ELECTRONICS FOR YOU� ❚❚❚❚❚ �FEBRUARY 2001

C I R C U I T I D E A S

S.C. DWIVEDI

This circuit is able to handle nineindependent telephones (using asingle telephone line pair) lo-

cated at nine different locations, say,up to a distance of 100m from eachother, for receiving and making outgo-ing calls, while maintaining conversa-tion secrecy. This circuit is useful whena single telephone line is to be sharedby more members residing in differentrooms/apartments.

Normally, if one connects ninephones in parallel, ring signals are

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heard in all the nine telephones (it isalso possible that the phones will notwork due to higher load), and out ofnine persons eight will find that the callis not for them. Further, one can over-hear others’ conversation, which is notdesirable. To overcome these problems,the circuit given here proves beneficial,as the ring is heard only in the desiredextension, say, extension number ‘1’.

For making use of this facility, thecalling subscriber is required to initiallydial the normal phone number of the

called subscriber. When the call is estab-lished, no ring-back tone is heard by thecalling party. The calling subscriber hasthen to press the asterik (*) button onthe telephone to activate the tone mode(if the phone normally works in dial mode)and dial extension number, say, ‘1’, within10 seconds. (In case the calling subscriberfails to dial the required extension num-ber within 10 seconds, the line will bedisconnected automatically.) Also, if thedialed extension phone is not lifted within10 seconds, the ring-back tone will cease.

The ring signal on the main phoneline is detected by opto-coupler MCT-2E (IC1), which in turn activates the10-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 beenused to connect +6V rail to theprocessing circuitry and theother has been used to provide220-ohm loop resistance to de-energise the ringer relay intelephone exchange, to cut offthe ring.

When the caller dials theextension number (say, ‘1’) intone mode, tone receiverCM8870 (IC3) outputs code‘0001’, which is fed to the 4-bit BCD-to-10 line decimal de-coder IC4 (CD4028). The out-put of IC4 at its output pin14 (Q1) goes high andswitches on the SCR (TH-1)and associated relay RL1. Re-lay RL1, in turn, connects, viaits N/O contacts, the 50Hz ex-tension ring signal, derivedfrom the 230V AC mains, tothe line of telephone ‘1’. Thisring signal is available to tele-phone ‘1’ only, because halfof the signal is blocked by di-ode D1 and DIAC1 (which donot conduct below 35 volts).

As soon as phone ‘1’ islifted, the ring current in-creases and voltage dropacross R28 (220-ohm, 1/2W re-sistor) increases and operatesopto-coupler IC5 (MCT-2E).This in turn resets timer IC2causing:

(a) interruption of thepower supply for processingcircuitry as well as the ring

DHURJATI SINHA