We will discuss LDR darkness sensor – LED OFF as light falls on LDR and Turn ON in the dark here, it has a previous part which is LED ON as Light falls on LDR and Turn OFF in the Dark



As we can see the output of bridge is fed into an op-amp. The output of terminal A fed to inverting (-) input of op-amp and output of terminal B fed to non-inverting (+) input of op-amp.

We know as light falls on LDR its resistance starts decreasing corresponding to intensity of light and in the dark resistance increases, we know current will follow low resistive path. So, if light falls on LDR it has low resistance means more current will pass through LDR (more at point A) while in the dark more current will flow through another path (at point B).

So in sunrise point A has more current and B has less. In dark B has more current and A has less.

We know op-amp compares inverting (-) and non-inverting (+) voltages and passes which one is greater. So


  • Voltage at inverting (-) is Higher. We know if signal fed into inverting (-) of op-amp it does change the polarity of the signal. So output of op-amp is negative
  • Voltage at non-inverting (+) is Lower.

That implies LED will get this negative voltage and remains OFF as it is reversed biased condition.


  • Voltage at inverting (-) is Lower.
  • Voltage at non-inverting (+) is Higher. We know if signal fed into non-inverting (+) of op-amp it does not change the polarity of the signal so the output of op-amp is positive voltage.

That implies LED will get this positive voltage and would turn ON as it is forward biased condition.


S.No. Item Quantity
1 LDR 1
2 Resistor 1K 3
3 Resistor 220 1
4 LED 1
5 OP-AMP LM358 1
6 Battery 5V / 9V 1
7 Male to Male jumper 8


As light falls on LDR output of op-amp is negative voltage means LED gets the negative signal that makes LED to be turned OFF and in the dark output of op-amp is positive LED will get the positive signal that needed by LED to be turned ON.

Total Visits: 459
0 0 vote
Article Rating
Notify of
Most Voted
Newest Oldest
Inline Feedbacks
View all comments
buy watch
6 months ago

696888 293838Attractive part of content material. I just stumbled upon your website and in accession capital to claim that I acquire in fact enjoyed account your blog posts. Any way Ill be subscribing to your feeds and even I achievement you get entry to constantly speedily. 262053

6 months ago

591665 176412I just added this webpage to my feed reader, fantastic stuff. Cannot get enough! 227228

design your own vehicle graphics
6 months ago

299573 184486Its like you read my mind! You appear to know so much about this, like you wrote the book in it or something. I believe which you can do with some pics to drive the message home a bit, but instead of that, this really is amazing blog. A fantastic read. Ill surely be back. 481042

tailgate mural
6 months ago

848743 678793Its onerous to search out knowledgeable individuals on this subject, even so you sound like you already know what you are talking about! Thanks 761240

đăng ký fun88
6 months ago

952738 380298As I internet website possessor I believe the content matter here is rattling amazing , appreciate it for your efforts. You should maintain it up forever! Excellent Luck. 311677

poker 99
6 months ago

109016 472403This will be a terrific blog, might you be involved in doing an interview regarding just how you designed it? If so e-mail me! 397537

6 months ago

449925 242438Flexibility indicates your space ought to get incremented with the improve in number of weblog users. 722178

طراحی سایت
6 months ago

131489 332305Some really marvellous function on behalf with the owner of this internet website , perfectly excellent content material . 710378

Gold Silver and Stock Market Economy in 2020
6 months ago

105696 852665Currently it seems like BlogEngine is the very best blogging platform out there right now. (from what Ive read) Is that what you are using on your blog? 283658

6 months ago

556102 494393Exceptional weblog here! following reading, i decide to buy a sleeping bag ASAP 603302

6 months ago

424405 192087Wow, great blog layout! How long have you been blogging for? you make blogging look easy. The overall look of your web web site is fantastic, let alone the content! 408395

6 months ago

649337 475620I likewise conceive so , perfectly written post! . 565039

6 months ago

713353 244308Valuable data. Fortunate me I discovered your web web site by chance, and Im surprised why this twist of fate didnt happened earlier! I bookmarked it. 547613

duratrans printing
6 months ago

203736 728172Bookmarked your great internet site. Fabulous function, exclusive way with words! 605341

fowling board pin layout
6 months ago

981279 327207There is noticeably a bundle to know about this. I assume you produced certain nice points in attributes also. 818153

6 months ago

99365 669064Hello! I would wish to supply a large thumbs up for your excellent info you can have here about this post. Ill be coming back to your weblog site for further soon. 70129

6 months ago

815508 433113Fantastic post, thanks so considerably for sharing. Do you happen to have an RSS feed I can subscribe to? 91990

buy vape carts online
6 months ago

57386 821879This is 1 really fascinating post. I like the way you write and I will bookmark your weblog to my favorites. 298018

6 months ago

573478 4270Fantastic beat ! I wish to apprentice even though you amend your internet site, how could i subscribe for a weblog internet site? The account aided me a acceptable deal. I had been just a little bit acquainted of this your broadcast provided bright clear idea 336855

Pauly d t shirt
5 months ago

939338 746105You produced some decent points there. I looked on the net towards the issue and discovered a lot of people go together with together along with your internet site. 809463

Translate »
Would love your thoughts, please comment.x