For making use of this facility, the calling subscriber is required to initially dial the normal phone number of the called subscriber. When the call is established, no ring-back tone is heard by the calling party. The calling subscriber has then to press the asterik (*) button on the telephone to activate the tone mode (if the phone normally work
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s in dial mode) and dial extension number, say, 1`, within 10 seconds. (In case the calling subscriber fails to dial the required extension number within 10 seconds, the line will be disconnected automatically. ) Also, if the dialed extension phone is not lifted within 10 seconds, the ring-back tone will cease. The ring signal on the main phone line is detected by opto-coupler MCT- 2E (IC1), which in turn activates the 10-second on timer`, formed by IC2 (555), and energises relay RL10 (6V, 100- ohm, 2 C/O). One of the N/O` contacts of the relay has been used to connect +6V rail to the processing circuitry and the other has been used to provide 220-ohm loop resistance to de-energise the ringer relay in telephone exchange, to cut off the ring. When the caller dials the extension number (say, 1`) in tone mode, tone receiver CM8870 (IC3) outputs code 0001`, which is fed to the 4- bit BCD-to-10 line decimal de- coder IC4 (CD4028). The output of IC4 at its output pin 14 (Q1) goes high and switches on the SCR (TH-1) and associated relay RL1. Relay RL1, in turn, connects, via its N/O contacts, the 50Hz extension ring signal, derived from the 230V AC mains, to the line of telephone 1`. This ring signal is available to telephone 1` only, because half of the signal is blocked by diode D1 and DIAC1 (which do not conduct below 35 volts). As soon as phone 1` is lifted, the ring current in- creases and voltage drop across R28 (220-ohm,...
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