Telephone off-hook detector circuit

Abstract

A circuit for connection into a telephone line for detecting when a telephone instrument has gone off-hook and which is unaffected by the presence of a ringing signal on the telephone line. The circuit comprises a transistor switch, a first plurality of series connected diodes connected in series with the telephone line for developing a first voltage thereacross when the telephone instrument has gone off-hook, an RC circuit coupled to the first plurality of diodes for charging to the first voltage when the telephone instrument has gone off-hook after a time delay and for activating the switch, a second plurality of series connected diodes, the second plurality being greater than the first plurality, the second plurality of diodes being connected to the switch and to the first plurality of diodes for developing a second voltage thereacross during one half cycle of a ringing signal on the telephone line. The first plurality of diodes develops the first voltage during a second half cycle of the ringing signal on the telephone line. The first and second voltages are of opposite polarity, such that the charging circuit alternately charges toward the first and second voltages during a ringing signal. The charging circuit has a time constant such that it never charges to a voltage during a half cycle of the ringing signal sufficient to activate the switch and on the average, the charging circuit maintains the control input of the switch substantially intermediate the first and second voltages.