| 摘要 |
PURPOSE:To speak through at least two telephone lines by applying impedance between the lines by depressing a call switch to change inter-line voltage, detecting the voltage change and turning a ringing tone generating circuit to a voltage source to drive a speaker. CONSTITUTION:When a call is generated from a central telephone office, a 16Hz AC signal is superposed to the telephone lines L1, L2 at a DC level. Call generating circuits Ga, in respective answering buzzers A1, A2 generate ringing tones by any one of AC and DC. Receiving the 16Hz AC singal, the call generating circuits Ga drive respective speakers Pa. If a telephone receiver is hooked off in one telephone set T1, a call signal from the office is stopped and an inter-line voltage is set up only to a fixed DC level. When an affair switch Sw of an opposite side is closed, the inter-line voltage is dropped, voltage change detecting circuits D in both the answering buzzers A1, A2 detect the voltage drop, turn switching contacts from aa to bb and generate call tones. |
