small signal Activity detector

Discussion in 'The Projects Forum' started by picaboo, Sep 26, 2011.

  1. picaboo

    Thread Starter New Member

    Sep 26, 2011
    3
    0
    Hi,

    Glad to find this site. I had read lot of postings here to get some help on what I am trying to do, but it is all messed up in my head now.

    I have a small digital table alarm clock, the kind with big black-white LCD readings.

    The clock runs with a single AA battery. I measured the signal at the buzzer when it beeps. It is badly shaped sine wave with 1v p-p. The alarm is on for entire 1 minute. During this 1 minute, the beep happens once every second. The beep sound frequency is 500Hz or so.

    I am trying to get a level high (anyting between 5v-12v will do) when alarm beeps. I mean that I want to see the output of my circuit goes high when the buzzer starts beeping, stays nicely high when it is beeping and goes low when the buzzer stops and 1 couple of seconds pass.

    My circuit will have separate power source.

    This was my approach.

    Since the signal is driving a buzzer, I can connect this signal to a 555 trigger input directly (similar to touch switch project) and get a square wave (rather series of pulses) at the 555's output. I need to make sure this output is high when alarm is not activated.

    This output is then connected to another 555 in "missing pulse detector" configuration.

    Will this approach work?

    If there is any easier approach, may be using only a few discrete components, that would be great. Please guide me.
     
  2. picaboo

    Thread Starter New Member

    Sep 26, 2011
    3
    0
    Actually, after some more thought, it seems better to detect the envelop of the buzzer signal first and then convert it to a nice shape.

    Here is my new thought.

    1. Amplify the buzzer signal (using Transistor or 741)
    2. Use a cheap envelop detector (like http://www.st-andrews.ac.uk/~jcgl/Scots_Guide/RadCom/part9/page2.html) with approrpiate cap and resistor values to hold the charge during 1s off duration.
    3. Feed output of envelop detector to a schmidt trigger

    Feedback, suggestions please?

    Thanks.
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    What about a simple monostable 555 timer with a 1minute delay. The delay will have your 5V High signal driven by a transistor off pin 3 of the 555. Perhaps having the 555 trigger off the first high pulse from the clock's alarm would work better.
     
    picaboo likes this.
  4. picaboo

    Thread Starter New Member

    Sep 26, 2011
    3
    0
    Love the idea. Thanks a lot Ionic.

    I was not sure if the other pulses except the first one would affect the 555 trigger. I did a little more search and found that once triggered, 555 will ignore additional triggers till it comes back to its stable state.

    http://www.kpsec.freeuk.com/555timer.htm


    I have to invert the buzzer signal though. It is 0V when inactive and 555 gets triggered with 0V.

    Should not be an issue. A PNP in switching mode. Base is driven by buzzer signal, collector is connected to 555 trigger.

    Thanks.
     
  5. iONic

    AAC Fanatic!

    Nov 16, 2007
    1,420
    68
    Nice research... You told me everything I could have told you and you found it yourself!
     
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