Aug 312015
 

This night light does not begin glowing until the ambient light in the room is very dim. Others 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.

power_supply

The completed circuit on bread board with LM7805 based 9v to 5v voltage regulator

 

 

 

 

 

 

 

 

Step 1: What You Will Need

partsComponents:

  • SPST on/off switch (optional)
  • LM7805 Voltage Regulator
  • 0.33uF ceramic capacitor
  • 0.1uF ceramic capacitor
  • 820R ohm axial-lead resistor
  • 100k axial-lead resistor
  • LED – Bright red is easier on sleepy eyes but white will probably be brighter
  • TRANSISTOR 2N3904 NPN
  • GL5528 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
    a 2N2222 NPN transistor will
    work in this circuit just as well as the 2N3904. The main difference
    between the two transistors is the amount of amperage they can handle.
    If you’d like to see a page comparing the two in depth visit:2N3904 vs 2N2222

Step 2: The Circuit Schematic

schematicThis 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. Since my voltage to LED1 is 5 volts and that particular LED is rated at 2.0 – 2.4 volts and 18 to 20 mA you could place a resistor (R1) as low as 150 ohms, if I use 2.2 volts as my estimated voltage drop in the equation. It’s the R2 100k resistor that controls how much ambient light will set off the night light. If you replace it with a lower value resistor (for example a 47K resistor) the LED will glow even though the room light is still plenty bright enough for you see around the room.

Step 3: The Voltage Regulator
power_supply2

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
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.

 
circuit_lit

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