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Audio sensing.

  1. May 2, 2007 #1
    This is my first post and I joined this forum to elicit some help with a aggravating situation. There is a public address speaker in the shop. It has a 1000HZ annunciator signal that lasts for app. one second before the human speech announcement. This frequency must be designed to be the most offensive to the ear and I wish to eliminate it.

    I want to detect the 1000HZ tone to turn on a relay. My problem is I don't remember how to detect this and convert it to a DC to energize a relay to complete my timing circuit. There isn't any DC or AC on the speaker wires before the public address system is activated. Then there is a app. 1000HZ (may be 950HZ) that is at 16.5VAC. My Fluke meter is a little slow for the one second annunciator but that is the best I can read and my readings may be a little off.

    I thought about a band reject filter but 1000HZ is right in the middle of human speech so I just want to detect it to start the operation of my timing circuit.

    Thanks for any and all suggestions.
  2. jcsd
  3. May 2, 2007 #2


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    Is the system used for any emergency notification or other safety function? If so, OSHA would frown on you messing with it.

    Have you looked into the public address equipment to see if there is an option to turn off the beep, or change the tone frequency?
  4. May 2, 2007 #3
    The location is in a electrical shop in a large bakery. There is a potential for a safety function but I only want to block the tone at one speaker in the shop. There is almost constant usage of the PA calling for ingredients and such so the this tone is going on quite often. I do not want to mess with the overall function of the system because the rest of the bakery is loud and the tone is needed to attract attention. My idea with the audio detect timing relay is an improvement to the local off switch I installed seven years ago in which large blocks of speaker time are just simply cut off. I have the timing function figured out and only want to detect the initial use of the system so I can use the timer to open the local speaker circuit for one second during the tone. Thanks
  5. May 2, 2007 #4


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    You can use a diode and capacitor to detect the start of audio. A one second delay timer to latch a relay. When the speach ends the cap will discharge to 0v for a signal to release the relay.
  6. May 2, 2007 #5
    Would a general purpose diode work? What size and type of capacator? I have been out of electronics for decades and am strictly electro/mechanical now.

    I tried using a general purpose bridge rectifier in parallel with the speaker coil to try and extract a DC but was unable to detect any DC with my fluke meter with a loading resistor of 500 ohms on the bridge output when the circuit was energized.
  7. May 2, 2007 #6


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    The bridge rectifier should work fine unless the signal is lower than the combined diode drop (about 1.2v).
    A 10uf electrolytic should do it.
    You don't want a loading resistor here or a very large value like 1 meg ohm.

    Without the capacitor the bridge output is still very much AC to the meter. So getting a DC reading seems unlikely.
  8. May 3, 2007 #7
    Tried it with the bridge rectifier and cap but still no go for developed DC. I put a 12V lamp in series with the speaker coil to see how bright the bulb glowed when the annunciator was on and it did not even glow. I have never worked with audio but thought that the bulb should at least glow a little. If it was RF I could use a pin diode to extract the signal.
  9. May 4, 2007 #8


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    Try just one diode and the cap then.
    There may not be 1.2v acrost the speaker terminals.
    If that doesn't give you a reading then you will probably need an op-amp.

    Some arrangements are fairly efficient and don't take much drive. Not enough to light a lamp anyway.

    See what an AC peak hold on your meter reads connected to the speaker trminals.
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