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 addition 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 the –ve lead of buzzer B2, thus ‘buzzing’ the other phone.
When that phone is picked up off hook, OC2 and OC4 also form a DC loop along with the 1k winding of Tx2 and the leds inside the optocouplers. The transistors inside OC1 and OC2 now form a ‘link’ (no pun intended) from the +12 volts supply (shown at left near phone #1) through to the junction of pins 12 and 8 of IC1, via diode D2. This effectively halts the ringer, and both ring pulses from pin 9 and ring tone from pin 5 cease for the duration of the call.
When both phones are hung up back to the on hook position, all four optocouplers (OC1 to OC4) are switched off. IC1 will now resume pulsing and providing ring tone, but neither one of these outputs will have any effect on either phone handset, until one or the other is picked up off hook again, to make the next call. The ‘transmission bridge’ mentioned earlier, comprises of Tx1 and OC3 along with Tx2 and OC4, as well as C5 and C6. Basically, the 1k windings prevent speech signals (which are an AC signal) from being grounded through the earth connection. The two capacitors allow these signals to pass between the handsets, but block any DC voltages from interfering with them. This circuit is known as a ’Stone Transmission Bridge’ and although I have employed a simplified version of it, it’s quite suitable for our purposes. Happy chatting on the line… ©Austin Hellier 1996 – 2003. 
D1,2 1N4148 or 1N 914
Q1 BC 547 (NPN small signal transistor) or equivalent
IC1 NE 556 dual timer chip
OC1-4 4N25 or 4N28 optocouplers
Tx1,2 1k/8R matching transformers
B1,2 9 volt DC buzzers
Miscellaneous Two phone handsets (preferably the same make and model – stay away from plastic one piece ‘cheapies’ Plug pack power supply (see diagram below) 15 volts DC at 200 mA output. Wire, solder, pc or prototyping matrix board, four wire cable, phone connectors, etc.
Download the project in .doc format
author:Austin Hellier, Wollongong City, Australia © 1996-2003,