Light/Dark Detector


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This handy little circuit can tell the difference between darkness and light, making it very useful for switching on and off signs, porch lights or other things when it gets dark or light.

Schematic

Schematic for Light/Dark Detector

Parts

Part
Total Qty.
Description
Substitutions
R11100K Pot
Q112N3904 NPN Transistor2N2222
Q21NPN Phototransistor
RELAY19V Relay
MISC1Board, Wire, 9V Battery Snap (if battery used), Knob For R1

Notes

  1. R1 Adjusts sensitivity

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Comments

Add A Comment

EMMAn
Light/Dark Detector
Monday, January 21, 2013 8:43:36 AM
Hey.. i'm just thinking.. how can i use this as a switch for a 220v AC powered Fan.??? should i just put the load in the Relay ????
anonymous
Light/Dark Detector
Friday, June 24, 2011 6:50:15 PM
Thanks a lot for posting this. but i have one question. How can it be a dark detector since darkness doesn't really exist?

(Editor's notes: Sigh...)

Chelsea
Light/Dark Detector
Sunday, October 31, 2010 4:48:42 PM
Do i have to get a knob for R1, will it still work without one?
Michael
Light/Dark Detector
Saturday, February 13, 2010 10:04:49 PM
Thank you for this simple but very effective circuit. It was the answer to my automatic switch machine control problem I was having on a model railroad layout that is still in the design stage. I used a 12 volt relay instead of the 9 volt one listed and it worked the very first time it was powered up. Thanks again
fakher
Light/Dark Detector
Wednesday, February 10, 2010 8:10:59 AM
it is good simple circuit. but it is not as sencitive as it should be.but still it works fine
anonymous
Light/Dark Detector
Wednesday, December 16, 2009 7:20:31 AM
oh! god ..........i jus can't get an idea of the significance of this detector...................
anonymous
Light/Dark Detector
Friday, October 23, 2009 9:41:04 PM
Please tell me in one paragraph what it toke you 4 years to learn. God people , give the guy a break. It works fine. It don't get any simpler than this. You could buy a book on the subject you know. That's if you can read of course. Oh,.... 2N3904 worked well for the transistor too. Only had a dozen or two laying around.
anonymous
Light/Dark Detector
Friday, September 11, 2009 12:50:27 AM
is it ok if i dont use 9v relay? I use 5V relay and my supply is 9V.. But it doesnt work.
acme fixer
Light/Dark Detector
Friday, April 24, 2009 11:02:42 AM
NPN Photodetector? One could never tell from the outside whether it's NPN or PNP. Assume it's a typical relay, say 200 ohm coil. If you connect this circuit to a 9V battery, it will draw three times as much current as a 9V battery usually supplies. The battery will last only a few hours. The coil should have a 1N4002 rectifier across it, cathode to the positive end.
anonymous
Light/Dark Detector
Friday, April 24, 2009 5:55:16 AM
it's vrey useful for students
Raheem
Light/Dark Detector
Thursday, April 16, 2009 5:05:13 AM
can any give me the schematic for controlling the dim and dipping of the headlamps of the vehicle when it detects the vehicle coming in the opposite direction to it.. so as to make the people see the roads properly during night
anonymous
Light/Dark Detector
Monday, April 13, 2009 11:34:25 AM
where can i get a device that can detect whether the street light is out during the day? my job is to locate streets lights that are out so i can report to the electric company for them to replace the lights. I dont want to have to go out at night and locate the lights that are out and during the day all lights are off so i can't tell. thanks
john
Light/Dark Detector
Tuesday, March 03, 2009 8:44:51 PM
i wasn't able to get a phototransistor so i used an LDR and a 50K preset, works fine. the diode didn't make much difference,it's only for theory buffs. pcb is so easy to design.....duh!
anonymous
Light/Dark Detector
Friday, February 20, 2009 11:58:09 AM
Hello Im i beginner and am finding it very hard to translate this circuit onto a strip-board. Please can you provide a strip-board layout for me to follow. Many thanks, Jethro
Sajjad Haider
Light/Dark Detector
Thursday, December 18, 2008 8:50:11 AM
Its a good and easy to make Circuit. Its so easy to make this circuit on bred board. I will recommend to all readers of this page to make this circuit. I hardly took 5 minutes for me to make it on bred board. "It really Works".
Shahab
Light/Dark Detector
Sunday, November 23, 2008 3:43:57 PM
It doesn't work. I tried it.. it works if you use an LED instead of a relay. but in case of a relay there is a huge voltage drop at the emitter of the 2n2222 transistor and there isn't enough to energize the relay coil.
Joseph KD5KZL
Light/Dark Detector
Thursday, July 24, 2008 4:33:52 PM
While the comment about the flyback diode is correct this circuit is in about every entry level electronics book. You don't need a PCB for this circuit as it is easy to layout on a perf board, but Dave Cutcher might have the board. He wrote the book Elelctronic Circuits for the Evil Genuis and used such a board for the projects in the book.
vinaya kumar
Light/Dark Detector
Monday, May 05, 2008 6:14:10 AM
can any give me the schematic for controlling the dim and dipping of the headlamps of the vehicle when it detects the vehicle coming in the opposite direction to it.. so as to make the people see the roads properly during night..
anonymoussai kiran
Light/Dark Detector
Friday, May 02, 2008 6:27:37 AM
i want to start this one but reading above comments it's not good to start i think for any one is it works if mail me plzzzzzzzzzzzzzzzz
anonymous
Light/Dark Detector
Tuesday, April 08, 2008 10:45:23 PM
Your circuits are totally crap, you need major adjustments, I see that theory is a completely ignored subject here. Not to mention that in the real world this circuit alone would fail, and electronics 101 you need a diode to take back voltage as the coil discharges, duh! Using the Diode toward the VCC end will not allow the coil to discharge fast this will prevent the back voltage.

(Editor's notes: I've been using this circuit for over 20 years to switch an EXIT sign on and off. But yes, I agree it should have a flyback diode.)

ROFL
Light/Dark Detector
Tuesday, April 08, 2008 10:55:11 AM
Nobody is going to make the circuit PCB for you, all your going to do is use it as your College Project. Do not build this circuit, it is poorly designed.
rohaan
Light/Dark Detector
Monday, April 07, 2008 9:43:32 AM
hey! did any of u work out this circuit..? i hv to do some thign like.. anyone wants to tell me bout.. and ya MuhaMMAD HASSAN what ur talking bout.. would u like to share? please help..its urgent!!! rohaanpathan_ekumeed@hotmail.com
Bande
Light/Dark Detector
Thursday, March 27, 2008 10:16:27 AM
Can u pls give me the god dam PCB of this god dam circuit... pls pls give it 2 me.i need that very badly. its a matter of life and death...
anonymous
Light/Dark Detector
Thursday, March 20, 2008 4:07:37 AM
i want to know can you protect your home with out using alarm
shaf
Light/Dark Detector
Monday, March 17, 2008 5:29:22 PM
can someone plz help me with light/dark detector by providing me a clear detail of how it works.for my AS level electronics course work i decided to design a security light which is going to place at the front door so when it is dark, it has to light up automatically as something moves, plz help, thanks.(karimmohammad108@hotmail.com).
SeanFalloy
Light/Dark Detector
Thursday, March 13, 2008 2:00:47 PM
Very Nice .. simple but usfull
shaf
Light/Dark Detector
Tuesday, March 11, 2008 2:43:13 PM
can someone please help me with clear explanation on how the light/dark detecter works, it would be very kind of you. it will really help with my school project, caz i am trying to design a security light as my year project, so this will be one of the sub system.
freeieda
Light/Dark Detector
Tuesday, March 04, 2008 1:25:42 AM
why your light /dark detector don't use IC??
anonymous
Light/Dark Detector
Wednesday, February 13, 2008 5:46:56 AM
juz 1 2 know how does the light dark automaticcally funct.
muhammad hassan
Light/Dark Detector
Monday, February 11, 2008 4:31:03 PM
i want a detail project explanation on light/dark detector which on automatically at dusk and off at dawn.it uses phototransistor as its sensor,4 transistors(2 NPN transistors,2 PNP transistors),4 resistors,9V DC supply and a bulb.The transistors are treated as darlington pair.
muhammad hassan
nifty night light
Tuesday, January 29, 2008 5:01:10 AM
i want a clear explanation on nifty night light using a phototransistor, 4 transistors,3 resistors and a bulb using a 9v supply.The transistors are treated as darlington pair
norcwiz
Light/Dark Detector
Friday, January 18, 2008 6:38:47 PM
If this will work with 12 volts is my question I guess only building one to see if it works . The N23404 will work but getting the correct NPN phototransistor is the question?
POGI
Light/Dark Detector
Friday, January 11, 2008 1:35:34 AM
can you give me an abstract of your experiment..........
anonymous
Light/Dark Detector
Tuesday, January 08, 2008 1:26:17 PM
how does it work
anonymous
Light/Dark Detector
Wednesday, December 05, 2007 6:29:01 AM
It just doesn't work as the relay is connected directly to the power source and it would be activated without the part of phototransistor.
kareem khaled
Light/Dark Detector
Saturday, November 10, 2007 5:29:21 PM
plzzzzzzzzzz every one have any idea about sensors plz contact with me
SASHI
Light/Dark Detector
Saturday, October 27, 2007 3:51:08 AM
I WANT TO KNOW ABOUT THIS DETECTOR APPLICATIONS AND its WORKING PROCEDURE Plz send me
Mike-HBS
Light/Dark Detector
Friday, October 26, 2007 11:01:19 AM
Can this work with (say) a 6v load?
seesy
Light/Dark Detector
Friday, October 26, 2007 6:02:21 AM
what use for light/dark detector...where we use for
Ral
Light/Dark Detector
Thursday, October 25, 2007 11:50:46 AM
Excellent post! I am thinking, If I substitute the phototransistor with voltage A and B, Could I alternate this project to make it Voltage Detector? What I want to make is a a battery backed up solution in case the primary voltage has been cut off or dropped. Thanks
anonymous
Light/Dark Detector
Wednesday, October 17, 2007 4:07:54 AM
i want details about dark and light dectector Advantages and Disadvantages
anonymous
Light/Dark Detector
Thursday, October 11, 2007 2:27:09 AM
i would like to say i made this circuit n it was enuf interesting as we can learn more new thing by dng or making these small circuits in our practical life...........
anonymous
Light/Dark Detector
Monday, October 01, 2007 10:26:32 AM
how dose the circuit works and how can it be connected so as to detect light and dark.
anonymous
Light/Dark Detector
Wednesday, September 26, 2007 6:51:39 AM
HEY GUYS, U HAVE SO MANY QUESTIONS.. UR NOT ENG'G PEOPLE, RIGHT?? THAT IS A VERY SIMPLE DIAGRAM... EASY TO CONNECT, EASY TO MAKE... ITS MAIN PURPOSE IS TO DETECT LIGHT SINCE IT USES A PHOTOTRANSISTOR.. IT CAN BE USE AS AN AUTOMATIC SWITCH, JUST CONNECT UR LOAD INTO THE RELAY...
bishop dele
Light/Dark Detector
Thursday, September 20, 2007 11:54:58 AM
how does a dark/light detector work. where can it be used? and the application/significance.
Tiago
Light/Dark Detector
Thursday, September 20, 2007 5:04:24 AM
Hello, can i change the battery 9v to 12v? what changes i must do in the circuit. I want make this circuit but with one 12v battery and 12v Relay. Thank you!
anonymous
Light/Dark Detector
Tuesday, September 18, 2007 12:18:36 PM
absolutely fantastic
MABILA SAKHI
Light/Dark Detector
Saturday, September 15, 2007 5:39:23 AM
I want to know the name of all the components needed, their values and the quantity, and also how to connect on the circuit board. also if u can supply me with the short explanation how its functioning.
anonymous
Light/Dark Detector
Saturday, September 15, 2007 1:34:57 AM
How could you connect it to a Fibreoptics example to control a pc keyboard?
antony
Light/Dark Detector
Wednesday, September 12, 2007 8:22:37 AM
Explain the uses of the dark light detector

(Editor's notes: To detect light and dark.)

anonymous Okley
Light/Dark Detector
Thursday, September 06, 2007 5:10:12 PM
can you explain the circuits on a the picture of a real board? It may be easier
anonymous Okley
Light/Dark Detector
Thursday, September 06, 2007 4:55:13 PM
I want to know how to connect all the componets on the board and what to do after that
Vaal Tech
Light/Dark Detector
Wednesday, September 05, 2007 10:02:23 AM
Whow does it work?Where can I use it?Send me all the details
Vaal Tech
Light/Dark Detector
Wednesday, September 05, 2007 9:38:32 AM
Can this Be used to activate electronic systems please give examples and brif explanatin
salahuddin
Light/Dark Detector
Saturday, August 11, 2007 7:29:31 AM
Initially it is given that we can use as per our requirement. But here only schematic picture is given. It does not gives full details like the series of transistor(or number).

(Editor's notes: It's listed in the parts list.)

DINESH
Light/Dark Detector
Tuesday, August 07, 2007 9:49:59 AM
PLEASE INCLUDE THE NUMBER OF NPN TRANSISTOR............
john lopez
Light/Dark Detector
Tuesday, August 07, 2007 4:26:40 AM
what is the most affordable light detector
SharkFeece
Light/Dark Detector
Thursday, August 02, 2007 1:22:01 PM
I say chaps, this could be a dangerous circuit if you go connecting it up to the mains. Most electronic circuit diagrams are for people who have at least a basic knowledge of electronics.
Benjamin
Light/Dark Detector
Friday, July 27, 2007 8:11:29 AM
what is R1? Is it a resistor? What type of resistor is it and what is its value? Pls, give me every detail about it.
Benjamin
Light/Dark Detector
Friday, July 27, 2007 8:00:24 AM
I want to know the name of all the components needed, their values and the quantity, and also how to connect on the circuit board.
samridh
Light/Dark Detector
Wednesday, July 18, 2007 7:53:22 AM
could you please send me the details of how to connect this stuff on the circuit board
anonymous
Light/Dark Detector
Friday, July 13, 2007 10:47:02 PM
will this circuit work wid a speaker an will it be able to be used as a laser trip wire?
abd_el_monam
Light/Dark Detector
Saturday, July 07, 2007 11:17:07 AM
I have trayed this circuite but the npn phototransistor not good can you give me the best senstivity transistor
mohamed
Light/Dark Detector
Tuesday, July 03, 2007 12:49:34 AM
I would like to know how this system can be used, and details
tunde tairu
Light/Dark Detector
Thursday, June 28, 2007 8:26:56 AM
can i have the diagram on how to connect this stuff on circuit board
tunde tairu
Light/Dark Detector
Thursday, June 28, 2007 8:20:59 AM
I WANT TO KNOW DETAIL ABOUT SCHEMATIC LIGHT AND DARK DETECTOR AND ALSO OTHERS LIGHT AND DARK DETECTORS . YOU CAN ALSO GIVE ME EXPLANTION ON SOLAR ENERGY
abdul rahamn
Light/Dark Detector
Monday, June 25, 2007 12:23:14 PM
I want to know what the variable resister is.
anonymous
Light/Dark Detector
Monday, June 25, 2007 12:20:22 PM
I know what the circuit is but i want to know can we use domestic supply e.g bulbs etc at the relay with the same circuit.
AM JUSTINE
Light/Dark Detector
Wednesday, June 20, 2007 9:57:08 AM
I WANT TO KNOW DETAIL ABOUT SCHEMATIC LIGHT AND DARK DETECTOR AND ALSO OTHERS LIGHT AND DARK DETECTORS . YOU CAN ALSO GIVE ME EXPLANTION ON SOLAR ENERGY
Abdus Sattar
Light/Dark Detector
Wednesday, June 13, 2007 1:47:48 PM
Thanks for ckt .Pls Send me the working and few notes about this project .
geoff
Light/Dark Detector
Wednesday, June 13, 2007 8:34:14 AM
i "get" how this circuit works but where is the negative terminal for the battery and also on a lot of your schematics you have a "relay" just what is a relay. thanx
keturm
Light/Dark Detector
Friday, June 08, 2007 12:24:36 PM
can i have the diagram on how to connect this stuff on circuit board
shyirambere
Light/Dark Detector
Tuesday, May 29, 2007 11:38:29 AM
I would like to know how this system can be used, and details
Mian Rizwan Arif
Light/Dark Detector
Friday, May 25, 2007 12:57:33 AM
i really need these circuits but not find accurate in searching but today i m glad to hav this site to get any circuit which i want .thanks lot..........
asif
Light/Dark Detector
Sunday, May 20, 2007 12:58:15 PM
please write the principle and working of this project
Santu
Light/Dark Detector
Saturday, May 12, 2007 3:12:04 AM
Thanks for the circuits but please send the working.And few notes about this project
M. A. Bari
Light/Dark Detector
Friday, May 11, 2007 12:04:00 AM
Dear Sir, Its more complex if we use a 9V battery, can this be substituted with the working A/C voltage?
Leon NTABOMVURA
Light/Dark Detector
Wednesday, May 02, 2007 3:57:03 AM
Thank you so much for this, if this can be practical in public suitching,can you provide me enough Notes so that I can implement it for our pour african countries?
imam yustiarto
Light/Dark Detector
Thursday, April 26, 2007 9:52:49 AM
i think its simple circuit
ahmad
Light/Dark Detector
Friday, April 13, 2007 5:26:15 PM
thanx for your effort, can you tell us more information I am a student in university and it seems that am gonna take that simple project for my electronics lap so please more details about Light/Dark Detector so as to be able to build it ...many thanks
can you plz explain it more
Light/Dark Detector
Sunday, April 01, 2007 6:13:44 AM
can u plz explain it more that hw the ckt works?....in detail plz?
anonymous
Light/Dark Detector
Friday, March 23, 2007 11:34:14 AM
could you plz explain it more
hussain jaorawala
Light/Dark Detector
Wednesday, March 21, 2007 8:40:05 AM
can u give me more information about this device
Dannywest
Light/Dark Detector
Saturday, March 10, 2007 6:42:37 AM
why did u descide to use a photo transistor instead of a photo resistor,as was used in WWW.OURWORLD.COMPUSERVE.COM?
yards
Light/Dark Detector
Wednesday, March 07, 2007 11:35:42 PM
may i know if this circuit is tested and function well? so i mean this circuit is detecting the light and dark with 1 circuit only? i need it and be prove so i can create this kind of circuit.
mk
Light/Dark Detector
Tuesday, March 06, 2007 11:29:36 PM
give more infor mation about it
Harrisons
Light/Dark Detector
Tuesday, March 06, 2007 9:25:43 PM
Hi, I am trying to build the light/dark detector. I am wondering what kind of NPN Phototransistor and 9V relay is good to use? Can you please give me the part number as well. Thank you.

(Editor's notes: Any standard NPN phototransistor and any 9V relay with a low current coil (less then 300mA).)

abel
Light/Dark Detector
Thursday, March 01, 2007 3:38:04 AM
thank u and al the web members
anonymous
Light/Dark Detector
Wednesday, February 28, 2007 5:45:02 AM
what is the use of relay in light/dark detector???i need an explaination... what is the specific number of the phototransistor/....

(Editor's notes: Your best bet is to ask technical questions in the forum instead of here.)

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