Baby monitor operated remotely via public telephone network - has amplifier for microphone and time-delay clock releasing relay

摘要

This baby monitor is used to check for noise from a nursery via the public telephone system. A detector, connected to the telephone lines, detects the ringing tone and operates a lock-on relay. The relay starts a clock and switches the output of an amplifier onto the line, stopping the ringing tone by the change in impedance. The amplifier amplifies the baby noises, if any, detected by the microphone. After a predetermined time the clock releases the relay disconnecting the amplifier from the line and finishing the call. An alternative version has a balance transformer between the detector system and the telephone line.