World’s Smallest Programable Indus Phone Design (Part 3)

In the previous article of this DIY Indusphone series, the smallest touchscreen phone was created using the SIM800L for the 2G cellular network and a round touch display.

Check Part 1 and Part 2 of this project.

This design is now finalised by integrating the previously developed user interface (UI) with its core functions and consolidating everything into a single system powered by a real-time operating system (RTOS).

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The cellular network compatibility is also enhanced by replacing the 2G GSM module with a 4G GSM module, enabling support for LTE and 4G networks (see Fig. 2).

Additional applications, such as a calculator and other basic apps, are included. The entire system operates on the RTOS platform. Fig. 1 illustrates the IndusPhone calculator application.

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Fig. 2 illustrates the 4G GSM module (left) and MIC (right). The MIC used in this device is an electret microphone capsule, which is commonly used in small electronic devices like mobile phones, hearing aids, and various audio recording devices.

These microphones are compact, sensitive, and ideal for small form-factor applications. The tiny holes on top allow sound to enter, where a diaphragm inside the capsule converts sound waves into an electrical signal.

The components required to build the IndusPhone are listed in the table above.

Circuit Diagram and Working

Fig. 3 presents the final circuit diagram for the world’s smallest programmable IndusPhone device. It is built around the 4G GSM module (A7670C), self-capacitance touch chip (CST816), microphone (GMI6027P), SMD speaker, LCD screen (FC9A01), antenna, and display.

The entire system is powered by a 3.3V battery. The buzzer will ring when an incoming call is detected and serves as an input to the IndusBoard for interrupts related to the incoming call.

The 0.5W SMD speaker is used for both calls and loudspeaker functionality. The condenser mic, in SMD format, is also used here. Both the mic and speaker are connected to the GSM module.

The IndusBoard sends the AT command for attending or making calls, sending or receiving messages, while the 4G LTE GSM module operates independently to process these functions.

As in the earlier design, functions and a UI have been developed for various phone features, such as the dial screen, call-dialing function, incoming call screen function, calculator function, and home screen function.

Now, all of these functions are integrated into a single code, which is scheduled and managed by the RTOS. The RTOS switches each function and runs them in threads.

In real time, the phone checks for incoming calls, battery percentage, touch data on the UI, network connectivity, and new messages, all while running the current screen’s main function.

Fig. 4 shows a snippet of the home screen function, Fig. 5 depicts the contacts function, and Fig. 6 demonstrates the calculator application running under RTOS.

Construction and Testing

The complete set of functions is now combined into one RTOS program. Upon powering the device, the system displays battery status, signal strength, and time on the screen.

Double-tapping on the display opens the app screen where options such as the calculator, dial pad, and contacts are available.

When a call is made, the buzzer sounds until the call is either answered or disconnected. The phone interface switches to the incoming call UI. Fig. 7 shows the final IndusPhone home screen displaying the calculator, call-making, and incoming call functionality.

Bonus: Watch the step-by-step video tutorial for this project-

Ashwini Kumar Sinha, an IoT and AI enthusiast, is Tech Journalist at EFY.