AUX output to Analog Phone signal

Discussion in 'Analog & Mixed-Signal Design' started by mecky.reuss, Sep 9, 2017.

  1. mecky.reuss

    Thread Starter New Member

    Sep 9, 2017
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    Hi,

    I want to play a recording from a mp3 player and translate it to an analog phone signal. I want to pick up the phone that is connected to the mp3 player via a regular RJ45 plug without having to modify the phone (it is a collectors piece) and hear the recording from the mp3 player (on loop).
    The mp3 player has an AUX conector. Anybody who can help me with the circuitry?

    Thank you,
    Mecky
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Is the phone connected to the telephone network or is it a stand-alone phone? If standalone, do you have a power source? What is it and how is it connected?
     
  3. mecky.reuss

    Thread Starter New Member

    Sep 9, 2017
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    Hi,

    It is a standalone phone that at the moment is not connected to anything. I want to connect it directly to the Aux with something in between that takes care of power and getting the signal from the aux into it. Does that make sense?
     
  4. dendad

    Well-Known Member

    Feb 20, 2016
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    Try something like this...
    PhoneAudio.jpg
    You will have to fiddle with values to suit.
     
  5. mecky.reuss

    Thread Starter New Member

    Sep 9, 2017
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    I am not too fluent in electronics. Would you mind being a little more detailed about the components?
    Thanks!
     
  6. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    If it is a standard old phone, one with no electronics in it, then you don't need anything other than a low-power audio amplifier. No power supply, no transformer, nuttin. A phone needs a DC bias current to run the microphone, but not the earpiece.

    Driving the tip and ring of the phone jack (the two center pins of an RJ-11 (not RJ-45) jack) will go straight to the earpiece. An MP3 player earphone jack should make enough output voltage and current to do this. If not, then there are small audio amplifier boards on ebay for about $2.

    Separate from that, if you want to detect when someone picks up the phone so the recording always starts at the beginning, that is a bit more complex. It certainly can be done, but detecting the change from on-hook to off-hook and turning that into something the mp3 player understands will take some circuitry.

    ak
     
  7. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    You either need a way to make the MP# player to start or make it continuous.

    The circuit in post #4 is about right. I'd put a 40 mA current regulator in the line. I've used an LM317. You can use a smaller part. I'd probably use this https://www.digikey.com/product-detail/en/triad-magnetics/TY-303P/237-1126-ND/ transformer with the 600 ohms secondary to the phone.

    A few other components round out the design.

    The 4K input Z should be easy to drive. Meed more specs about the MP3 player.

    This https://www.eeweb.com/blog/circuit_projects/off-hook-telephone-line-indicator can provide the basis for an OFF Hook detection if needed.
     
  8. mecky.reuss

    Thread Starter New Member

    Sep 9, 2017
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    Thanks! I don't have specs about the mp3 player yet. Let's consider it to be an old iphone. It will be started manually and put on loop. Where in the circuit would the current regulator go?
     
  9. AnalogKid

    AAC Fanatic!

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    Why? How does 40 mA of DC make a magnetic headphone function?

    ak
     
  10. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
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    Don;t really know, but I did use an xformer and LM317 as a current regulator to debug a touch tone phone.

    I never played with off hook and on hook detection, but for the most part it's voltage.

    This http://www.cermetek.com/catalog/Telephone-Line-Interface/DataSheet/CH1808_607-0012.pdf chip which I could not find easily detects below 18V as off hook. I think that's usually around 5.

    So, >18 V and about 23 mA.

    I now have a Temp Side Kick. http://www.prc68.com/I/SidekickTandN.shtml

    Makes troubleshooting telco a lot easier. I just don;t have the exchange test phone numbers.

    See http://www.sandman.com/loopcur.html for some info.

    Not sure if you need a Hybrid or would something like this https://www.bswusa.com/Hybrids-Broadcast-Tools-TT1-PLUS-P10765.aspx be a closer option and work better.
    manual: http://broadcasttools.com/wp-content/uploads/2014/10/Manual_TT-1Plus.pdf

    There's not enough info to know if it would "work" in some shape or fashion mostly by supplying loop current only.

    Idea: tap mode and use the hook relay to seize and drop the line based on Hook condition.

    It's needs a bit of extra logic, like a 5 V power supply, but it looks doable. Just verify with manufacturer.

    For line in, you would make a 48 V power supply, maybe less and a current regulator and a 600 ohm transformer. Probably a 600 ohm resistor would work.

    You'll need a source of 5V for the logic and a few other parts to make it all work.

    There could be reasons it won;t work.
     
    Last edited: Sep 10, 2017
  11. AnalogKid

    AAC Fanatic!

    Aug 1, 2013
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    Different problem. A TT keypad has an electronic circuit in it, while the rest of the phone does not. BTW, the original TT circuit is one of the great analog designs of all time.

    Note that this application uses only the earpiece; no talking, no signalling. Because of that, there is not need for a hybrid, DC bias, etc.

    ak
     
  12. KeepItSimpleStupid

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    Mar 4, 2014
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    @AnalogKid

    You could be right. Here's http://www.classicrotaryphones.com/wd_we500a-b-j-k_big.jpg a model 500 set. I have a few. it would be nice to see if I connect an audio oscillator to tip and ring what would happen? Too much going on right now to do the experiment. First problem, my 9V transistorized oscillator may not be able to drive it. I haven't used my antique HP 200C oscillator for a while. The 600:4K x former would help here.

    Thanks.
     
  13. AnalogKid

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    Imagine that.

    As shown in your schematic, there is a direct AC path from L1 and L2 to the receiver when the handset is off-hook and the dial is in the rest position. When the receiver is on-hook or when the rotary dial is in motion, the receiver is shorted out to protect both the receiver element and the user's hearing from various transients on the line.

    ak
     
  14. mecky.reuss

    Thread Starter New Member

    Sep 9, 2017
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    Worked like a charm! Without amplifier, no nuttin!!! I could make do with that but have a couple of keypad phones that I also would like to use. How difficult would it be to get those working?
     
  15. KeepItSimpleStupid

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    Mar 4, 2014
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