How to turn a 7 inch Elecrow pi terminal into a SDR Radio

Today I received the Pi Terminal-7” IPS HMI CM4 Panel All-In-One Module Raspberry Pi Computer from Elecrow. The shipment arrived in about a week in a branded box well protected from mechanical damage. The package contains:

• 7 inch Pi Terminal with user manual
• Wi-Fi Antenna
• 12V/2A Adapter
• ad three Phoenix Type Connectors

First I would like to describe some of the features and functions of this small but powerful computer. It is equipped with the powerful Raspberry Pi Compute Module 4 as the main control, with excellent real-time performance, and can quickly respond to and process real-time control and monitoring tasks. It has a 7-inch display with a resolution of 1024 x 600 pixels and a touch screen with 5-point Capacitive Touch. A highlight is the rich industrial interfaces and functional modules, including CAN bus, RS232/RS485, isolated digital IO, differential input ADC, relay control, etc., which fully meet the communication and control needs with various sensors, actuators and other devices.

Communication capability is another highlight of Pi Terminal. It integrates WiFi, BLE, and reserves Mini PCIE interfaces for 4G and LoRa. At the same time, it provides Gigabit Ethernet to ensure high-speed data transmission and remote monitoring between devices. Pi Terminal supports a rich software ecosystem based on Raspberry Pi and is compatible with the Node-RED programming tool, giving users the ability to easily create and manage IoT applications.

Now let's look at the input-output interfaces:

• RS232 port
• UART comunication port
• Gigabit Ethernet
• HDMI output
• audio IN/OUT connector
• Two USB-A ports
• On/Off switch
• Micro DS Card slot
• SIM card slot
• LED Indicators
• Four SMA antenna conectors
• 2*20 PIN GPIO
• Two Camera slots
• another GPIO connector
• Relay control
• DO & DI
• and CAN, RS485, and ADS connector

Next let's briefly look at the inside where the motherboard is located. Boardf consist the Raspberry Pi CM4 board cooled by small fan, relay, speaker connector and the two PCIE slots in which the LORA and GPS modules are mounted which I additionally ordered. This time I want to present you a simple way to make a portable Software Defined Radio (SDR Radio) that would work independently of a PC. For this purpose, besides this panel display, I will also use a cheap RTLSDR USB dongle.

A software-defined radio (SDR) is a radio communication system that uses software to process various signals (modulation, demodulation, decoding, etc.). A typical SDR setup involves an RF front-end connected to a computer that will perform the conversions from digital to analog, and vice versa. Because of rapidly developing digital electronics, the capabilities of SDR continue to increase. In this case, the RF front end is the RTLSDR USB dongle, and the signal processing and visual presentation is the task of this beautiful display module.

In one of my previous videos I presented a way to make a similar device like this, but then some hardware work and modifications were needed and some more advanced knowledge in this area was required to make it. Unlike then, this time making the device requires almost no knowledge in the field of electronics, and the SDR radio works flawlessly and stably. This time I present to you one of the possible solutions, which is box that I specially made for this display module, but also the device can be made without a box so that the rtlsdr dongle can be glued directly to the back with double-sided adhesive and we can use a wireless tuning knob if needed.

The complete device is very simple to make and consists of only a few components:

• Display module which is the basis of the project and whose characteristics I presented at the beginning of the video
• small speaker
• Rotary encoder
• RTLSDR USB dongle
• USB Hub
• and a modified PCB from a cheap PC mouse

The modification consists in desoldering the original miniature rotary encoder and soldering a more robust one in its place. The speaker is connected to the appropriate place provided for it on the board of the display module. As you can see, the sides of the display module are open, out of the box so that you can easily put for example an SD card, or some other external device. On the outside of the box I made a socket for a Wi-Fi antenna, although we don't need it for this project.

Now let's dwell a bit on the software part.

In fact, it is the software that in this case replaces a huge hardware part of the radio and is also highly adaptable and responsible for the professional characteristics of this relatively cheap radio. The display module itself comes with a recorded Raspbian Pi operating system, which start with a Demo program that describes some of the panel's capabilities.

I didn't want to modify this factory-set OS, so I decided to use another cheap micro SD card to record the whole new setup. To prepare new operating system we need a SD card with a size of 32GB or more. On the Sourcefodge site you can download a Raspbian Pi image with a huge number of programs from the field of HAM radio called "HAM Pi", including SDR Radio softwares that we will use for the needs of this project.

I will briefly describe the method of preparing the SD card:

First we need to download Blenda Etcher and Ham Pi image from the given pages.

Next we install and run the Balena Etcher software. Press "Select image" and select the Ham Pi image that we downloaded earlier. Now we connect the SD card to the card reader and to the USB port of the PC. Next, press "Select Drive", and select the drive with the SD card. Finally, press "Flash" and wait for the procedure to finish. With that, the card is ready and we can mount it on the Display Panel and start it.

The operating system starts immediately, and no special settings are required.

As we can see, all important Ham radio applications are pre-installed on it, and we are especially interested in SDR software, specifically SDR++ and GQRX. GQrx is a well-known Unix software, but at least for me, SDR++ is even better thanks to its low hardware requirements, multi-platform support, and high similarity to sdr#, which is otherwise a commonly used Windows software. For smooth control I use miniature bluetooth keyboard/mouse combination which fits this device perfectly.

Now I start SDR++, on the left side is the part for setting the receiver functions, and on the right side is a visual presentation of the radio signal. As we see the RTLSDR dongle is detected, which means we can start receiving signals immediately. I will start with the broadcast FM band, because there are many strong active radio stations here and so the possibilities and the way of working with this receiver can be easily presented.

After starting the software, the majority of functions can be controlled through the rotary encoder and the touch screen, so there is almost no need for a keyboard and mouse. I will avoid commercial music stations for copyright reasons. We can move through the entire displayed range by touching the screen, or more sophisticatedly with the rotary encoder.

We select the movement step by pressing the appropriate digit where the frequency is displayed. We can also use a wireless tuning knob, the making of which I described in one of my previous videos.

The following is a short demonstration of the receiver's capabilities and operation on different wavebands.

Just don't forget to mention that I used the RTLSDR dongle as the most suitable variant as far as the price-quality ratio is concerned. Otherwise, we can use many other SDR radios of this type, because the software has support for almost all popular models.

And finally a short conclusion. First of all, let me tell you that I am really surprised by the construction, possibilities, and stability during operation of this display terminal. I previously had some experience with this type of device powered by a Raspberry Pi 3B with 2GB ram, but I can comfortably tell you that the difference is incomparable, starting with the robust and quality build, flawless display and touch panel, and most importantly, stable and fast long-lasting operation without any signs of heating, blocking, lack of power, etc., which were common in my previous project of this kind.

And this is just one small project with which I turned this display terminal into a quality SDR radio with professional capabilities and features. In the future I will try to describe many more simple ways to use this piece of hardware to build other quality devices.