telephone sharer circuit 9


Posted on Feb 7, 2014

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


telephone sharer circuit 9
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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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