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 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 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 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 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 the telephone button is converted into binary values by MT8870. The binary values are fed to driver IC 7447 to display corresponding number on the seven-segment display.
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 you dial this dedicated extension number, the ring will cease in a few seconds. After the ring ceases, press a digit on the telephone to indicate your location (say, 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 S2 to 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 a general-purpose 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.
