Easiest way to sound an alarm when no voltage present

Discussion in 'The Projects Forum' started by danwheeler, Jan 12, 2010.

  1. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0
    Hello, i'm new here. I'm a member of a different car-related forum where we were discussing ways to create an alarm when an inertia switch is triggered causing a circuit to open which kills the fuel pump in the event of an accident.

    I know I can use a SPDT relay and the normally closed terminal to power an alarm whenever voltage is not present on the coil terminals of the relay but I was wondering if there was an easier way to do this?

    I took a look at some battery monitoring circuits but they all seem to rely on the power source they are monitoring to activate an alarm which won't work in this case because the circuit being monitored will have no power when the circuit is opened.

    thanks in advance for any ideas

    dan
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    U can do it using motion or shock sensors

    Rifaa
     
  3. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0

    I already have the inertia sensor - it's a standard Ford part for fuel injection systems. I'm trying to create an alarm when the inertia switch is triggered which causes the circuit to be opened.
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    If so, can u identify inertia sensor connections?
    Post with pics and details

    Rifaa
     
  5. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0
    There are just 2 connections. it's essentially a SPST switch. The circuit is opened when a significant impact occurs.

    +12v is sent through the switch.
     
  6. eng1ne

    Member

    Dec 4, 2009
    97
    3
    Where are you anticipating the power for the alarm will come from?

    The way I interpret your requirements, you want a alarm to sound with no power. No power, no alarm I'm afraid.

    Although, surely this is not the case in a car, with a 12 V battery?

    You could use a simple transistor arrangement, with a separate battery for safety reasons.

    Wihout knowing more detail, I can give you little more detail.
     
    Last edited: Jan 13, 2010
  7. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0
    sorry i didn't mean to imply that there was no power at all... just not on the circuit I am trying to monitor. 12v is of course available from the car battery. I'm just trying to find an easier way to activate a buzzer or led when an unrelated circuit has last power.

    let me put it this way - let's say I have a SPST switch that has one terminal connected to the battery and the other terminal connected to a fan or other device.

    What i'm looking for is a way to activate a buzzer or led when no power is being sent to the fan or device.

    i know i can do this with a SPDT relay and the normally closed terminal but I was hoping for an easier way that doesn't rely on a relay being continuously energized (concerned about burnout because relays aren't typically designed for continuous duty)
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    If that is your impetus, it's not a problem. High power devices like solenoids are limited duty, but is is a rare relay that can't handle continuous power to the coil.

    A Omron G6H-2-DC12 sugar cube relay will handle 1 amp through the contacts, but only need 11.7 ma to hold the coil. Mouser has them as 653-G6H-2-DC12 for $3.41.
     
  9. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0
    i guess that was most of my concern, and I was hoping for something a little more simple and with less moving parts but if nobody can come up with anything easier then I guess a relay is the way to go.
     
  10. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    let me get this straight,
    Ur inertia sensor is a simple switch, which when activated closes the contacts, and u want to sound an alarm when it does.
    If this is the case then u have to find a 12V buzzer with a current rating that does not go beyond the inertia switch ratings.
    then u can wire a simple series circuit with the switch and buzzer to the battery with an appropriate fuse.

    Rifaa
     
  11. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0
    no its the opposite. the inertia switch is normally closed and I want an alarm that sounds when it opens.
     
  12. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    Ok.. simple enuf.
    What type of alarms and components do you have. I'm asking this cause this is really simple if you have some components to start with.
    This way you don't have to buy anything

    Rifaa
     
  13. danwheeler

    Thread Starter New Member

    Jan 12, 2010
    7
    0
    all i really have is a 12v piezo alarm. I dont mind buying things. This scenario pops up quite a bit so I'm just looking for a solution I can use when I need to sound an alarm when a device has lost power.

    thanks
     
  14. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    8,750
    760
    OK! let's see.
    Answer me these and I will give u a schematic.
    1. Do you have access to the car battery?
    2. Are you confident enuf to meddle with a car battery, knowing the dangers of high current and possible fire accidents, that might arise due to careless wire routing through the car chassis.
    3. I presume you have a 12V piezo for the alarm.
    4. You want the alarm to sound when the switch opens.
    5. Give me the buzzer specs.
    6. should the alarm switch off when the car is switched off or u need to manually shut it off.

    This much will do for now.

    Rifaa
     
Loading...