# Electronic Circuit: Dark Activated LED light with photoresistor

Dear friends welcome back! In this video we learn how to build a dark activated light.When the light disappears the the LED goes on. It is a very easy project, so let’s start! The parts needed in order to
build this project are these: a 2N3904 NPN transistor a photoresistor, we need a 2K resistor a 100Ω resistor for the LED and ofcourse a bright LED. We also need 4 AA batteries in order to power the project. Of course this project can work with other parts as well. For example you can use any NPN transistor you like and another combination of resistors and input voltage. Before connecting all the parts together let’s see how the transistor works in this
particular project. In this project the transistor works as a digital switch. If we push a small amount of current through its base, the middle leg the transistor switches on so a big amount of current can flow from this leg to this leg, from collector to emitter. So what we
want to do is to turn on the transistor when it is
dark. When the transistor turns on current can flow through it and the LED turns on. If there is no current to transistor’s
middle leg the transistor is off, so it won’t let current flow through it so the LED is off as well. We are going to use the photoresistor in order to turn the transistor
on or off. Now let’s connect all the parts together.
We connect all the parts according to the circuit diagram. First we connect the transistor. This is the collector, the base and the emmitter. According to the diagram we connect the collector to 5V so we connect it to breadboard +. Next we connect the 2K resistor to breadboard + as well, according to the diagram. You can replace it with a trimpot if you like. Next we connect the photoresistor, from the base, to ground. Next we connect the 100Ω resistor for the particular LED that we are using. The long leg of the LED connects to the resistor and the
other one to Ground. Now we connect power and as you can see the project is working. If I cover the photoresistor with my hand, the LED goes on and when I remove it the LED goes off. Let’s now see in detail how the circuit works. When it is day the photoresistor has a very low resistance lower than the base-emmiter resistance of the transistor. so the current when it has to paths to
choose from it will choose the path with the least resistance. In this case it will choose the path from the photoresistor. Only a tiny amount of current will go
through the transistor, less than the value required to turn
the transistor on. So it will remain off in the LED will remain off as well. If it is night, the photoresistor, has a huge resistance. So the current will try to avoid it. So it will choose the path through the transistor, which now has less resistance than the photoresistor. the amount of current that these flowing
through the base of the transistor is enough to turn the transistor on, so a big amount of current will flow from collector to emitter and that
current flow will turn the LED on. As you can see with the use of a few parts we can build a quite interesting device. If
you understand how this project works you have gained a basic understanding of
how transistors work as well I believe that true knowledge only comes by doing, so I encourage you to build this
project if you want to better understand transistors and photoresistors. If this is your first time here I would love to have you subscribed, In this channel I post DIY hardware and software projects every Saturday. I believe that knowledge and knowhow must be shared to as many people as
possible. Together we can make impressive things.By combining our skillesets we can impove our lives.I encourage you to join our community, and post your ideas in the comments
section below. I would love to hear you. The next video will be posted online at
the first Saturday or September. I am taking a few weeks off. So, I wish you the best and I will see you in September with more exciting projects. Bye!

## 54 thoughts on “Electronic Circuit: Dark Activated LED light with photoresistor”

1. srul76 says:

Thanks, it is great example, simple to understand, please keep creating new videos… 🙂

2. GnobarEl says:

Great Video! Keep posting more videos please.

3. Regi Pavan says:

I am always watching your nice vídeos, please keep making them… Have a good vacation!

4. Raccoon Replica says:

These videos are really helpful and nice small projects to explore the world of electronics.

5. MKme Lab says:

Another great tutorial!  Cheers

6. Dog Rox says:

I totally agree with the rest of the commenters. a simple but yet very good tutorial explaining the concept of transistors and current flow. Good job!! oh, and have a great vacation! 🙂

7. Dimitris Vlamis says:

Καλησπερα.! Μπορω να χρησιμοποιησω το 2N2222? Νομιζω ειναι πανομοιοτυπα αυτα.

8. cpobyrne1 says:

nicely explained – thanks.

9. Kamal Aziz says:

nice project! with best explain…
how can we make same project but with 12v battery? we use mosfet or just n transistor?

10. Santosh Shetty says:

hi

i want to make LED flash using a 20 or a 30 watt w LED…. i wish to have to trigger the LED ON/OFF using a phototransistor as a trigger….can u suggest a circuit…. (am trying to build a simple LED FLASH for cheap diy photography)

11. Danny Martin says:

Great tutorial I had tried several other web based tutorials but your's seemed to be the one that worked for me, got the project up and running in no time. Can you please demonstrate how to add addition LEDs in either series or parallel?Thanks

12. Vincent Wiedermann says:

Sloud it work with bc547?

13. Sudhanshu Sekhar Jha says:

What changes do I need to make in ratings of the components if I want to use a 10W LED???

14. Baren Mohanlal says:

My led turns on when there is light present and turns off when no light is present.. what is the problem?

15. DjLennox Shaon says:

can I use 9V battery??

16. Mo Do says:

That's loved this video got it straight away thanks

17. S.M. Bahauddin Bakhtiar says:

Excellent Tutorial. I finally successfully implement Thanks a lot. I search all the net but not work any implementation but your tutorial is finally worked. Thanks again. Now i have a question that how can i connect PIR sensor with this diagram. Because when dark come then pir sensor activate the light . So how can i do this.

18. Savita Dhiman says:

in series or parallel

19. νταν νταν says:

your enlish is verry charm and beautiful …btw can i use 9v batery ?

20. Pedro P says:

Thanks for sharing your knowledge. I swicthed the 2.2k resistor for a 200k trimmer resistor. When the trimmer is on its maximum there is't current to the base and the led don't light up. But when the triimmer is on its minimum the led is always light up. The trimmer controls the sensibility of the circuit. Near the led I put a 470 ohm resistor. Here's my scheme: https://s16.postimg.org/ny5jso12t/circuit.png

21. Tayyab Hasan says:

how does i control the sensitivity?

22. ManNexusX says:

how could you add a pot to make the led dimmable

23. ABC 123 says:

Thank you!

24. Drashti Modasara says:

Thanks a lot!!!! I had a lot of trouble with all the connections. But after watching this video, I could make the circuit and now my project is actually working. Thanks for your help! 🙂

25. Pratik Shelke says:

can i use more than 1 led and what resistor would i need if i use 9v battery?

26. Dipan Patel says:

Thank you so much. Your explanation, Diagram and video is very accurate and working. Thank you.

27. Pratik Shelke says:

can i add buzzer to it?

28. Atik Mahbub says:

Not working 🙁

29. Ain Ramlan says:

hi!
when i connect the component, the LED already turn on without/with me covering the LDR. So, any suggestions?

30. Claudio Nelli says:

Thank you, Learned so much

31. Kombiice says:

A switch? That black thing?

32. Savaş Yatmaz says:

oreo binteo!

33. Steffen Grund says:

Danke die Skizze mit den Pfeilen erklären das super. Kein anderes Video bei YouTube erklärt es besser.^^

34. vignesh lingineni says:

not working waste

35. HCL Service Center in Mumbai says:

Hello Everyone !!!! What a awesome video, i like it
Bol7 Have Updated Automatic Day Night Switch, Light Sensor Switch Services. ✌️
you can ping me on skype id : HEMANT_136 , 💐👌 or whtsapp : 91 7838888 080
OR You Can Check My Website :BOL7 on google🙂🙂🙂💯🙂🙂🙂thanks

36. Alisa Shell says:

Don't waste your time. This circuit does NOT work.

37. Illiminatie overlord Gurglekoks says:

I'm only just getting into this stuff and this video is very helpful! Thank you so much for taking the time to make it! 🙂

38. Sameer Khan says:

hey! what should i use to light up 10 leds using ldr?

39. Mchael Wagner says:

so what if I want to do the opposite? what if I want the led to be on when it is light or what I really want to do is power a solenoid with a laser pointer.

40. Aryan Maurya says:

Is it necessary to use the resistors and the transistor

41. RodebertX says:

What is the Power consumption when it's sunny/ Led off?

42. G RP says:

What would be needed to create a 120V system? and how would that circuit look?

43. sandim koushik says:

Thanks Sir! You saved my day!

44. Crazy Couple says:

Amazing

45. Jimi Grant says:

Hi, great vid, I would like the LED to go on when the LDR goes from dark to lit. Is this as simple as reversing the LDR?, thanks

46. Jhana says:

Great Lesson. Thans ! Like !

47. Stan Chion says:

Helpful and well explained- thank you.

48. Timo Möbes says:

Thank you so much for this wonderful video. I understand the electronic circuit now so much better! 🙂 can you tell me which program you use to draw the circuit on the computer? Thanks in advance and good luck with everything.

49. Amlan Chinya says:

If I use different npn transistor then will the setup of registances change?

Why is the LED not bright when using this circuit? If using a 1k trimpot with LED, LED is super bright, but with your circuit even with 100 ohm resistor from transistor to led its weak. Help please.

51. Steve Burch says:

What do I need to add or change to make this work with two LEDs?

52. Ed Coulbourne says:

CONGRATS finally I understand this circuit you taught a Brick something you are a very good Teacher Thx, Ed

53. Peter Retief says:

How do you calculate the resistor values if I used say 3.3V input?