How can a LED lamp turn on automatically when the light go off ?

Discussion in 'General Electronics Chat' started by Michael George, Sep 20, 2015.

  1. Michael George

    Thread Starter Member

    Feb 8, 2015
    52
    2
    I have a lamp that contains many small LED's and two rechargeable batteries. The lamp is always connected to AC mains but it is off and no light comes out of it. When the light go off or no electricity in the AC mains, the lamp turns on automatically using the battery. What is the circuit that can do that ? and How it knows that there is no electricity in the AC mains ?!

    One more thing, Do automatic start up diesel generators use the same technique ?

    Thank you very much, I appreciate your help :)
     
  2. peter taylor

    Member

    Apr 1, 2013
    106
    5
    With an automatic light turner-on-erer
     
  3. recklessrog

    Member

    May 23, 2013
    338
    102


    awww don't be unkind, If the thread starter is a novice, at least lets give him useful info.
     
    Michael George likes this.
  4. dl324

    Distinguished Member

    Mar 30, 2015
    3,211
    619
    The two turn on methods are separate and unrelated.

    Ambient light is typically sensed with a light dependent resistor (LDR). Loss of AC power can be detected in many ways.
     
    Michael George likes this.
  5. peter taylor

    Member

    Apr 1, 2013
    106
    5
    sorry
     
    recklessrog likes this.
  6. recklessrog

    Member

    May 23, 2013
    338
    102
    They use devices that are sensitive to light and in an appropriately designed circuit can do many thing LIKE :- turn on/off a relay that is then used to switch on the lamp depending on the light level.
    Also, a very simple circuit can monitor if the mains voltage is present,EG :- when it is off, can operate a relay to turn on the light.
    the combinations of possibility are enormous, and yes a light or power sensing circuit can (and do) control start up of generators.
     
    Michael George likes this.
  7. peter taylor

    Member

    Apr 1, 2013
    106
    5
    There will be a circuit that is connected to the mains, that keeps the battery charged and keeps the LED switched off. When the mains disappears (during a downpour normally in my area), this circuit no longer functions, and the LED and battery will run normally.

    Regarding diesel generators, the circuit is the same in theory, but way more complex, involving batteries to start the engine, and charging circuits that 'sense' the mains and monitor it for fluctuations or spikes.

    Those generators are HUGE, and I have worked on one that backed up an entire 33 story office block. Scary.

    There are more fail-safe circuits than I'v had McDonalds Filet of Fishes.

    And we are talking 100's of peoples lives relying on this circuit to run properly.

    The circuit you have is very simple in comparison.

    I would imagine now they have Embedded Micro-controllers sensing the mains and deciding when to the BEAST over.

    Just my 20 cents
     
    Michael George likes this.
  8. shteii01

    AAC Fanatic!

    Feb 19, 2010
    3,376
    494
    My dad got a couple similar units, but I don't think they have batteries, just AC, they plug into wall outlets. They do have LDR sensors. At night they are ON. During day they are OFF. I am not sure if they have AC failure detection. They are basically safety lights for kids or for adults to get around the house without turning main lights on.
     
  9. Kaiser Ed Bayawa

    Member

    Mar 19, 2015
    60
    6
    You can make a dark detector circuit.
    When the light is out it will automatically turn on.
     
Loading...