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

Parts

Part	Total Qty.	Description	Substitutions
R1	1	100K Pot
Q1	1	2N3904 NPN Transistor	2N2222
Q2	1	NPN Phototransistor
RELAY	1	9V Relay
MISC	1	Board, Wire, 9V Battery Snap (if battery used), Knob For R1

Notes

R1 Adjusts sensitivity

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Comments

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oil burner Light/Dark Detector Wednesday, September 06, 2017 10:53:57 AM

Hey please tell me i am used Q2 > LDR without flame: 1Mohm or above, with flame 2 to 5kohm at 10 lux and relay 24vdc normal, operating voltage 24vdc i am like used that circuit but needed component to 24vdc circuit.

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.

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oil burner	Light/Dark Detector	Wednesday, September 06, 2017 10:53:57 AM
Hey please tell me i am used Q2 > LDR without flame: 1Mohm or above, with flame 2 to 5kohm at 10 lux and relay 24vdc normal, operating voltage 24vdc i am like used that circuit but needed component to 24vdc circuit.
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.