Mine paging and telephone system

Claim 1

A paging and telephone communication system especially useful in mines, comprising a plurality of stations interconnected in parallel by a pair of metallic line conductors, a dry cell battery energizing said system, each station including a handset comprising a receiver, microphone and a Push-to-Talk switch, each station also including a handset amplifier controlled by said switch, and energized by said dry cell battery, an audio, bridge type speaker amplifier also energized by said dry cell battery and driven by voice signals appearing on said line conductors and comprising a first pair of complementary symmetry transistors connected together at a first mid-point, a second pair of complementary symmetry transistors connected together at a second mid-point, a loud speaker directly connected between said first and second mid-points, a voltage divider comprising a pair of resistors connected together at a third mid-point which is electrically connected to said second mid-point, the other terminals of said speaker amplifier other than those connected to said first, second and third mid-points of said first and second pairs of transistors and of said voltage dividing resistors being connected respectively to said line terminals to provide a double bridge circuit, means for maintaining said first and second midpoints at a constant voltage value, and a paging circuit having a Push-to-Page switch associated with said handset for applying D.C. bias voltage through said line conductors and line conductors of another station to complete an energizing circuit in said other station to a loudspeaker of said other station and to amplify any voice signals appearing in said line conductors, a release of said Push-to-Page switch removing said bias voltage and deactivating said loudspeaker of said other station and allowing conversation to continue only on said handsets of the respective stations, the entire system becoming dormant upon release of both said handset switches, said entire system, including the connection between said first and second mid-points and loudspeaker, being devoid of large capacitors and inductors that store energy in amounts sufficient for initiating ignition, and means for limiting the paging current to less than 1 ampere under short circuit conditions to prevent ignition in an atmosphere of methane-air mixtures and coal dust.