Fake car alarm blinking led

Discussion in 'The Projects Forum' started by pete1986, Jul 22, 2018.

  1. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    Just wanting to make a flashing led that blinks when you remove key from ignition using a transistor. I have it working just wanted to ask if anyone would look over it and see if its ok.greatly appriciate any help
     
  2. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    If you are sure that the IGN signal goes to an open dircuit when the ignition is off (as opposed to some other load on that line pullint it to GND), then things look ok.

    For cleaner switching action:
    tie the emitter directly to the battery
    Increase the base resistor to GND to 10K

    Not that the LED will not blink. It will be on continuously when the IGN signal is floating.

    ak
     
  3. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    The led is one if them blinking ones you just give ut voltage and it flashes. Ign signal is connected to cigarette lighter power and when key is removed this 12v cigarette power turns off. Can the emitter handle 14v
     
  4. dl324

    AAC Fanatic!

    Mar 30, 2015
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    You can eliminate the resistor in series with the emitter. The base resistor could be a larger value to reduce battery drain by about 10mA. If that makes the LED too bright, you can increase the resistor in series with it.
     
  5. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    What resistor wattages should i use?
     
  6. Hymie

    Active Member

    Mar 30, 2018
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    You do not need any additional components, other than the flashing LED and 1kΩ resistor.

    Connect the resistor to a permanent 12V, and connect the cathode of the LED to a switched 12V ignition circuit.

    With the ignition on, both sides of the LED will be at 12V (LED off), once the ignition is switched off, the impedance of the ignition circuit will provide a sufficiently low impedance path to operate the LED flasher circuit.

    A ¼ Watt resistor will be OK.
     
  7. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Certainly worth a try.

    For protection against all of the nasty things in an automotive electrical system, consider placing a 12 V to 15 V zener diode directly across the LED, cathode to GND. The resistor will limit operating current to the LED and transient current through the zener.

    ak
     
  8. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    Going to use transistor as feels safer.just wondering what wattage resistors to use
     
  9. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Watt's Law or Joule's Law. What is the voltage across the LED when it is operating? Or, what is the current?

    ak
     
  10. pete1986

    Thread Starter New Member

    Oct 12, 2013
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    I was thinking in building a pcb board SMD style using 2n2907 transistor and 1206 resistors. wouldi need to change anything in my orignal design to make this transistor work?
     
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