The Original 2 Phone Link=

Posted on Feb 4, 2014

This is the circuit that sparked off all the other Link Telephone Intercom designs. Originally designed back in 1996 with heavy duty relays and their contact banks, it was updated late last year with the addition of IC1 in place of a simpler transistor multivibrator, the replacement of all relays and contact banks with optocouplers, and the additi

The Original 2 Phone Link=
Click here to download the full size of the above Circuit.

on of the two transformers as part of the transmission bridge (see explanation below) which was comprised partly of the relay coils. If you just want simple ‘no frills ’ communication from one phone to another, then this is the right circuit for you! This version is much more compact, more economic on power drain (yes, you can run it off of batteries – eg: a 12 volt Gel Cell!) and has no moving parts, except for the switch hooks inside each of the phone handsets. This was one of the earliest Link designs. It works by either caller simply picking up their handset and ‘buzzing ’ the other phone. There are no numbers to dial (in fact, no dial tone either – just ring tone, which pulses on and off in unison with the buzzer at either end when a call is in progress. ) Let ’s assume that #1 picks up their handset. This forms a DC loop between the handset, the 1k winding of Tx1, the 0v_ earth terminal, the +12 volts terminal and the leds inside OC1 and OC3. IC1 (an NE556 dual timer chip) is always ‘on ’ –that is, it ’s always pulsing on and off at output pin 9, which then drives the second half of the timer chip on and off via D1 to pin 4, producing ring pulses, and ring tone from pin 5 via C3 and C4 to the 8R windings of Tx1 and Tx2. The transistor inside OC1 switches on hard, taking its collector lead low, and ring pulses are fed from pin 9 of IC1 via R5 and Q1, to the emitter of OC1, through to its collector lead, and then on to...

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