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 for Light/Dark Detector


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


  1. R1 Adjusts sensitivity

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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 ????
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...)

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?
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
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
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...................
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.
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.
Light/Dark Detector
Friday, April 24, 2009 5:55:16 AM
it's vrey useful for students
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