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Help needed to design and build trigger for Ambulance siren

Discussion in 'Electronic Basics' started by Les Desser, May 6, 2007.

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  1. Les Desser

    Les Desser Guest

    Not sure if this is the right place to ask, but give it a go anyway.

    I need a small switching device to trigger and stop an ambulance siren.
    Can anyone help? I am totally ignorant in this matter and would be
    willing to pay for the finished product. We are in London, UK.

    The existing siren is a sealed unit and is triggered by the vehicle horn
    button. The first tap on the horn starts it off with tone 1 which
    continues until the next tap moved it on to tone 2 and then again for
    tone 3 and round again.

    A double-tap on the horn switches it off.

    The next tap starts the cycle from tone 1.

    What we need is a momentary activation switch pressed by the driver
    which will:-

    1. Simulate the horn tap (presumably just need to close a contact for a
    fraction of a second)

    2. Start a count-down timer (preferably user changeable) for x seconds
    (probably 3-5 seconds)

    3. After the x seconds it should simulate a double tap (close-open,
    pause, close-open) to shut off the siren automatically.

    The unit would need to be suitable to mount under the dashboard of the
    ambulance with a push-switch on the dash. Working off 12-16V vehicle

  2. You could use 3 12vdc delay relays if you don't want to do electronics.
    Otherwise there are a number of electronic solutions including a BASIC stamp
    microcontroller and related components:
  3. Les Desser

    Les Desser Guest

    I could cope with stringing relays together! The only hit on Maplins is
    <> which is
    not what I think you had in mind.

    Other hits on Google also do not seem relevant.

    What exactly did you have in mind?
    Anyone prepared to build one for us?
  4. Actually, yes. You may find it is cheaper and easier to use three of these
    than to engineer a fancier solution.
  5. Les Desser

    Les Desser Guest

    I think I understand what is being suggested:-

    1. Momentary push button triggers the siren and closes a normal relay to
    supply continuous power to 2 delay 'on' relays. The 1st relay is wired
    to maintain its own supply - to be subsequently broken in step 3.

    2. The timing on the two relays to be set to within 0.5 seconds or less
    of each other to simulate a double press on the horn.

    3. The NC contact on the last relay to fire will open and cut off the
    power supply to all relays. This assumes that the NC opens after the NO
    contact closes.

    Am I getting warm?

    On further though that is not good enough. The first delay relay must
    close and then immediately open. So it must cut off its own supply.

    I do not have the spec of the siren as to how close the two pulses have
    to be but would assume that 0.5 sec should do.

    Could two delay relays be set to reliably fire that close together?

    Links to suitable relays (preferably UK) would be appreciated. I cannot
    quite see how I could use the Maplin offering.

    Thanks Homer for your responses.
  6. IMO it's worth trying. Does this whole thing have to be triggered by the
    horn - it can't be operated by a separate switch?
  7. Les Desser

    Les Desser Guest

    I'm keen to give it a go as long as I can get hold of suitable relays.
    It is to be used separate from the horn. The horn functionality must
    stay unaltered.

    This new device would be triggered by a non-latching push-to-make

    I am waiting for a spec of the siren system to see the min/max timings
    and whether voltage needs to be applied or just a circuit closed.
  8. Ross Herbert

    Ross Herbert Guest

    There may be some statutory regulations regarding equipment which can
    be used on emergency vehicles such as ambulances. The authorities may
    not allow DIY sirens or controllers so you had better check to see if
    only approved types (UK legal) can be used in your case.

    It may be more economical to "go the whole hog" and lash out on a
    controller off-the-shelf rather than DIY. Woodway Eng'ng in the UK
    make a whole range of systems including sirens and controllers

    eg. (couldn't get sample sound#2 to
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