Power Outage Light

When the electric utility power goes out and it’s night outside, it’s nice to have a light to prevent stumbling in the dark.

Commercial power outage/night light are readily available. These typically have a rechargeable battery kept on trickle charge to light a LED when the power goes out. Several reviews mentioned that the rechargeable battery overheated and scorched the unit and wall but thankfully did not start a fire.

Since power outage lights are (hopefully) seldom activated,  it is practical to use standard alkaline AA or AAA batteries and replace them once a year. This will obviate the need for a charging circuit.

You can make your own power outage light with a few components.

  • A power source – any 5 volt wall wart charger.
    • 110v ac to 5v dc plug = eBay = $1.00
  • A 5 vdc relay – any SPDT relay will do. You will use the Normally Closed (NC) contact .A 50 ma capacity is adequate. (eBay $1.00-$300)
  • Any spare white LED
  • AA or AAA battery in a holder ($1.00)
  • A Joule Thief circuit from:
    • A YX8018 Solar Light Joule Thief DC DC Converter – ($1.00-$3.00)
    • Or a solar garden light to disassemble for the circuit and LED
  • Misc wire, solder, glue, double-stick tape, etc.

 

top: AAA cell holder; white LED; joule thief circuit board
Middle: pc board with SPDT 5 VDC relay
Bottom: 5 vdc wall wart power supply

Construction.

  • Glue / double-stick tape the relay to the wall wart.
  • Wire the wall wart to power the relay.
  • Using the solar garden light board or Joule Thief board:
    • Cut off and discard the solar panel, if any.
    • Cut off the rechargeable battery, if any. Tag the positive and negative connections.
    • Connect the negative / ground terminal to the battery holder negative end.
    • Connect the positive terminal to one side of the relay.
    • Connect the other side of the relay to the positive end of the battery holder.
    • If needed, relocate or replace the LED.
  • Package it up neatly.
  • Insert the AA or AAA battery. it should light up.
  • Insert the wallwart into a wall outlet. The LED should extinguish.
  • Done.

Theory of Operation.

  • The wall wart provide a constant 5 volts to keep the relay activated.
  • The NC contacts in the relay is kept open preventing the battery current from flowing
  • When power is lost, the relay contacts close, connecting the battery to the LED circuit.
  • The solar light circuit acts like a Joule Thief and boosts the 1.5 vdc AA or AAA battery enough to light the LED.
  • A Joule Thief circuit will keep the LED lit until the voltage drops below approx 0.6 vdc which should last for several days.
  • To test it anytime, unplug the wall wart and the LED will light.

JY-3/2018

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