Instructables
Picture of LED nightlight that turns on in the dark
My first instructable! This is something I originally made for a friend who's still using it. It works very nicely at adding a nice ambient light to a dark room. I made this instructable because I decided to make one to stick in the bathroom at my house.

It's a small easy to build circuit that will, as the title says, turn an LED on when it's dark and turn it off when there's light, making it a perfect night light. It runs on a 12v wall adapter, so you won't need to worry about replacing a battery.
 
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Step 1: What you need

Picture of What you need
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So here's what you need for this project:

[Soldering iron, of course... and solder]

- 100k Resistor
- 1K Resistor
- NPN Switching transistor (2N4401)
- Photo Cell
- 3v LED
- 12v wall adapter (look around... I'm sure you've got one somewhere!)

Optional:

- Small circuit board (makes it easier!)
- SPDT Switch (for complete disconnection)

Step 2: The schematics

Picture of The schematics
As said previously, the circuit is very simple.

The transistor switches what happens with the LED. Without it the LED would turn on when the lights are on and that'd be useless. The 100k resistor limits how much light is needed to turn it off, and the 1K resistor limits the amount of voltage going to the LED.

You can experiment with different resistor values. In this project I used two 100K resistors because I wanted the night light to be more sensitive and stay off in most ambient light. You could very easily replace these with a variable resistor, like a small trim pot, to let you determine the sensitivity whenever you'd like. You could also change the 1K resistor, but a lower value may make the maximum brightness a little too bright for a night light.

I also used 3 white LEDs for this project instead of one (which didn't work out as well as I'd hoped, stick to one).

Step 3: Putting it together

Picture of Putting it together
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Following the schematics is pretty straightforward. The pictures below show the process, including the traces on the bottom of the little circuit board I used.
Manmeswar3 months ago

Hey the transistors very difficult to get as they are very rarely available. May i have options for any other alternative transistor?

screasy9 months ago
I made a similar project for my electronics class, but it uses a potentiometer to adjust the brightness/light sensitivity from to photocell, two red LEDs, similar resisitors, 9volt battery for power, and I created and etched my own board. All together it's inside a case with a battery snap I cut out of plastic. I can't add any photos right now because my teacher didn't grade it yet.
screasy9 months ago
I made a similar project for my electronics class, but it uses a potentiometer to adjust the brightness/light sensitivity from to photocell, two red LEDs, similar resisitors, 9volt battery for power, and I created and etched my own board. All together it's inside a case with a battery snap I cut out of plastic. I can't add any photos right now because my teacher has it.
Anyone know what changes i will need if i use 30+ LED's for this circuit? Does each LED require a 1k resistor coming from a 12v 1800 MaH Ni-Mi Battery?
cabor1 year ago
this circuit works great for my solar led windows
any one needs one solderd up let me know
agis685 years ago
usually the white striped cable in the packed adapters is the negative one...
Roaraar agis681 year ago
Of all the powerpacks I have worked with, all of them had solid black as negative, and white stripe as positive. Maybe you have a very unusual powerpack
Locutis5 years ago
I'm going to try to build this with 3 LR44 batteries instead of a 12V wall adapter. If it works I'll post it.
Any luck with 3LR44 batteries ?
pandyaketan2 years ago
Nice work! 

Issue of 3 leds:

The reason why you are not satisfied with results using 3 leds is because:

a) you should have trippled the resistor values - 3 1k resistors in series and three 100k in series or
b) you should have used "H bridge" to increase the sensitivity.

Alternative voltages:

a) AC Mains & EL wire:

Instead of using a 12v adaptor, you can use a diode bridge with capacitor (see my Instructables).

EL wire can be used effectively with my  diode bridge and capacitor power circuit along with your front end sensor circuit. 

Or 

You can also use an old plone charger, negative of the main circuit touches the outer side of the charging pin, while the positive of the circuit goes inside the pin. 

b) DC voltage:

you can also use 9V, 6v or other battery pack variations without changing the main parts of the circuit!

Hope it helps...

reg
ketan
CivicSiR2 years ago
Thanks for sharing your design. I followed your method, but I'm using a 9V battery and 16 smd leds (it's a led for car dome rated at 12V). I got the photocell from ebay, but they don't seem to work as well. The leds only turn off when it's 10cm away from light source (i.e. yellow ccfl light or white led flash light).
What am I doing wrong here? Any way to increase the sensitivity of the photocell, so that it can turn off the leds with small amounts of light (clear sky during the day)?

Photocell specs:
size: 5.00x4.00mm
resistance on light: 5k-20k ohm
resistance in darkness: 1M ohm
max voltage: 150V (DC)
Peak sensitivity: 560nm

Thanks,

Jeff
nodoubtman2 years ago
do you use 3 leds or 1 led?

thank you!
marC:)
Thegame9953 years ago
Hello!

Do you know much much money it will cost me in electricity each month? Not exact, but is it expensive to have on like 7-8 hours a day? Does it use a lot of electricity? Thanks!
Well, it it uses 12 volts so a nine volt battery in series with a 3 volt battery pack would work but an ac adapter shouldn't cost you too much...of course I not sure about the close count...
Thegame9953 years ago
Could you please please please PLEASE! Upload a video where you do this? Im very new and I would like this to be my project! I really want this as I have the same PCB board! Please tell me a bit more detail/more pictures/a video!
Thanks for the instructable!
can someone help me, I can't seem to make it so that if its past the "dark" threshold it goes on and if its brighter that that it goes off it doesn't have just 2 solid states it has this big gray area were its only partly on.
You can also do this with the same el wire inverter that was mentioned above, from http://www.ellumiglow.com.

You can add the light resister to the inverter to make this work.
dawisch3 years ago
I was wondering if it would be possible to setup something just like this but with EL wire instead of LED's.

I want a strip of blue EL wire going around the top of my room that turns on when the lights are off.
dawisch dawisch3 years ago
I looked into the EL wire a little and saw that they need inverters.

I found this one that runs at 12v: http://www.ellumiglow.com/Electric-Optics-EL-Wire-Inverter-p/eo25ei.htm

So now my question is, could I simply remove the 1K resistor all together so that a full 12v from the wall outlet is flowing through then solder the inverter leads into the circuit instead of the LED? Or does their need to be a resistor there still?
20cookies3 years ago
put the switch in a break in the pos wire off the battery
simdude2u3 years ago
How would you change the circuit where the led turns on when there is light instead of darkness?
joasan184 years ago
can you help me out please, where would i put the switch?
zack2474 years ago
how do i make it so the light turns on when no power comes out of the wall? could i use say... a 5v adapter and put that where the photocell would go?
zack2474 years ago
awesome! i found this instructabe a long time ago but i forgot to bookmark it. it took me about 3 months to finally find it again! (in my spare time, of course)
Lucasc954 years ago
where do you take that photo cell from
?

i bought a pack at the source (radio shack for those of you in the U.S.) now i can actually use them...
joasan185 years ago
is the NPN Switching transistor (2N4401) necessary? what is i want to use a 9v battery?
kind of. its what makes it turn on in the dark and not during the day. for a 9v battery use different resistors
JustXtreme4 years ago
if i had a circuit in place already could i just add a photocell to the - side of the circuit?
mhkabir5 years ago
Can 30-40 LEDs be used with this circuit???? Any modifications needed???Which resistor to change etc.
Eddiev19855 years ago
Thank you very much for posting this! I was just playing around with a photo cell or light dependent resistor and an led. Sure enough, the less light the LDR received the dimmer the led became. I've been promising my wife a purple LED night light. You just gave me the answer I've been looking for. Very cool project! Well done! Thanks again. Eddie
b3nj4m5 years ago
What changes did you make to the circuit (if any) in order to use 3 LEDs?