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 This is something I originally made for a friend who's still using it.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.
Step 1: What You Need
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!)
- Small circuit board (makes it easier!)
- SPDT Switch (for complete disconnection)
Step 2: 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
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.
Step 4: Give It Power!
Now you'll want to take your 12v adapter, and measure out the length of wire you want. The way I set it up, the little circuit board rests right on top of it when it's plugged in so the LED shines up the wall and gives the room a nice glow. Once you've got it at a good length, strip the wire a bit so you can easily solder it in place.
This is important! You'll want to make note of the polarity of each wire and where they go. The one with the white stripe is the positive and the solid black is the negative. Once you've established polarity, just wire it up according to the schematics.
[If you choose to add an on off switch, simply wire the switch between either the positive or negative wire and the board)]
Step 5: Plug It Up - You're Done!
Plug it in and cover up the photo cell, unless you're in a dark room. The LEDs should come on! Whoooooo. Now you can just find a way of getting the board on top of the adapter. I drilled two holes in the top of the adapter and screwed it in place, but I can't say I recommend this :P Hot glue works just fine.