| 摘要 |
A telephone test set that performs dry loop testing of telephone lines has a passive speech network, a speakerphone circuit, and an amplifier and a speaker coupled to the speakerphone circuit. The passive speech network and the speakerphone circuit area isolated by the use of latching relays which are switched by a relay driver circuit that provides a short current burst. Line power can be used to power the speakerphone, so as to extend battery life of a battery which can augment the line power as needed.
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