9-Line Telephone Sharer
By Dhurjati Sinha
This circuit is able to handle nine independent telephones (using a single telephone line pair) located at nine different locations, say, up to a distance of 100m from each other, for receiving and making outgoing calls, while maintaining conversation secrecy. This circuit is useful when a single telephone line is to be shared by more members residing in different rooms/apartments.
Normally, if one connects nine phones in parallel, ring signals are heard in all the nine telephones (it is also possible that the phones will not work due to higher load), and out of nine persons eight will find that the call is not for them. Further, one can overhear othersТ conversation, which is not desirable. To overcome these problems, the circuit given here proves beneficial, as the ring is heard only in the desired extension, say, extension number С1Т.
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 works in dial mode) and dial extension number, say, С1Т, 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 Сon timerТ, formed by IC2 (555), and energises relay RL10 (6V, 100- ohm, 2 C/O). One of the СN/OТ 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 deenergise the ringer relay in telephone exchange, to cut off the ring.
When the caller dials the extension number (say, С1Т) in tone mode, tone receiver CM8870 (IC3) outputs code С0001Т, which is fed to the 4- bit BCD-to-10 line decimal decoder 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 С1Т. This ring signal is available to telephone С1Т only, because half of the signal is blocked by diode D1 and DIAC1 (which do not conduct below 35 volts).
3669 22 August 2007
The circuit presented here can be used for connecting two telephones in parallel and also as a 2-line intercom.
The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply.
This ring generator will ring a telephone once every 10 seconds.