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