Simple Calling Bell Using an Intercom

Posted on Feb 5, 2014

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 up to nine locations where extension lines are installed. The system is connected to a dedicated extension for the office boy. Whenever someone needs the office boy`s assistance, he can

Simple Calling Bell Using an Intercom
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dial the office boy`s 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 a bell will sound to alert the office boy. Pressing a switch will clear the display. The ring cease and bell timer section is built around optocoupler MCT2E (IC3), NE555 timers (IC4 and IC5) and a few discrete components. The optocoupler is used to trigger the timers (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 preset VR1. If you want a longer time period, increase the value of capacitor 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 output of IC4 at its pin 3 makes transistor T1 conduct, putting resistor R11 across the telephone line to cease the ring. At the same time, the high output of IC5 (pin 3) drives relay RL1 to enable the bell for a preset time period. The decoder cum display section is built around DTMF IC 8870 (IC1), IC 7447 (IC2), common-anode 7-segment display LTS542 (DIS1) and some discrete components. The main IC of this section is MT8870. It is widely used in telephones and a variety of other applications. The DTMF (dual-tone multi-frequency) tone pair generated by pressing...

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