Schematics > Telephone related > The Link 4+0 – Internal Intercom
The Link 4+0 – Internal IntercomThis version of the Link provides four internal intercom
phones with no outside line access.
See also the
other Intercom circuits from
Austin Hellier
listed here
The Link 4+0
This version of the Link is for those who really need a
good cheap intercom that will work reliably, but without access to an outside
Telco line. This could be in a pre school, a hobby farm or a small workshop or
factory, where external phone traffic needs to be kept separate and protected
from phone abusers and kids playing around… It follows mainly the principals of
the Link pulse dial circuit, but with the modifications and additions of the
Link A2B+1 for DTMF dialing.
Circuit Explanation
Basically what I’ve done is remove the OSL relay and the
flip flop (FF2) that controlled its activation and release, and added an extra
two internal handsets and their associated relays and components. FF1 (as part
of the Ring Trip Circuit or RTC) is still there, but is now shown down the
bottom of the diagram for simplicity’s sake. Internal wiring for the RTR relay
is as per the Link A2B circuit. When any phone is picked up off hook and a
number from 1 to 4 on the keypad is pressed, the DTMF decoder chip (IC2) decodes
this into IC3 (1 x 16 decoder) and the output of IC3 is then fed to the
appropriate base resistor of Q1 to Q4 (R11 through R14).
Pin 11 of IC3 goes low, removing the high from pins 12 and
8 of IC1 and impulsing of the selected line relay (LR1 to LR4) begins, via
driver transistor Q5 and the appropriate buffer transistor (Q1 to Q4). When the
called party answers the call, the RTC circuit ‘trips’ the ring current,
impulsing along with the ring tone is halted, and the conversation can proceed.
When the conversation is completed and both phones are hung up, the collector of
OC1’s phototransistor goes high and resets both FF1 and IC3. Pin 11 of IC3 goes
high again, halting the impulsing action of IC1 but providing dial tone from pin
5 in the reset state, ready for the next call. One extra nifty little feature
involves the # output (pin 14 of IC3). If you connect this to the set (S) input
of FF1, you can halt the ringer if someone calls your extension while you’re
away. You can pick up another extension anywhere else and press the # key and
effectively ‘pick up’ the call. This feature prevents the other phone from
continuing to ring and also halts the ring tone from overpowering your
conversation. In principal, it follows very closely the features of the Link
pulse dialing version, except that there’s only a two wire circuit between each
handset and the ‘black box’ switcher. In reality, tone dialing is more
efficient, and you could easily use some of the other unused outputs of IC3 for
remote control purposes (turning on sprinklers or low voltage garden lights,
door entry systems etc.)
I hope that you have as much fun building and using these
circuits as I did in designing them AH – Downunder.
Download the project in .doc format
See also the
other Intercom circuits from
Austin Hellier
listed here
Title: The Link 4+0 – Internal Intercom
Source: www.electronics-lab.com
Published on: 2005-02-10
Reads: 218
Print version:  Other schematics from Telephone related
Telephone Hold Button Phone line indicator Multipurpose Circuit for telephone The Original 2 Phone Link Design Audio Visual Ringer Two line intercom plus a telephone changeover switch The Link A2B+1 (the Link Telephone Intercom - DTMF version) Telephone Ringer using 556 dual timers Cut Phone Line Detector Ringing Phone Light Flasher
Schematics > Telephone related > The Link 4+0 – Internal Intercom | 
Home
