I have an old phone made in the 1980s and would like to know what the wires do.?

There are two wires, a green and a red. I would like to use this in a project but cannot find infomation on what these two wires do and how much voltage/amps they take. Can anyone here help me out?

11 Answers

  • Steven
    Lv 7
    3 weeks ago

    A "POTS" phone uses 3 different interfaces.

    1. When on-hook, the phone is a capacitor coupled ringing circuit that blocks DC but accepts a ~20Hz high voltage ~90VAC to ring the bell, ~5KOhm.

    2. When off-hook, the phone draws about 30mA DC to activate the phone exchange. This drops the -48VDC open voltage to about -7 VDC. Pulse dialing interrupted this current one pulse per number, and 10 times for zero. "Touch-tone" dialing uses two tones of 8, selected by the keypad matrix. Negative DC voltage is used to reduce  corrosion.

    3. During the call, a "hybrid" circuit partially separates local and remote audio signals on a 600 Ohm  audio transmission lines, although the impedance is actually frequency dependent because long phone lines are actually about 100 Ohm.

    Rural phone lines typically use 3 wires, ie 2 wires + ground. The ground is used for the ringing circuit only.

    The choice of these voltages etc are historical, dating back long before your 1980 phone to a time when the entire system was electro-mechanical and no electronics were available.

  • 3 weeks ago

    The red and green wires carried the signal on the system. They are very small wires and do not carry Voltage and Amperes (well, they do but at very small amounts). A phone has a microphone and a speaker connected in series between the red and green wires. When you talk into a microphone, it converts the sound waves into small electrical signals on the red and green pair of wires. The speaker will convert those electrical signals into sound waves. The telephone does not generate power, it generates an electrical signal on the telephone line.

  • 3 weeks ago

    In the US, POTS phones (Plain Old Telephone Service) were driven by a 20 mA loop.  The phones had an impedance of 600 ohms, so the voltage across tip and ring with the handset offhook was about 12 volts.  (anything between 6 and 24 volts will work OK)

    If you have two phones and want them to talk to each other...  Connect the green wire or one to the red wire of the other, then connect the remaining green and red wires to a 12 - 24 volt source. 

  • 3 weeks ago

    They are the tip and ring wires of the phone and operate @~32V.

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  • 3 weeks ago


  • oyubir
    Lv 6
    3 weeks ago

    Phones, from 19th century to the 21st (that is all analogical phones) follow a very simple protocol: just a pair of wires connecting everything in parallel (microphones of both sides, speaker of both sides, and later, keyboard of both sides).

    Since all technological evolution appeared progressively, they had to be compatible with older technology. So, even the latest analogical phones were still following model of a point to point communication.

    You pick up the line, and your microphone and speaker are connected, in parallel with the one of the switchboard operator, until the operator physically connect your line to the one of your peer.

    Later, automatic switch replaced operators. You could operate the switches remoteli through impulses sent on the same line as the voice (each impulses trigger a rotation of a sort of step motor, physically positionning a swictch). And to create those impulses you used a dial keyboard. But from the protocol point of view, it was exactly the same as picking the line, being connected point-to-point to an operator, and saying "Operator, connect me to 1414, DC". Except that the operator was a bunch of steps motors, and what was "said" on the lines was a steps signals for those motor.

    Later those were replaced by dtmf or similar signal for electronic switches. But again, from the communication point of view nothing has changed: point to point communication with an electronic switch, listening to your vocal order. That order is not a human voice, but a series of DTMF sounds. But still, it is just sound (and the keyboard is just an alternative device playing the same role as the microphones. If you were able to sing DTMF tones, you would not really need a keyboad to "speak" to the "electronic operator".

    The point is, from an electrical point of view, phones were just a point-to-point interphones. Everything that made them apparently more sophisticated came from the network.

    So, practically the line is supposed to be powered by a 48V DC (also for historical reason: at first, network were not providing the tension, and people were not supposed to have electricity at home. So phones came with 48V DC batteries, and each devices were powering the line). 

    On which are connected, in parallel (when phone is picked up, otherwise, it is an open switch) a speaker, a microphone, a keyboard, a bell, and the other side.

    There is not really a + and - (it is DC current. But nevertheless phones contained a diode bridge. So you can basically ignore wire color)

    Norm impose that any device connected to the line never take more than 60mA.

    So if you want to use one of those old phones, there is not really any need to understand any protocol (it is not the devices that were following a protocol, but the network). Just plug approx 48V, modulate around those 48V any sound you want and speaker will play it. Bell is just another speaker, very louder, but connected in serial with a band-pass filter, so that it really only "play" 50 Hz. 

    And the 48V tension you will connect to the line will be modulated by the phone also.

    It you want to use the keyboard, you'll have to analyze the signal (how the 48V are consumed by the phone), for example (I take you want to connect that old phone to some modern electronic) by fourier transform, and react when you see a peak in 2 of the DTMF set of frequencies

  • Andy
    Lv 6
    3 weeks ago

    The two conductor were commonly known as "tip" and "ring". See below for information on this and how telephone circuits work.




  • 3 weeks ago

    If you cut the red wire, the phone will explode.

    Source(s): Terminator 2
  • Jay
    Lv 6
    3 weeks ago

    positive and ground

    • Mark
      Lv 7
      3 weeks agoReport

      I think about 12 V in the "talk" phase, 50 in the "ring" phase.  If you held on to them when it was ringing, you'd feel a zing, but you wouldn't die.

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