Abstract: 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.

Abstract: A key telephone system comprising a main equipment and a plurality of key telephones connected to the main equipment and comprising elements for operatively connecting another similar key telephone system. The main equipment in the key telephone system comprises a system call transmitting interface and a system call receiving interface which are connected through connecting lines to a system call receiving interface and a system call transmitting interface in a main equipment of another similar key telephone system, respectively. Each key telephone in both systems comprises an additional key for selecting the system call transmitting interface of its own main equipment. By use of the additional key in one key telephone, a speech path can be established between the one key telephone and another key telephone in another key telephone system through both main equipments and the connecting line.

Abstract: In a selective calling circuit including a band-pass filter which may be constituted by an active filter, and a power source for energizing the filter, a control signal is generated which has a predetermined duration in accordance with the output of the band-pass filter at a point of time whereat the output of the band-pass filter reaches a predetermined level. In response to the control signal, there is established a state in which the center frequency of the band-pass filter can be changed, and the output of the power source is changed from a first level to a second level for a period of time substantially corresponding to the predetermined duration of the control signal. At the aforementioned first level, the band-pass filter may be energized, while at the second level, it may be deenergized.