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. 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! 🙂

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

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

  4. 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)

  5. 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

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

  7. 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.

  8. 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

  9. 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! 🙂

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

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

  12. Hello Everyone !!!! What a awesome video, i like it
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  13. 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! 🙂

  14. 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.

  15. 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

  16. 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.

  17. 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.

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

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