Here is a simple calling bell circuit that can be used in small offices with an existing intercom system to call the office boy. There are up to nine places where extension lines can be used to call the office boy. The system is linked to the office boy's own phone line. When someone needs the office boy's help, they can use the intercom to call the office boy's extension number and then press a key to let the office boy know where they are (say, 5). This number will be shown on a seven-segment display, and an office boy will be notified by a bell. If you press a switch, the display will go blank.

The whole circuit can be broken up into two parts: the ring stop/bell timer and the decoder/display. The ring stops and bell timer section is made up of an optocoupler MCT2E (IC3), two NE555 timers (IC4 and IC5), and a few discrete components. The optocoupler is what makes the timers go off (IC4 and IC5). Both timers are set up to work in a single-stable mode. The time period of IC4 is always between two and three seconds, while the time period of IC5 can be set with preset VR1 to be between three seconds and one minute. Increasing the value of capacitor C6 or resistor R16 will make the time period last longer. When the office boy's phone rings, the LED inside optocoupler IC3 lights up. This turns on timers IC4 and IC5. When IC4's pin 3 has a high output, transistor T1 conducts, which puts resistor R11 across the phone line to stop the ringing. At the same time, the high output of IC5 (pin 3) turns on the bell for a set amount of time through relay RL1. The decoder cum display section is made up of a DTMF IC 8870 (IC1), an IC 7447 (IC2), a common-anode 7-segment display LTS542 (DIS1), and a few discrete components. The most important IC in this part is MT8870. It is used a lot in phones and a wide range of other things. MT8870 turns the DTMF (dual-tone, multi-frequency) tone pair that is made when a phone button is pressed into binary values. The binary values are sent to the driver IC 7447, which then tells the seven-segment display what number to show. How the circuit works is easy to understand. Connect the circuit you just put together to the office boy's extension. Use a 5V power adaptor to connect 5V to the circuit. Now, when you call this dedicated extension number, the ringing will stop in a few seconds. After the phone stops ringing, press a number on the phone to tell them where you are (say, 5). This number will show up on the seven-segment display, and the office boy

will hear the bell for a set amount of time. The office boy can wait for the next call after helping out in this room. If you want to use the phone as a regular intercom, just connect switch S2 to position 1. The circuit will no longer work. You can now use the phone to call any extension number. If you want to use the call bell again, just move switch S2 to position 2. Put the circuit together on a PCB that can be used for many things and put it in a suitable cabinet. At the back of the cabinet, connect the plugs for the phone line and the AC power. Install the 7-segment display, the buzzer, and the S2 switch on the front of the cabinet.