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LED lights when circuit broken

Discussion in 'Electronic Basics' started by [email protected], Dec 31, 2012.

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  1. Hi,
    What is the simplest (cheapest, lowest energy) way to get an LED to
    light if the circuit from a 1.5v battery is broken?
    Thanks,
    Rod
     
  2. Thanks for your reply, I should explain more. I want a simple circuit
    to detect when a switch is open. Normally it is closed. If I connect
    an LED in series with the switch then it will go out when the switch
    is open. However I want it to come on when the switch is open. If I
    connect the LED in parallel to the switch it will come on when the
    switch is open but my battery will run flat quickly. There must be a
    very simple solution to this, which will preserve the battery life
    when the switch is closed but light the LED when it is open.
    Rod
     
  3. Well an LED + resistor in parallel with the switch might work.

    You are basically stuck with the current drain of the LED, you can use
    a big R to limit current...(and light) or some more complicated pulsed
    gizmo.

    George H.
     
  4. Phil Allison

    Phil Allison Guest

    "[email protected]"


    Thanks for your reply, I should explain more. I want a simple circuit
    to detect when a switch is open. Normally it is closed. If I connect
    an LED in series with the switch then it will go out when the switch
    is open. However I want it to come on when the switch is open. If I
    connect the LED in parallel to the switch it will come on when the
    switch is open but my battery will run flat quickly. There must be a
    very simple solution to this, which will preserve the battery life
    when the switch is closed but light the LED when it is open.


    ** Errr - is a slowly blinking LED OK ??




    .... Phil
     
  5. Hi Phil,
    Yes, a blinking LED would be better!
    Rod
     
  6. amdx

    amdx Guest

    What else is the switch supplying and what is the voltage?
    AC or DC?
     
  7. P E Schoen

    P E Schoen Guest

  8. Guest

    Use an LM3909 cct across the switch.
     
  9. P E Schoen

    P E Schoen Guest

    wrote in message
    I already suggested that, but they are rare and quite expensive. They also
    draw about 550-750 uA. A PIC10LF320 draws only 25 uA. However, it is only
    rated to 1.8V minimum. You could build a relaxation oscillator that slowly
    charges a capacitor and then discharges it quickly through an LED. You can
    use a low voltage OpAmp such as the TS1001 which works from 0.65 to 2.5 V
    supply:
    http://touchstonesemi.com/wp-content/uploads/2012/07/ts1001ds r1p1 - dec 11.pdf
    http://www.digikey.com/product-detail/en/TS1001IJ5T/TS1001IJ5TCT-ND/3622637

    You can also use an LMC555 which works down to 1.5V.
    http://www.digikey.com/product-detail/en/LMC555CMM/NOPB/LMC555CMM/NOPBCT-ND/270765

    Actually, it will be hard enough to light an LED on 1.5V unless you make a
    boost circuit. Perhaps something like a Joule Thief.

    Paul
     
  10. The National LM3909 was an LED flasher, ran off 1.5v. I don't recall an
    inductor, it did put a capacitor in series with the LED, so I guess it
    charged up and then discharged into the LED. Lots of secondary uses were
    found for the device, but it was intended as an LED flasher, and I think
    disappeared long ago from the catalog, not enough use for it.

    Michael
     
  11. ehsjr

    ehsjr Guest

    Exactly.

    He might be able to use John Fields' idea by putting the
    Joule Thief across the switch, like this:

    /
    +-----o o----------+---[Load]---+
    | | |
    +---[Joule Thief]---+ |
    | |
    [Bat] |
    | |
    +--------------------------------+

    If the load is low enough impedance the thief will run fine. He
    indicated that the battery ran down quickly when he put an LED
    across the switch and opened it, so presumably the impedance is
    low enough to allow the thief to run. If it isn't, he could go
    to a SPDT switch and run the thief this way:

    +-----o o-------------[Load]---+
    | \ |
    | o-------[Joule Thief]---+
    | |
    | |
    [Bat] |
    | |
    +-------------------------------+

    Interesting Joule Thief info here:
    http://quantsuff.com/LED2.htm

    Quantstuff did a lot of experimenting with the things,
    and his site shows a blinking thief as well as info
    on circuit efficiency.

    Ed
     
  12. Guest

    Puzzling. Supply == 1v5, LED VF >= 1v8. How did the battery go flat?
    Unless it was assumed to be flat because the led didn't light? Maybe it did light initially as some primary cells initial voltage is above 1v5.
     
  13. Jasen Betts

    Jasen Betts Guest

    yeah, I heard LM3909 was End of life
    that doesn't seem to be a 1.5V pic, and in any case for 1xAA operation
    you want chip that'll run on a 0.9V supply.

    http://www.digikey.com/product-detail/en/TLC551CP/296-10333-5-ND/380508
     
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