Undesired listening-in to phone conversations may be prevented by the present circuit, which requires only two components to protect a telephone.
In each telephone to be protected, a traic is in each telephone to be protected, a triac is inserted in series with the line. A diac is connected between the anode and the gate of the triac. The trip voltage of the diac is about 25 V, which is appreciably lower than the line voltage of about 50 V, and considerably higher than the potential across a telephone is set with that is in use (5-12 V). Thus, the triac is switched only if the receivers of all the parallel telephone are the hook. As soon as a receiver is lifted, the voltage across all sets drops from about 50 V to at most 12 V, so that no diac can be tripped. If after the receiver has been lifted, the polarity of the line voltage reverse, the diac is not affected. When a call signal is received, all triacs conduct, so that the bell on each-telephone will ring.
An added advantage of the preset circuit is that it suppresses the parallel thinking of telephone when a number is dialed on one of them (this happens only where older telephone exchanges are still in use).
The circuit is small enough to be built into the telephone line outlet box.