Abstract: Two-wire television interphone system is arranged to discriminate by a line voltage discriminator incorporated in a master device a line voltage variation due to a calling operation on the side of a branch device, to actuate a voltage controller in the branch device to shift the system from standby state to operation state, and to drive a call signal generator in the branch device for actuation of a call signal receiver in the master device. The system is thus made to have a power supply circuit energized for a power supply to the system only when required for effecting the calling operation, talking between the both devices, and so on.

Abstract: A passive infrared detector including an optical collector releasably attached to a base with a sensor on which incident infrared radiation directed through the optical collector is focused. The sensor is held by a joint member which is pivotally supported on the base and to which said optical collector is releasably attached, whereby the optical collector is rotatable together with the joint member relative to the base for adjustment of its angular position and the optical collector can be alone replaced as necessary without having to replace the base, the sensor and its associated electric circuitry.