A power switch device is used with a telephony instrument that is connected to
a local phone loop via two telephone wires and that has two power lines for connection
to a power source. The device includes a rectifier connected to the telephone wires,
and a latching relay having an exciting coil connected to the rectifier via a differentiator,
and a switch unit. The coil is excited by current flowing through the differentiator
when a ring signal is present at the telephone wires so as to enable the switch
unit to make connection between the power lines and the power source. An integrator
is connected between the rectifier and the coil, and discharges when neither the
ring signal nor a talking signal is present at the telephone wires so as to excite
the coil and enable the switch unit to break connection between the power lines
and the power source.
