Dark Sensor With LDR, Transistor and a LED

Introduction: Dark Sensor With LDR, Transistor and a LED

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

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.

Step 2: 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

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

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59 Discussions

I am doing the same simple project but stuck in it since days. I am using bc 548 and have tried almost all the youtube tutorials showing diagrams which are closely similar with just different resistor values. But my led stays on in light and turns off in darkness plus the brightness of the led increases if light is put on the ldr. What am i doing wrong?

What's the reason to change the emiiter and collector since it's not working


2 years ago

Hi, How can I use a power supply of only 5v?

1 reply

you can use ordinary batteries

why only bc547 type of transistor is used why cant we try with anyother one?

use npn transister what you want it makes you job ,like 2n23904,2n2222,bc547,bc548,bc550,mbs8090,2n2369 ,etc

to check use protuse software

no more 680k is used use only 680k===10-50k;1k,or less than that

arduino with ldr

Hello. I just wanna ask, what are the use of every component in this circuit. Like why it has to be a 1k and 680k resistor? I need this for my finals. Thank you.

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?

1 reply

Is this can be used as study lamp or light house with some modification , basically I want to know how it can be modified for some other purpose, can you suggest some ? I do not have a good idea of all these things, just want learn and play, I am not from a electrical background .

1 reply

It works very nice. Though I'd like to know if there is any way to switch the LED on and off suddenly and not just gradually, depending the amount of light. Hope it makes sense.


what's the use of transistor in this actually???

1 reply

it acts like a switch (as in like a mechanical switch).

Uhgh. Can someone help me? Instead of LED, i want to use a motor(that dynamo thing.) Is it possible? Can u show me how?

how can i make this circuit more sensitive? please reply fast as much as u can becoz i have to give my model on 30 jan