Party Line Intercom

Discussion in 'The Projects Forum' started by mark63534, Nov 24, 2011.

  1. mark63534

    Thread Starter New Member

    Nov 24, 2011
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    http://www.opamplabs.com/plinter.htm

    I need a little help with this one. I really don't understand what op-amp is used in the circuit. More importantly, I want to make this circuit normally closed (N.C.), so taking out N.O. (normally open) would mean taking out the DPST switch.

    I really need some help.

    [​IMG]
     
    Last edited: Nov 24, 2011
  2. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This must likely uses a small audio amplifier IC, as op-amps do not normally have enough current output capacity to drive a loudspeaker.

    If you take out the switch, depending how you wire it you will only be able to talk or listen, not both. Is that really what you want?
     
  3. mark63534

    Thread Starter New Member

    Nov 24, 2011
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    Well, I was planning on talking and listening at the same time. I would like to make multiple intercoms.

    How does the switch work then in this situation?

    Very confused..... I just want to start out with 2 - way intercom with this circuit and then develop it to 3 or 4 way. I picked this circuit because it said "up to 100 sets."
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    It's push-to-talk. The speaker is also used as the microphone. You cannot talk and listen at the same time. If you want simultaneous talk/listen capability, you will need a very different circuit.

    When the switch is not depressed, the input to the amplifier comes from the party line, and the output of the amplifier is connected to the speaker.

    When the switch is depressed, the input to the amplifier comes from the speaker, and the output of the amplifier goes to the party line.

    If more than one station tries to talk at the same time, the outputs of the audio amplifiers will be connected together. This will cause high power dissipation in both amplifiers. It would be better if there was a ~8 Ohm resistor between pin 4 of the amplifier and the common of the DPDT switch; that way the amplifier output won't be damaged.

    There is no cap between the NC contact and the speaker. There should be one to block DC from flowing into the speaker; otherwise the speaker and/or amplifier may overheat, and there will be excessive power consumption.
     
    Last edited: Nov 24, 2011
  5. mark63534

    Thread Starter New Member

    Nov 24, 2011
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    A speaker as a microphone? Never heard of that one...

    It is true that it is a DPDT Switch. I was wrong with saying DPST.

    So then, how would I go about making this with 3.5mm inputs(one for the headphones, the other for the mic)? Is this possible?

    Also what do you mean by no cap for the NC?

    I'm assuming the "switch (push-to-talk)" is the DPDT switch. please tell me if I'm wrong.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    That's how many intercoms work. A speaker has a movable coil attached to the cone, and a large fixed-position magnet. Passing current through the coil moves the cone. Moving the cone causes the coil to move by the magnet, generating electricity in the coil.

    The wire going from the speaker to the left N.O. connection on the DPDT would come from the microphone instead. If the mic is an electret mike, it will need a preamplifier circuit - or at least a voltage supply.

    Cap = short for capacitor. 10uF would probably be adequate.

    Yes, that is the DPDT switch. You could also use a SPST N.O. switch, and a DPDT 24v relay - or a 12v DPDT relay and a resistor to limit coil current.
     
  7. mark63534

    Thread Starter New Member

    Nov 24, 2011
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    So then, in this case, my line in (input) would come from the mic. the speaker (to hear) would be the other man's headphones.

    You are spot on with the electret mic. I'll have to find a simple audio amp to amplify that before feeding it into the intercom. Hopefully, by using an available usb port (+5v Bias) that will be able to be powered.

    I'm still confused about the switch. If the switch is N.O. then how does the push-to-talk option work?

    Sorry if I have been too nagging. I really just want to understand this so that when I go about building it I will understand it more.

    EDIT: I understand what you mean with the push-to-talk application. What ic or amplifier do you think I can use for this circuit? The # 34 doesn't give me a clue as to which one to use.
     
    Last edited: Nov 24, 2011
  8. SgtWookie

    Expert

    Jul 17, 2007
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    OK, here is the original schematic:

    [​IMG]

    The signal comes in from LINE, goes through the volume pot, then through the NC side of the switch to the common terminal, through the amplifier, then through the other switch common to the NC terminal, and finally to the speaker.

    Here is what happens when the switch is pressed:

    [​IMG]

    The amplifiers' input (red arrows) now comes from the speaker, the amplifier output (orange arrows) gets routed out to LINE.

    A TDA2030 might work for you.
    You have to keep in mind that 24v is specified; and that eliminates a lot of amplifiers.

    I don't know what the "34" means.
     
    Last edited: Nov 24, 2011
  9. mark63534

    Thread Starter New Member

    Nov 24, 2011
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    Wow. Nice illustration.

    So then, I guess I'm back where I started. Here's how I see it:

    I'll assign some values.

    H1, H2 = Headphone 1 and Headphone 2
    M1, M2 = Mike 1, Mike 2

    Lets assume that I have two of the same circuits already developed.

    If I was to place mike 1 into one of the circuits (Line 1) and the speaker output goes to H2, when the switch occurs, the whole operation will be reversed, right?

    vice versa for the other circuit (Mike 2 as line, h1 as speaker output).

    So, if it is reversed, then the mike becomes an output(?) and the headphones become an input(??)

    This is why in the beginning I thought to omit the DPDT switch and just make the whole thing closed....
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    If you go the headphone/mic route with this circuit, you will need a preamplifier for the mic, and you won't be able to talk and listen at the same time.
     
  11. SgtWookie

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    Jul 17, 2007
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  12. mark63534

    Thread Starter New Member

    Nov 24, 2011
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    It looks like I'm going to make that one (although it looks like its gonna be much more expensive and time-consuming).

    Thank you for all the help, though. I appreciate it.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    The ComClone intercom is going to be pretty complex for someone new to electronics.

    If you have not done much with electronics at this point, I strongly suggest that you buy several inexpensive kits and assemble them. That will give you practice in assembly, soldering, and troubleshooting if it comes to that.

    If you try something complex too soon, you will feel very lost if something goes wrong. You may damage the project if your soldering skills are not very good. This kind of stuff takes some time and practice to get good at; it doesn't happen overnight.
     
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