Radio to telephone interface

Discussion in 'The Projects Forum' started by basildane, Apr 11, 2011.

  1. basildane

    Thread Starter New Member

    Mar 31, 2011
    3
    0
    I have to interface a radio (for emergency alerts) to a POTS telephone jack.
    When the radio squelch opens, it needs to take the phone "off hook" and send the sound into the phone line, then disconnect when the radio goes quiet.

    I built this circuit and it is working great. It does have a small issue with picking up noise from the building power line and false tripping, but it is rare.

    I would appreciate any critiques. Especially on my false tripping problem.

    http://dl.dropbox.com/u/18786384/alert.png

    Speaker level audio comes in to the audio jack, and the phone line connects to J3. The xformer is a standard 600 ohm telephone transformer.

    Thanks.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Have you tried adjusting the feedback on the first, or adding negative feedback to the second opamp to "desensitize" the triggering?

    Using a LPF to filter out the noise peaks could also help with false triggering.

    Elsie is a free filter design tool, give her a whirl:
    http://www.tonnesoftware.com/elsie.html
     
  3. basildane

    Thread Starter New Member

    Mar 31, 2011
    3
    0
    The feedback on the first is set to what I think is optimal to detect an event.

    Strange thing is, it was perfect for a year, then something changed (maybe some new noise source) and started tripping randomly.

    That filter site is very cool - but the specs for a filter are beyond my skill level at this moment.
     
  4. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Well, you have DRIFT to deal with. The resistors or POT in your feedback loop have likely drifted in the past year.

    Use higher quality resistors (like 1% or better) or perform a twice-yearly maintenance to adjust the trigger point to optimal.
     
  5. basildane

    Thread Starter New Member

    Mar 31, 2011
    3
    0
    I'm going to check that out tonight. Also, the LPF sounds like a great idea too.
    I wish I still had a scope...

    Thanks.
     
  6. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    Ahh.. yes, you should get yourself a scope.

    do a search... There are ways of using your soundcard and PC or LAPTOP as an o'scope.

    If you have a spare PC sitting around, you may wanna give it a whirl.

    Also look for projects for building a "frontend" for soundcard scopes. These will give you some attenuation and safety so you dont kill your soundcard or motherboard.

    If you follow the directions, you'll be fine ;)
     
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