Bulk SMS Sender

How We Built an SMS Backup Module with Raspberry Pi Pico W and GPRS SIM800L in MicroPython

At Woopi, we are always looking for innovative solutions to ensure the continuity and efficiency of the services we provide to our clients. One of the recent challenges we faced was how to maintain smooth communication in our mass WhatsApp messaging platform in case of service interruptions. This is where the idea of developing an SMS backup module was born, using accessible and highly customizable technology.

This project was an exciting challenge, and I developed it entirely from scratch using MicroPython and components such as the Raspberry Pi Pico W and the GPRS SIM800L module. In this article, I want to share the technical process behind this development, highlighting the most important steps and decisions I made to make this module a reliable backup.

The Problem to Solve

Our mass WhatsApp messaging platform is critical for many companies that rely on us for their essential communications. However, as an external service, WhatsApp can occasionally experience interruptions. We needed a system that could automatically take over to ensure that messages reached their destination.

Sending SMS emerged as the ideal solution: it is a proven, reliable method widely supported by mobile operators.

The Module’s Components

To build this system, we selected cost-effective and easy-to-integrate components:

Raspberry Pi Pico W: I chose this board for its power, versatility, and low cost. Additionally, its MicroPython support significantly facilitated code development.
GPRS SIM800L Module: This module enables GSM/GPRS communication, ideal for sending SMS quickly and efficiently.
Power Supplies: Since the SIM800L can be demanding on power, I used a voltage regulator to ensure stable operation.
External Antenna: To maximize signal reception in areas with poor coverage.

Development Step-by-Step

1. Hardware Setup

The first step was to correctly connect the SIM800L to the Raspberry Pi Pico W. It’s essential to consider that the SIM800L operates at 4V, so the following were required:

A logic level converter to communicate the module with the Pico, which operates at 3.3V.
A stable power supply with a capacitor to prevent unexpected reboots.

2. MicroPython Installation

The Raspberry Pi Pico W has native MicroPython support. I uploaded the latest firmware, set up the environment, and tested the GPIO pins to ensure they were functioning correctly.

3. Configuring the SIM800L

The SIM800L module is controlled via AT commands. These commands allow for tasks ranging from checking the signal to sending SMS. Some of the key commands I used were:

AT+CSQ: To check signal strength.
AT+CMGF=1: To configure text mode.
AT+CMGS: To send a message.

4. Coding the Solution

In MicroPython, I wrote a script to:

Read a list of numbers from our platform.
Send SMS to each number using AT commands.
Update the status of each sent message through a PUT request to our API.
Introduce a random delay between messages (1 to 6 seconds) to avoid being flagged by the mobile operator.

The code also includes error handling to ensure that, if a message fails, it retries before reporting it as undelivered.

5. Platform Integration

To connect the module to our mass WhatsApp messaging platform, I used an endpoint that:

Receives the list of numbers and messages to be sent.
Returns an updated status of the deliveries made.

This ensures that if WhatsApp is unavailable, the system automatically switches to SMS sending.

Challenges and Solutions

1. Weak SIM800L Signal
The SIM800L module is sensitive to GSM signal quality. To address this, I added an external antenna, which significantly improved reception.

2. Power Consumption
The SIM800L has power spikes that can restart the module if not adequately supplied. I used a dedicated voltage regulator with a 1000 μF capacitor to stabilize the current.

3. Server Response Latency
To avoid delays, I implemented a random delay system and optimized API queries, reducing overhead.

Results

This module became a robust solution ensuring service continuity. Now, even if WhatsApp temporarily goes offline, our clients can continue sending SMS messages without interruptions.

Conclusion

Developing this module was a gratifying experience that demonstrated how accessible tools like the Raspberry Pi Pico W and SIM800L can solve critical business problems. I hope this article inspires other developers to explore similar solutions.

At Woopi, we remain committed to innovation and quality. If you want to learn more about this development or implement a tailored solution for your company, don’t hesitate to contact us.