alarm extension - over the mains

alarm extension ... The garden party is in full swing .. . Amidst all the happy noises and chatter it is difficult to hear when the telephone rings or a late guest rings the front-door bell. The alarm extension described here will enable you to hear the phone ringing wherever you are, provided there is a mains socket close . . . over the mains at hand. The principle is well-known: an intercom which uses the mains wiring as the trans- mission channel. This is a very handy gadget which can be used wherever a mains socket is available. Speech facility is not provided: the receiver merely indicates that the trans- mitter has "detected" a certain sound, which may come from the telephone bell or from another source. General principles The transmitter and receiver contained in the alarm extension are shown in block form in figure 1. The signal detected by the transmitter is amplified, rectified, passed through a high-pass filter, and then used to switch an astable multivibrator (AMV). Th...