# Personal Silent Alarm System Diagram Project

Posted on Feb 6, 2014

The TTY and the detector-transmitter unit are connected to separate phone jacks on the same line. When the ring sensor detects an incoming phone call, it activates the transmitter, which sends an RF signal to the receiver in the pager device. The receiver activates intermediate circuitry which drives the vibrating motor and thus alerts the user to

the incoming call. In this section we describe the design and implementation of the ring detector and its connection to the transmitter. We also describe the circuitry in the pager that connects the receiver to the vibrating motor. We required an understanding of the signals and standards used in telephone communication in order to build the interface between the telephone line and the transmitter. Figure 2. 2 shows the signals on a silent and a ringing telephone line. When the line is silent, a DC voltage of Vl = 48 V is present. During ringing this voltage is superposed with a sine wave of considerably large amplitude. Often the ringing amplitude is 80 Vrms, which gives a peak voltage of Vp = 48 V +/- 1. 4 * 80 V = {+160 V; -64 V}. Not all telephone companies use the same standard for their ring signals, so the amplitude and frequency can vary over a wide range. A telephone line in use has a small DC voltage of approximately Vl = 4 V with a superposed current signal (from a voice, fax, or modem) which corresponds to a voltage change of up to 2 V in amplitude. In order to make our product compatible to every telephone ring standard, we used the amplitude of the line voltage to distinguish a ringing line from all other possible telephone line states. All telephone standards have one property in common: The peak voltage of a ringing line is significantly higher than the DC voltage for a silent line or a line in use. Using...

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