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Add a delay to a 12V circuit

  1. Apr 29, 2010 #1

    I'm looking for some help with a (hopefully) simple EE question regarding a DC circuit. I'm a chemical engineer and it's been a while since elec/mag physics for me :-) Here goes:

    In my life outside engineering, I run a small ambulance service. Our ambulance has a rather annoying piezo buzzer that sounds any time the parking brake is in the "wrong" position (i.e. parking brake set and vehicle is in drive, or vice-versa). The problem is that it sounds every time the vehicle is shifted, since there's no delay (i.e. you'd have to simultaneously shift and adjust the parking brake to avoid the buzzer).

    I'd like to add something to the circuit that adds about a 1 to 2 second delay before the buzzer sounds - however, the buzzer must turn OFF immediately when the voltage is removed. Ideally, I'm looking for something cheap and simple (as always)...

    I seem to remember that inductors allow current flow in a time-dependent nature (i.e. ramp up from zero current) - am I on the right track?

    Any help would be great!

    P.S. The voltage is around 13.5 V DC, and I'd imagine the total current draw is < 200 mA. I have access to the buzzer (+13.5 and ground).


  2. jcsd
  3. Apr 29, 2010 #2
    An inductor would not be practical because of the amount of time delay you want ( 1 to 2 seconds). How about a time delay relay such as this one: http://www.ics-timers.com/op4142.html
    The OP412N8 would probably fit your needs. However, I notice that the input voltage spec is 12V +/- 10%. Going a little above that may only affect the timing accuracy, which is not an issue for you. But it's still something you should ask about. Or just google "time delay relay" for more options.
  4. Apr 29, 2010 #3
    Excellent - I'll give that try.

    So - I take it there's really no easy way of doing this with solid-state components?

    Thanks again for the fast response!

  5. Apr 29, 2010 #4


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    Hi Brian,

    Not sure what you mean by "solid state components" that just means you aren't using vacuum tubes. Do you mean with unpowered components (like R, L, C components)?

    Also, remember that whatever you do put on these ambulances really needs to be fail-safe, and needs not to piss off your insurance carrier. If you put a circuit in and it fails later, and an accident results ("Hey, I thought we parked the ambu right here...."), that would be bad.

    So a reasonable alternative would be to install a momentart push-button switch in series with the power wire for the buzzer. You can mount it in some convenient place that makes all the shifting and pulling or whatever easy to do while you hold the button in. The wire should go through the Common and Normally Closed contacts of the switch, so that the buzzer will operate normally unless the button is held in.

    Would that work instead? It's a whole lot more fail-safe, and I doubt your insurance carrier would mind. I bet the medics will appreciate being able to keep the buzzer from going off -- just make sure they understand that they can't be taping the button down with their med tape.... :biggrin:
  6. Apr 29, 2010 #5
    Sure there is. Google is your friend. Just add "solid state" to the search query. You should be able to find solid state modules for "on delay". An example: http://relays.tycoelectronics.com/datasheets/VTM1.pdf The VTM1QCD looks like it would fit your application. You set the time delay by adding a resistor with value determined by the formula given in the spec sheet.
  7. Apr 30, 2010 #6
    Thanks for the great replies!

    Berkeman: Good points with liability concerns. However, this circuit (reminder buzzer) is not a requirement and is something that we add to our ambulances as a convenience. So, in the worst case the buzzer simply would fail to operate and it would be like every other ambulance... Also - you were right in that by "solid state" I simply meant 'no moving components' (i.e. RLC type circuit). Perhaps it is the wrong terminology... As far as the button - it would work, however that's yet one more thing the driver has to keep in mind - it really needs to be automated to make everyone happy.

    Turtlemeister: This is exactly what I was looking for!!! It goes right inline with the circuit, it's not a relay, and it has variable time delay. I see this being the most reliable type of setup and easiest to install. Perfect!!! I had Google'd this before and I found more complex units with relays, etc. Great find!

    Thanks again to everyone for all the help!

    - Brian
  8. Apr 30, 2010 #7


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    Why not use a thermal delay? The sort that brings courtesy lights on slowly in motor cars.
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