Project Idea
The project displays the telephone number dialed using a 7-segment display for improved visibility. It uses the DTMF (Dual-Tone Multi-Frequency) decoding technique, where each telephone button press generates a unique tone. These tones are decoded and processed by an 8051 microcontroller, which drives the multiplexed 7-segment displays to show the digits instantly.
 
Project Description
This project leverages the DTMF concept where pressing a telephone button produces a unique frequency pair. The DTMF decoder (MT8870) receives these tones through the telephone’s earphone socket and converts them into 4-bit binary signals. The signals are buffered and sent to an 8051 family microcontroller. The microcontroller processes this binary data and multiplexes it across several parallel connected 7-segment displays, showing the dialed numbers in real time. Multiplexing ensures fewer pins are used and the digits appear simultaneously via persistence of vision.
 
List of Components
8051 Microcontroller (AT89S52 or similar)
MT8870 DTMF Decoder IC
Dual-tone telephone handset with earphone socket
7-segment displays (common cathode/anode, typically 4 units)
NPN transistors (for driving the 7-segment displays in multiplexing)
Buffer circuits (like 74LS244/245 or similar)
Resistors and capacitors as per circuit design (for biasing, filtering)
Crystal oscillator (12 MHz for 8051 clock)
Power supply (5V regulated DC)
Connecting wires and PCB or breadboard
 
Features
Real-time display of dialed telephone numbers
Uses DTMF tones for signal reception and decoding
8051 microcontroller processes and multiplexes data to 7-segment displays
High multiplexing mode for efficient display with reduced I/O pin usage
Receives signals via telephone earphone jack for easy interfacing
Low power consumption and compact design
 
Applications
Telephone number logging or display systems
Telecommunication training and learning projects
Landline activity monitoring for better call visibility
Embedded systems education focusing on DTMF and microcontroller interfacing
Remote control and signaling projects using DTMF tones
 
Project Includes:
Fully assembled and tested circuit
Circuit diagram
Source code 
(For DIY KITs - only the components will be provided)
 
Project Types:
DIY KIT(Do IT YourSelf KIT) - We are providing only components in this kit for this project
DIY KIT with Code & Circuit Connections - Along with the components, you will get the Code and the Circuit Connections file to help you build the project easily.
Readymade Breadboard - We are providing fully assembled, tested and ready-made project using plug-n-play jumper wires and breadboard with all the other components, code & circuit diagram.
Readymade Soldered - We are providing fully assembled, tested and ready-made project soldered on a PCB with all the other components, code & circuit diagram.
 
Dimensions
Approximate circuit board size: 12 cm x 8 cm x 2 cm (LxWxH)
Display unit size depends on the 7-segment modules used (typically 2-3 cm per digit)
 
Shipping Dimensions
15 cm x 10 cm x 5 cm (packaging with protective casing)
 
Weight
Estimated total project weight: 150 grams
 
Estimate Project Weight: 1kg
 
Notes: All the products are made by LearnElectronics India and not for re-sale.