How to trigger separate 12VDC circuit from 1.5VDC LED alarm light

Discussion in 'The Projects Forum' started by Blink101, Feb 15, 2015.

  1. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    I'm new to this site, and my electronic knowledge is amateur at best.
    I have an alarm LED (1.5 DCV input) that I want to use to trigger a separate 12 DCV circuit to light up an LED light.
    When the alarm LED is on, I want that to turn on the separate 12V DC LED Light.
    Is there a simple way to do this, and if so how?

    Thanks for your help.
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    image.jpg @Blink101
    Welcome to the site. One request: please use better titles for your thread. A PNP transistor with a light sensitive resistor works.
     
    Last edited: Feb 15, 2015
  3. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I think Blink means that in the existing alarm, the forward voltage drop across the LED (when lit) is 1.5V (likely a red LED)?

    In order to do the interface, we need some additional information:

    What power supply is the existing alarm running on? It might be batteries?
    Is either pin of the LED inside the alarm connected to either the + or - of the power supply?

    Can the new external 12V power supply be floating with respect to the internal batteries, or do the two supplies have to share some common connection?

    How much current do you want to drive the new external LED?
    What is its Vforward?
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Blinky,

    Welcome to AAC. Just a quick note on forum etiquette.
    The title you choose for your thread is very important to all members, readers, browsers and staff.

    "New and Need Help" is as useless as they come.

    "How to trigger 12VDC circuit from LED light?" would be a much better title.
     
  5. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    Thank You both for responding.
    The alarm (red/Green) LED (3 pin) shows 1.5 volts at the leads (+) (-) when the led comes on for alarm status. The alarm is from a Radar detector. I have 12 vdc powering up the radar detector. I want to use the Alarm LED lead voltage, and trigger a 12V DC LED light (3 lights on a strip) using 12v dc from the fuse box of the car.
    Here is a rough sketch of what I have.
    circuit1.jpg
     
  6. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    Thank You
    I will do better on my next postings
     
  7. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    I have a small LED alarm LED, it has 1.5VDC at the LED leads when the alarm is activated. The alarm voltage is from Red/Black source. The Green /Black is for a fake blinking security light.

    I would like to use the 1.5VDC from the alarm LED to trigger a separate 12VDC 3 led Strip light.
    The strip with (3 LED's) is rated at 12 VDC 6A working current 0.35 - 1.2A
    I have attached a rough sketch of what I'm trying to do.
    I'm not very savvy at Electronic circuits, so I will need a simple diagram with the components required.
    The reason I'm doing this is so I can get a brighter alarm light that be seen more easily.

    Thank You very Much
    circuit1.jpg
     
  8. crutschow

    Expert

    Mar 14, 2008
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    I don't understand what you mean by "rated at 12 VDC 6A working current 0.35 - 1.2A".
    When does it use 6A?

    You could use a low ON voltage N-MOSFET rated for at least 3A, such as one of those listed here.

    Connect the gate to the LED active lead,
    the source to the LED ground,
    and the drain to the negative terminal of the LED strip.
    Connect the positive terminal of the LED strip to +12V (negative of the +12V goes to ground).
     
    Last edited: Feb 15, 2015
  9. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    That is what the rating is for the strip light LED's. It's probably not important to include that info.
    I can hook into a car 12vDC circuit and make the lights work. I just need to be able to trigger that from the 1.5VDC source
    Any help will be appreciated
    Thanks,
     
  10. crutschow

    Expert

    Mar 14, 2008
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    You beat me to the punch.
    See my edits in post #2.
     
  11. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    Thank You, I'll give it a try
     
  12. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    Just one more question. When the Alarm LED alarm is activated, the LED blinks. Will the LED strip light blink as well?
    I just have to get a low-Voltage N-MOSFET, and hook it up as per your instructions, and I'm good to go?

    Thanks
     
  13. MrChips

    Moderator

    Oct 2, 2009
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    No point in running two threads. They are now merged into one thread.
     
  14. Evil Lurker

    Member

    Aug 25, 2011
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    Use a voltage comparator. Set it up to run off the LED strips power rail and set your reference voltage to half a volt or using a standard 2 resistor in series voltage divider. Hook up a jumper wire off "+" terminal of the LED and instead of directly connecting it to one of the inputs of the comparator use a 4148 diode and a current limiting resistor connected to an electrolytic capacitor. The idea is to have the comparator detect when voltage is applied to the LED, that will cause the output to go low (or high with a pull up resistor) thus driving either a FET or transistor powered mechanical relay whenever the LED switches on. What the diode and capacitor does is it keeps the voltage up on the input pin during the time the LED is off in between flashes. That said you may need a pull down resistor on the cap to turn the strip off. You will have to work out the values for yourself but if you use high trim pot and a halfway decent sized cap you should be able to control how long the strip stays on after the alarm has been turned off if you wish.
     
  15. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    I purchased a 1.5V gate 20V N-MOSFET rated for 9A, so with your clear instructions, I'm optimistic that my circuit will perform as intended. Those were not easily found in my area, so I had to order from Digi-Key

    Thank You,
    Paul
     
  16. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    Thank You very much for the diagram. Does the Alarm LED +1.5 V get attached after the Light sensitive resistor? Do I need a LSR, or just hook in the LED circuit. When the alarm LED comes on, I get 1.5 VDC between (+) and (-) wires on the LED leads.


    Thanks
     
    Last edited: Feb 20, 2015
  17. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The Alarm LED should just be pointed at the Light sensitive resistor. The circuit will need to be redesigned a bit because I see you bought an N-Mosfet. I post a new drawing shortly. I had also left out a second resistor to bias the LDR correctly - I will fix it.
     
  18. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    I'm not using the N-MOSFET, because it's too small to work with.
    I will try to work with your diagram if I can get that to work.
    Do I need a LSR, or can I just hook the LED (+) (-) leads of the LED circuit on the Base.

    I can certainly get a LSR resistor and point the LED at it, if that's the easiest thing to do.
    Thanks
     
  19. GopherT

    AAC Fanatic!

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    The LED (+)(-) lead can only be used to connect to the MOSFET Source and Gate pins if the (-) and (+) are at zero volts vs. ground when the LED is off. I don't think this will be true. I suspect that they will be at
    12 to 13 for (+) and something random between 0.5 and 10.5 when off and
    12 to 13 for (+) when and 10.5 to 11.5 when on
    Remember, measure from chassis ground to the pin in the DC voltage mode and report values.
     
  20. Blink101

    Thread Starter New Member

    Feb 15, 2015
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    I decided against using the N-MOSFET. I don't have the tools to solder to the N-MOSFET pins, as it's too tiny (pad) 0.7mm


    I would like to try to work with the diagram you posted for me.
    Can you give the specifics on that?

    Thanks,
    Paul
     
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