... tapping ormisuse of telephone lines by sound-ing a buzzer. In on-hook mode, 48V DC supplyis maintained across the telephonelines. In this case, the bi-colour LEDglows in green, indicating ... flowcharts for main program and various sub-routines are shown in Fig. 5.PCB and parts list are included only for the main interface diagram of Fig. 1.The actual-size, single-sided PCB for thesame ... time of open-ing or closing of the door. Thus, at pointA, a low-to-high going voltage transitionis available for every closing of the doorafter opening it. (See waveform A in Fig.2.)The...