Connect with us


Discussion in 'Electronic Basics' started by [email protected], Jul 16, 2006.

Scroll to continue with content
  1. Guest

  2. Alan B

    Alan B Guest

    Hope you didn't miss this bit:

    "The only thing your little intercom cannot do is ring the phone to tell
    the person at the other end to pick up. The "ring" signal is a 90-volt AC
    wave at 20 hertz (Hz)."
    Because you obviously missed this bit:

    "...a 9-volt battery (or some other simple power supply) and a 300-ohm
    resistor that you can get for a dollar at Radio Shack."
  3. Chris

    Chris Guest

    Yup. And if Radio Shack doesn't have a 300 ohm 1/2 watt resistor, then
    270 ohms or 330 ohms will do just fine.

    Have fun
  4. jasen

    jasen Guest

    I've done it.
    use a 9v battery. if you get tired of paying for new batteries get a plugpack.

  5. feebo

    feebo Guest

    it will.

    ummm, try a 9 volt battery

    In the pic, the 300 ohm resistor is a non-standard value - if you are
    using UK spec phones, go for a 470 ohm - this will extend the battery
    life too. Also if you are in the uk, and using modern phones with
    electronic ringers, you can make the bell "ring" by rigging a switch
    at each end to connect pin 2 & 3 of the standard line jack together.
    Press the button to attract the other ends attention then lift the
    reciever and wait for them - i.e. it won't work if you have already
    lifted the handset.
  6. Don Bowey

    Don Bowey Guest

    The article told you to use a battery. Do you mean how to get the 9V
    *without* a battery? You could use a wallwart.
    You do not need the resistor. Modern telephones have a DC resistance of
    about 430 Ohms, so the two phones in series will limit the current to about
    10.4 mA., which is a good value.

  7. Puckdropper

    Puckdropper Guest

    Obviously the 9V battery is too hard. The OP needs a simple solution
    like using 6 AA batteries, or 7.5 NiMH batteries. (Or 8 and a standard

    *removes tounge from cheek*

  8. Guest

    I wired it, and it works.

    Out of curiosity, does it matter if the power is on the red wire or
    green wire? The wires appear equal.
  9. Alan B

    Alan B Guest

    Terminology lesson: red is tip, green is ring. That means, on a bantam
    plug, red is the center conductor (the tip) and green is the outer ring of
    the plug. Your simple intercom is a current loop, so under normal
    circumstances the polarity of the voltage and the direction of current
    aren't important. Here's a crude drawing of how it works, ignoring the
    hook switch and ringer circuits:

    Just out of curiosity, how are you going to let the person on the other end
    know he or she is being summoned to the intercom?
  10. Puckdropper

    Puckdropper Guest

    Shouting "Hey! Pick up the phone!" would work. It at least saves you
    from having a shouting conversation.

    This is an electronics group, though, we can do better. Isn't the ringer
    activated by sending a higher voltage on the line?

  11. jasen

    jasen Guest

    yeah, it should be AC too... 25 to 20 Hz

  12. Guest

    Is there an easy way to get this 20 Hz without a frequency generator?
  13. Lord Garth

    Lord Garth Guest

  14. Alan B

    Alan B Guest

    Well, it would be an interesting project to produce 90VAC 20Hz from 120VAC
    60 Hz. Food for thought.

  15. Ma bell has done it for decades. The old 1A2 type phone system power
    supplies did it with no problems. Junk 1A2 systems can still be found
    cheap, or for free. They also supply the filtered DC for the talk
    circuit, and power for all of the lamps on the phones.

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  16. jasen

    jasen Guest

    some other sort of oscilator, like a 555, you'll probably need to boost the
    voltage a bit though,
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day