Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Audio annunciator suppression.

  1. Apr 28, 2009 #1
    I am searching for an easy solution for an exasperating nuisance.

    There is a public address system at work and before the start of the messaage a loud audio annunciator frequency has been added to warn people of the upcoming message and I want to locally supress it but still receive the message.

    The speaker is located just above my shop and do not want to interfere with the global annunciator but only this one speaker.

    I am an electrican who was trained in electronics decades ago and my idea is that when the annunciator tone is present, rectify it and pick up a relay that starts a delay on timer that blocks the signal until timing is completed.

    The annunciator tone is app. 900HZ to 1000HZ and centered around 950HZ as measured with an o'scope. The annunciator tone lasts for one second and is very disturbing and want to cancel it during that time and then have the timer close the circuit again to hear the message. That is it in a nut shell.

    I tried using a standard bridge rectifer powering a 5V relay but the relay would not pick up and if I can pick up the relay then the local problem will be solved.

    Please give me advice on how to accomplish my goal which will make all of the electricans happy in the electrical shop. Thanks for any and all responses.
  2. jcsd
  3. Apr 28, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    Would it be easier to just put a notch filter inline that attenuated 950Hz, wouldn't have to be very narrow so not hard to design.
    It would distort the voice a bit but should still be recognizable

    The trouble with detecting the tone and turning it off is that unless you do some clever analysis of rise time etc, just detecting the frequency would mean you would also turn off the speaker when the person talking happened to hit that note.

    - pf readers are all over the world in different time zones, but mostly in USA/Europe - so depending when you post you might have to wait 8-12 hours for anyone to see it.
  4. Apr 28, 2009 #3


    User Avatar

    Staff: Mentor

    The beeper tone should only be used in case of emergencies. Have you tried talking to the folks that manage the audio paging system? You should be able to make a case for only using the beeper for emergency messages. That's how many paging systems are run.
  5. Apr 28, 2009 #4


    User Avatar
    Science Advisor

    You could connect the speaker so that it only gets sound if a relay is pulled in.

    Then you would need the following:
    a source of regulated 5 volts.
    a 567 chip
    a Picaxe 8M chip
    a transistor to pull the relay in.

    Run the speaker input line to the 567 tone decoder chip.
    This will make its output go low when the input tone matches the 950 Hz expected.

    Take this to the Picaxe chip.
    Program this to look for this low input on one of its pins.
    Look again each 50 mS to see if it is still there.
    When it isn't there any more, pull in the relay via the transistor.

    Estimate the longest time anyone would talk on the PA and double it.
    Leave the relay pulled in for that amount of time. (Easier than trying to detect speech on the line.)

    Disconnect the relay.

    If the 567 detects another message starting while the relay is still pulled in, the Picaxe can disconnect the relay while the tone is present. However, you may hear the beginning of the tone if this happened because the Picaxe would have to allow a period of 50 mS to be sure the tone was a sustained tone and not an accidental 950 Hz in someone's voice.
  6. Apr 28, 2009 #5


    User Avatar
    Science Advisor
    Gold Member

    I thought I was having a time warp.
    Last edited by a moderator: Apr 24, 2017
  7. Apr 28, 2009 #6


    User Avatar

    Staff: Mentor

    Last edited by a moderator: Apr 24, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook