Introduction: Automatic Night Light
My night light does not begin glowing until the ambient light in the room is very dim. Others I've seen on here begin glowing when the light in the room is still fairly bright. If your night light comes on when the light in the room is still bright you're going to be wasting a lot of the electricity in your battery. Another feature is the use of the 9 volt battery using the built-in 5 volt regulated power supply. My thinking here is by powering the night light with a 9 volt battery it should operate for quite a while before the battery will need to be replaced.
Step 1: What You Will Need
- SPST on/off switch
- LM7805 Voltage Regulator
- 0.33uF ceramic capacitor
- 0.1uF ceramic capacitor
- 820R ohm resistor
- 100k resistor
- LED - Bright red is easier on sleepy eyes but white will probably be brighter
TRANSISTOR 2N3904 NPN
LDR: also known as a Photocell - Photo Resistor - Light Dependent Resistor
Bright Resistance (10Lux) (KΩ): 10-20Dark Resistance (MΩ): 1
9 volt battery snap
9 volt battery
- Breadboard for testing
Step 2: The Circuit Schematic
This is the complete circuit. I used CadSoft's Eagle schematic design software and the LDR symbol is a little different from the LDR symbols I've seen on other schematics. If you are interested in using the Eagle software I recommend reading the Instructable by westfw on where to obtain and using the software. And he also provides a couple of useful links.
Step 3: The Voltage Regulator
I built the voltage regulator first and then tested it to make sure it was actually putting out 5 volts. As the 9 volt drains it will maintain the correct voltage this night light needs to operate correctly. I really think it should be able to go for quite some time before the battery will need to be replaced.
Step 4: The Completed Circuit
Here's a close-up of the completed circuit. You can see it's really very simple. In the picture where the LED is lit I have placed a Sharpy pen cap over the LDR to put it in complete darkness. I plan to make some birthday and Christmas gifts using this circuit. Maybe find a Statue of Liberty model where the LED can be placed in the torch Lady Liberty is hoisting up in the air. Another couple of ideas I've had are to place the night light in a tiny house so the glow comes through the windows, or a little campsite with the LED in the campfire. You should be able to come up with some great ideas too!