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headlight alarm circuit

Discussion in 'Electronic Basics' started by CM, Jun 8, 2005.

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  1. CM

    CM Guest

    Found this circuit on the internet.. It is intended to be a headlight alarm
    (warning) to prevent one from leaving lights on by mistake.

    My question is, what is the resistor for ? do you need it and do you need
    the diode from the ign aux line? Can't we wire the negative from the buzzer
    straight to ign circuit minus the resistor to ground and diode?

    Help me undrstand the purpose of the added parts.


  2. Chris

    Chris Guest

    Hi, CM. You're probably not seeing it because it's so simple.

    Let's ignore the "To IGN-AUX" part of the circuit (which happens when
    it's at 0v, i.e. the ignition and aux are off, and the diode prevents
    current from flowing). The buzzer BZ1 is a low current device. When
    "To Lights" is high (i.e. the lights are ON), 0.7V will be dropped
    across the diode, and a volt or four across the 1K resistor, leaving 8
    to 11 volts across the buzzer, enough voltage to make the buzzer sound.

    Now let's assume the key is in the ignition and it's turned to AUX or
    ON. There's 12V there, so 0.7V will be dropped across the diode, and
    the rest will be dropped across the 1K resistor. That means there will
    be no voltage for the buzzer, and it won't turn on.

    Net effect: the buzzer will only turn ON when the IGN/AUX switch is
    OFF and the lights are ON. Simple & straightforward.

    A buzzer isn't specified. I'd recommend trying your buzzer or binger
    with 12V and a 1K series resistor before you install. It's possible it
    might not be loud enough for you.

    Good luck
  3. Byron A Jeff

    Byron A Jeff Guest

    A useful thing. I'm glad my car already has it.
    To limit the amount of current flowing through the circuit.

    Note that 12V is nominal. Probably closer to 14V when the engine is running.
    Here's why everything is where it is. When the car is running the IGN AUX
    line is at 12V. When the lights are on the lights are at 12V too. Since
    both terminals of the buzzer are at 12V it doesn't buzz.

    Now the resistor is required because the IGN AUX line is connected to the
    buzzer where ground is also connected. Without the resistor there would be
    a short between the IGN AUX line and GND blowing the fuse for the IGN AUX
    line rendering it useless.

    Now you turn the car off. The IGN AUX line doesn't go to ground. It's simply
    disconnected from the battery and is floating. So in your arrangement the
    lights are at 12V and the IGN AUX line is connected to nothing. No buzzer.

    In the circuit as specified when the IGN AUX line is disconnected then the
    other side of the buzzer is now connected to ground. If the lights are on
    then the buzzer has 12V on the lights side and GND on the other. So it starts

  4. Tom Biasi

    Tom Biasi Guest

    This is a common simple circuit to activate the buzzer when the lights are
    on. The diodes provide isolation. If the lights are on the buzzer will be
    activated from the lights plus through D1 and the resister. If the lights
    are on AND the ign is on the buzzer minus will be raised to plus and the
    buzzer will not have enough difference to buzz. If you tie the minus of the
    buzzer to the ign, the buzzer will not have a path when the lights are on.
    You don't want the ign plus tied to ground when the ign is on.
  5. Dan Hollands

    Dan Hollands Guest

    The circuit assumes the the signal from the Ign-Aux is +12v without the 1k
    resistor there would be a short from +12V to Ground

    The 2 diodes prevent the +12V from the Lights and from IgnAux from feeding
    each other thru the buzzer.

    The circuit works as follows:

    When the IgnAux is on there is 12V applied across the resistor

    If the lights are turned on the buzzer doesn't sound because both sides of
    the buzzer are at the same volts

    If you turn off the IgnAux current flows from the lights thru the buzzer and
    the 1k resistor causing the buzzer to sound



    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
  6. CM

    CM Guest

    Ok thanks to the responses below. Reason I asked original question was that
    I have seen similar schematics wired as follows:

    + headlights--->>>>| (diode)------+Buzzer-------IGN AUX

    This was described as headlight power and ign aux 12 V (no buzzer)

    Ign off, headlights on 12V will find ground path through ign aux circuit and
    buzzer will sound.

    Does the above also make sense then? I realize there are many ways to
    accomlish the same thing.

  7. Impmon

    Impmon Guest

    It may not work for some cars. Some car will ground the IGN AUX and
    other car it may be simply disconnected (floating). A quick way to
    check this is to turn the key to accessory or on and then use a meter
    to check for continuity from IGN AUX to ground. If you get zero, the
    above schematic might work. If you get infinity, you'll need to add
    another diode, 1k resistor, and ground connection as mentioned in the
    link in your first post.
  8. CM

    CM Guest

    Thanks that definitely clears the difference up completely.
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