Notes on the Link intercom

[Alpha]

Folks,
the transformer depicted in the diagrams is a simple audio style transformer, with a 1K centre tapped winding (for the phone handset side of the cct) and an 8 ohm winding, which is used to couple dial tone, ring tone and busy tone (where it's used) from IC1's output pin 5, to the caller.

The pin number on IC1 where D3 is connected is pin 4 - pins 3 and 11 aren't shown either, and they form the voltage control pins for each half of the NE 556 dual timer chip. You can connect 0.1uF greencaps from each pin to ground for a smoother tone output and more stability in the circuit, especially if your power supply arrangements are a little rough.

Connecting the buzzers inside older pulse dial handsets shouldn't be too much of a problem. You can use 2 sided tape to stick them to the inside of the base of each phone handset, and sonnect them via the phone's switch hooks. You may need to check the handset circuit diagram, or alternatively, trace it out on the pc board, to see where the AC ringer's blocking capacitor is. You will need to bypass it (use a jumper wire) as the DC signal from the Link's ringer circuitry won't pass trhough the blocking cap.

Hope this answers all current questions for now.

Austin Hellier
Downunder

[Ashfeel]

Hi sir,
        i tried your link intercom and got a perfect result for 9 phone handsets unless i connect the buzzer to the switch hook inside the phone handset. When the phone is off-hook then the collector of its associated transistor is getting a voltage due to the voltage at the pin of handset's switch hook in off-hook condition. Due to this there is an additional clock pulse to the counter for the output pins after that particular handset, for example if #4 is off-hook and we dial #6 then the ring goes to #7 instead of #6. Could u please help me out for connecting the buzzer properly to the switch hook so that i can get rid of that voltage. Please reply me as early as possible, its very urgent.
   Thank you..

[vihangas]

Hi There,
I have a question on design "The Link Telephone Intercom " The Qx transistor used has Collector pin open. Is it intended to tie up at GND?