Project Hub /Basic Analog & Power Electronics/Sequential, Timing & Pulse Circuits/Multi-User Call Bell For Up To Nine Persons

Intermediate

ANALOG / DISCRETE

Project Specific

3 – 6 Hours

Partial Kit

Multi-User Call Bell For Up To Nine Persons

A simple multi-user call bell system that allows calling an office boy or home help from up to nine different locations while displaying the caller's room number.

Originally published on ElectronicsForU

View original tutorial

Parts You'll Need

This project is sourced from another site, not written by us — this list is our best match of their components against what we stock, not a verified bill of materials. Always check the original blog (linked above) for the actual parts list before buying.

Piezo Buzzer (Big) 6-12V Rs. 45.00
2-Slot 18650 Holder with Protective Cover and On/Off Switch – Case Rs. 109.00
10pcs 5mm RED Diffused Light Emitting Diodes - LEDs Set Rs. 39.00
10pcs - 220 ohm 1/4 W Resistor Set Rs. 39.00
400 Points Mini Solderless Prototype PCB Breadboard High Quality Rs. 89.00
MB102 Adjustable Breadboard Power Supply Module (3.3V & 5V) for Arduino & Raspberry Pi Rs. 139.00
Jumper Wire 10 Pin Male To Male 20cm Rs. 45.00
Jumper Wire 10 Pin Male To Female 20cm Rs. 45.00

Selected: Rs. 0.00

This list includes a generic resistor set as a placeholder — it is not matched to the exact value or part number this circuit needs. The correct one is specified on the original source page linked above; check it before buying.

We don't carry yet: Voltage Regulator IC (7805 / LM317 / 1117)
Logic ICs (74xx / 40xx)
555 Timer IC
Transistors (BC547 / 2N2222 / TIP / MOSFET)
Diodes (1N4007 / 1N4148 / Schottky / Zener)
9V Battery + Snap
7-Segment Display
Capacitors
Custom PCB / PCB Fab

Browse all components

How It Works

The Multi-User Call Bell system is an intermediate analog and digital circuit designed to identify which of up to nine separate locations is requesting attention. Power is supplied by a 9V battery, and an LM7805 voltage regulator IC1 steps this down to a stable 5V rail to safely power the logic integrated circuits. When a user presses one of the input switches S1 through S9, it pulls the corresponding line low to signal an event. These switch inputs interface with a 74LS147 priority encoder IC2, which converts the nine active-low inputs into a 4-bit binary-coded decimal (BCD) output corresponding to the highest-priority active switch. To drive a common anode seven-segment display (LTS-6960HR/LTS542, DIS1) that visually indicates the caller's room number, the circuit utilizes a 74LS247 BCD-to-7-segment decoder/driver IC4. Meanwhile, a 74LS04 hex inverter IC3 helps condition and invert logic signals as necessary across the digital path. Concurrently, pressing any call switch triggers an audible alert: an NE555 timer IC5 configured as an oscillator generates a pulsed tone that drives a piezo buzzer PZ1 through switching transistors T1 and T2 (such as BC547). Signal diodes like 1N4148 (D1-D4) are strategically placed to isolate control paths and prevent back-feed, while discrete resistors and capacitors set timing parameters and pull-up levels. Individual indicator LEDs (LED1 through LED9) provide a secondary visual cue for each distinct calling station on the PCB.

Why Build This

Create a practical multi-room calling system for school offices, labs, or home environments to streamline communication.

Gain hands-on experience combining analog timing circuits (NE555) with digital logic ICs (encoders and decoders).

Learn how to interface discrete switches and transistors with digital displays and audio alerts on a custom PCB.

Real-World Application

Office buildings, homes, or hospitals where assistance needs to be called from multiple specific rooms to a central station.

Skills You'll Learn

Priority encoder logic operation

BCD-to-7-segment decoding

regulated power supply design using linear regulators

astable multivibrator timer configuration

transistor-based buzzer driving

Safety Precautions

No specific precautions noted for this build — always follow general electronics safety practices.

Frequently Asked Questions

Can I use more than nine switches with this circuit?

No, the 74LS147 is a 10-to-4 line priority encoder specifically utilized here for up to nine active-low inputs (plus a reset state), meaning expanding beyond nine stations would require a completely different logic architecture.

Why is an LM7805 voltage regulator needed if the battery is already 9V?

The logic ICs in the 74LS series operate strictly on a 5V TTL supply level. The LM7805 steps down the variable 9V battery voltage to a steady, protected 5V to prevent damaging the digital chips.

How does the system know which number to show if two buttons are pressed at once?

The 74LS147 is a priority encoder, which means it is internally designed to automatically prioritize and display the highest numbered input line if multiple switches are closed simultaneously.

Gallery

This page is part of Kitkraft's Project Hub — a curated collection of electronics & robotics projects gathered from the maker community to help students and hobbyists discover and learn to build. Kitkraft does not claim authorship of the original project, and makes no representation as to the accuracy, completeness or safety of third-party source material. The summary and "How It Works" explanation above are AI-generated, rewritten with reference to the original tutorial — not a reproduction of it. Full credit and a direct link to the original source are provided above. If you're the original creator and have a concern about how your work is represented here, please contact us and we'll address it promptly.

Kitkraft Project Hub — curated from the maker community, credited at the source.

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