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 Name | Manufacturer | Quantity | Buy KitBuy Kit |
|---|---|---|---|
| 0.1uF 25v Capacitor | 5 | Buy NowBuy Now | |
| 1uF 50v Capacitor | 2 | Buy NowBuy Now | |
| 20pF Capacitor | 2 | Buy NowBuy Now | |
| 2.2uF 250V Capacitor | 1 | Buy NowBuy Now | |
| 1N4001 Diode | 5 | Buy NowBuy Now | |
| IRF540N N-Channel MOSFET | 2 | Buy NowBuy Now | |
| 2N2904 NPN Transistor | 5 | Buy NowBuy Now | |
| HT9170D DTMF Decoder | Holtek | 1 | Buy NowBuy Now |
| CD4047 Multivibrator | Texas Instruments | 1 | Buy NowBuy Now |
| DC/DC Converter 5v / 1A | RECOM | 1 | Buy NowBuy Now |
| LM317T Adjustable Voltage Regulator | ON Semiconductor | 1 | Buy NowBuy Now |
| 3.579545MHz Crystal | ABL | 1 | Buy NowBuy Now |
| DPDT Relay | OMRON | 5 | Buy NowBuy Now |
| 75k 1/4 Watt Resistor | 1 | Buy NowBuy Now | |
| 220 Ohm 1/4 Watt Resistor | 2 | Buy NowBuy Now | |
| 22 Ohm 1/4 Watt Resistor | 1 | Buy NowBuy Now | |
| 56k 1/4 Watt Resistor | 2 | Buy NowBuy Now | |
| 100k 1/4 Watt Resistor | 1 | Buy NowBuy Now | |
| 150k 1/4 Watt Resistor | 1 | Buy NowBuy Now | |
| 68k 1/4 Watt Resistor | 1 | Buy NowBuy Now | |
| 1k 1/4 Watt Resistor | 5 | Buy NowBuy Now | |
| 300k 1/4 Watt Resistor | 1 | Buy NowBuy Now |
Additional Parts:
| Product Name | Quantity |
|---|---|
| RJ11 Telephone Jack - right angle | 5 |
| 120V / 18V Center Tap Transformer | 1 |
| Prototyping PCB | 2 |
| PCB Standoffs + Screws | 4 |
| Metal Enclosure | 1 |
| SOP-20 Breakout board | 1 |
 | Derek Brodeur - Schematic.pdf |