为了方便用户进行程控电话特服业务的登记、使用、注销,电话机中通常设有“R”键电路。其功能与单稳态触发器相同,电话摘机后,如不按“R”键,电话机可连续使用,不影响拨号及通话;按动“R”键后,会产生一个宽度数百毫秒的控制脉冲,迫使串接在直流主回路中的开关管(亦称电子门)截止,切断话路电流,暂停通话。控制脉冲消失后,开关管导通,直流主回路被通,可重新进行工作。该电话机“R”键电路如图1所示。本机“R”键电路 In order to facilitate the users to register, use and cancel the special service of program-controlled telephone, the “R” key circuit is usually provided in the telephone. Its function is the same as that of the monostable trigger. After the phone goes off-hook, if the “R” key is not pressed, the phone can be used continuously, without affecting the dialing and calling. Pressing the “R” key will produce a width of hundreds of milliseconds Of the control pulse, forcing the main circuit connected in series DC switch (also known as the electronic gate) cut off, cut off the channel current, call suspended. After the control pulse disappears, the switch is turned on, the main circuit of the DC is switched on and the work can be restarted. The phone “R” key circuit shown in Figure 1. Local “R” key circuit