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
[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
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).