CALLING BELL USING AN INTERCOM

Calling bell

Here is a simple calling bell circuit that can be used in small offices to call the office boy using an existing intercom system .the office boy can be called from upto nine locations where extension line are installed .the system is connected to a dedicated extension for the office boy.whenever some one needs the office ‘boy’ assistance,he can dail the office boy extension number through the intercom and then press a key to indicate his location number (say,5).this number will be displayed on a seven segment display and at the same time bell will sound to alert the office boy.pressing a switch will clear the display .the entire circuit can be divided into two sections:ring cease cum bell timer ,and decoder cum display .

The ring cease and bell timer section is built around optocoupler MCT2E (IC3),NE555 timers (IC4 and IC5) and few discrete components.the optocoupler is used to trigger the timer (IC4 and IC5).both the timers are wired in monostable mode.the time period of IC4 is fixed at 2-3 seconds,while the time period of IC5 can be set from 3 seconds to one minute using present VR1 .if you want longer time period ,increase the value of capicator C6 or resistor  R16.when someone rings the office boy’s phone,the internal led of optocoupler  IC3 glows triggering timers IC4 and IC5 .the high o/p of IC4 at it’s pin 3 makes transistor T1 conduct,putting resistor R11 across the telephoneline to cease the ring .at the same time,the high o/p of IC 5 (pin 3)drives relay RL1 to enable the bell for a preset time period.

 

    Working of the circuit is simple .connect the assembled circuit to dedicated extension for the office boy .connect 5v to the circuit using 5v power adaptor.now when dial this dedicated extension number the ring will cease in a few second after ring cease ,press a digit on the telephone to indicate your location (says,5).this number will be displayed on the seven segment display and the  bell will sound for a preset time period to alert the office boy after providing  the assistance to this room ,the office boy can wait for the next call.if you want  to use the telephone as a regular intercom ,simply connect switch to S2 position 1 .this will disable the circuit .now you can dial any extension number  through  the telephone .if you want to use the call bell again ,just turn switch S2  to position 2 .assemble the circuit on generalpurpose PCB and enclose in a suitable cabinet .fix the connectors to connect the telephone line and AC supply at the rear side of the cabinet .mount the 7-segment display ,buzzer and switch S2 on front side of the cabinet .

                                                           By :- om kumar gaurav