| 摘要 |
The purpose of the circuit arrangement is to automate the start of charging in a payphone. It contains a call signal detector (19, 20) and a logic circuit (21). The signal detector is connected to the wires (a, b) or to the secondary winding (17) of a transformer (16). The signal detector comprises a filter circuit (19) and a threshold value circuit (20). The filter circuit (19) contains a high-pass filter suppressing the ringing tone, the call-back tone and the busy tone. The logic circuit (21) operates automatically and initiates charging with the occurrence of the speech voltage when a call has been answered by the called party and activates the microphone (6) of the calling party. <IMAGE>
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