Version 4

    Multi-Line Telephone Intercom

    element14 Presents  |   Derek Brodeur's VCP Profile  |  Project Videos



    Modern smart phones have become so ubiquitous these days that old analog phones are getting left in the dust. However, you may still have a few of these old phones hanging around your house collecting cobwebs or you may have seen some lingering on the shelves at your local thrift shop. Now’s your chance to put them to good use. In this video, Derek takes you back to a time when stretching a phone cord across the room was the norm by creating a 4-line telephone intercom system with basic ringing capability. The circuit surrounds a DTMF decoder IC which handles keypress detection, a simple AC inverter to handle the ringing and a current source which allows analog communication between multiple phones.



    Bill of Materials


    Product NameManufacturerQuantityBuy KitBuy Kit
    0.1uF 25v Capacitor5Buy NowBuy Now
    1uF 50v Capacitor2Buy NowBuy Now
    20pF Capacitor2Buy NowBuy Now
    2.2uF 250V Capacitor1Buy NowBuy Now
    1N4001 Diode5Buy NowBuy Now
    IRF540N N-Channel MOSFET2Buy NowBuy Now
    2N2904 NPN Transistor5Buy NowBuy Now
    HT9170D DTMF DecoderHoltek1Buy NowBuy Now
    CD4047 MultivibratorTexas Instruments1Buy NowBuy Now
    DC/DC Converter 5v / 1ARECOM1Buy NowBuy Now
    LM317T Adjustable Voltage RegulatorON Semiconductor1Buy NowBuy Now
    3.579545MHz CrystalABL1Buy NowBuy Now
    DPDT RelayOMRON5Buy NowBuy Now
    75k 1/4 Watt Resistor1Buy NowBuy Now
    220 Ohm 1/4 Watt Resistor2Buy NowBuy Now
    22 Ohm 1/4 Watt Resistor1Buy NowBuy Now
    56k 1/4 Watt Resistor2Buy NowBuy Now
    100k 1/4 Watt Resistor1Buy NowBuy Now
    150k 1/4 Watt Resistor1Buy NowBuy Now
    68k 1/4 Watt Resistor1Buy NowBuy Now
    1k 1/4 Watt Resistor5Buy NowBuy Now
    300k 1/4 Watt Resistor1Buy NowBuy Now



    Additional Parts:


    Product NameQuantity
    RJ11 Telephone Jack - right angle5
    120V / 18V Center Tap Transformer1
    Prototyping PCB2
    PCB Standoffs + Screws4
    Metal Enclosure1
    SOP-20 Breakout board1