Frishauf, Holtz, Goodman & Woodward
Ringing signal sending-out circuit
 A circuit for sending out a ringing signal of a high voltage and a low frequency to a telephone set through a telephone line repeatedly with a predetermined time interval. In order to omit use of a high power oscillator and relays, a D.C. power supply of a high voltage is operatively connected to the telephone line through phototransistors of photocouplers being driven by a pulse signal having a low voltage and the low frequency under control of pulses occurring at the predetermined time intervals so that the ringing signal is sent out to the telephone set repeatedly with the predetermined time interval. 
Nitsuko Limited Nippon Telegraph and Telephone Corporation
H
H04M
inventional
Yoshiji Jouji Masaaki
Tanimoto Tanaka Saito