When I started tinkering with the JumboSpot and small OLED displays, it got me interested in larger and informative display options, so I quickly picked up a few Nextion style displays as covered a few months ago in the "Bigger Display for Jumbo/China/Covert" article.
Even though having a larger and more informative display to be used with the MMDVM based hot spot is great to show callsigns, name, location, IP addresse, temperature, etc, that means the hotspot can not be put where I want the hotspot for maximum RF range and still see the display.
So, what about making the display itself remote from the actual hot spot? Seemed like a good idea and here is what I have done to make this a reality.
Wireless Nextion Display
Getting the correct signal from the hot spot was easier than I thought after some research and decided instead of trying to send the signal over wireless serial blue-tooth connection to a remote receiver that feeds a Nextion display that using 70cm amateur radio spectrum would be just be a bit more fun to experiment with.
Inexpensive HC-12 modules allow serial based communication over a wide range of speeds and can be configured to operate on a number of predefined channels set by the manufacturer or configured by the end user. Here is a great user manual I found on the Elecrow website that will fill in some of the blanks on its capabilities.
The HC-12 can be driven from from the TXD, GND and VCC pins on the duplex or semi duplex hot spot boards easily. All I needed to add was a 1N4007 diode to drop the voltage a little in order to not over drive the HC-12 module since its sensitive to over voltage.
Since I was not interested in providing a two way wireless connection, there was no need for the RXD connection between the hotspot and HC-12.
After getting everything connected, crossing my fingers and apply power, I started to see pulsed signals being sent on the default HC-12 module frequency of (3) 433.400 MHz as shown on the screen capture by using an inexpensive SDR dongle sch as the NooElec Nano3 and the popular SDR# software package.
The other two signals shown is the output or transmit signal generated by my duplex hotspot that was connected to the HC-12 and was set to operate on (1) 432.525 MHz, only 875 kHz away.
The very narrow continuous unmodulated signal at (2) 433.063 MHz seems to be coming either from my cable modem or cable line, but does not seem to be an issue I can easily fix, so just ignore that.
Within the Pi-Star dashboard, I selected screen type as "Nextion", port as "Modem" and Nextion Layout as "ON7LDS L3" since that is what I use with my 4.3 Inch Nextion display that runs a modified version of one of the nice layouts found in the file section of the Nextion Ham Radio Screens Facebook Group
Connecting the HC-12 module to the Nextion display was a little more tricky to receive signals though. I connected the RXD and GND from another HC-12 to the Nextion display and ran +5V to power up the display.
I needed an isolated power supply with another 1N4007 diode to power up the HC-12. Everything worked on the second try! The first try was when I accidentally forgot to use a diode to drop the voltage and "smoked" an HC-12, but that is why I ordered a few spare just in case!
On the second try I thought I had a different issue, but as it turns out the default setting of the HC-12 is pretty high transmit power and I recall the manual saying to separate modules by a few feet.
Once I moved the contraption across the room, my display was updating with users chatting away on TG 91 worldwide DMR talk group!
An Additional Future Experiment
Here is where I hope to get some people interested and assist with an extra added feature/benefit. What about decoding the data packets that the HC-12 transmits by using the SDR?
Its possible to decode weather stations, door bells and other 433 MHz range devices with the rtl_433 application when used with an SDR dongle, so maybe someone can find a way to translate the pulses and view those on a computer or smartphone connected to an SDR?
There are likely many protocols that an HC-12 can send and receive, so why not figure out a way to add another to read what the Nextion display expects to see and render that in some sort of application.
To get anyone (and everyone) started, I did a base-band recording that can be easily played back in SDR#. You can play back the file as if you were in real time listening to the signals sent by the Nextion. Its even possible to use DSD+ to decode the active DMR audio signal too!
Just 37 seconds of 2 MHz wide of spectrum took up 225MB, so I have it hosted on Google Drive.
Even though having a larger and more informative display to be used with the MMDVM based hot spot is great to show callsigns, name, location, IP addresse, temperature, etc, that means the hotspot can not be put where I want the hotspot for maximum RF range and still see the display.
 |
| Be sure to purchase a Nextion display and not a TJC branded one. Only the Nextion version has English development software. TJC screens are designed for the China market or those that can read Chinese software. Otherwise, they are the same thing and would work fine with any MMDVM Pi-Star Hot Spot |
So, what about making the display itself remote from the actual hot spot? Seemed like a good idea and here is what I have done to make this a reality.
Wireless Nextion Display
Getting the correct signal from the hot spot was easier than I thought after some research and decided instead of trying to send the signal over wireless serial blue-tooth connection to a remote receiver that feeds a Nextion display that using 70cm amateur radio spectrum would be just be a bit more fun to experiment with.
 |
| Silicon Labs is just one of the companies with components that make this small data transceiver module a reality |
Inexpensive HC-12 modules allow serial based communication over a wide range of speeds and can be configured to operate on a number of predefined channels set by the manufacturer or configured by the end user. Here is a great user manual I found on the Elecrow website that will fill in some of the blanks on its capabilities.
The HC-12 can be driven from from the TXD, GND and VCC pins on the duplex or semi duplex hot spot boards easily. All I needed to add was a 1N4007 diode to drop the voltage a little in order to not over drive the HC-12 module since its sensitive to over voltage.
Since I was not interested in providing a two way wireless connection, there was no need for the RXD connection between the hotspot and HC-12.
 |
| Steve K2GOG SDR capture of MMDVM DMR audio and HC-12 pulses |
After getting everything connected, crossing my fingers and apply power, I started to see pulsed signals being sent on the default HC-12 module frequency of (3) 433.400 MHz as shown on the screen capture by using an inexpensive SDR dongle sch as the NooElec Nano3 and the popular SDR# software package.
The other two signals shown is the output or transmit signal generated by my duplex hotspot that was connected to the HC-12 and was set to operate on (1) 432.525 MHz, only 875 kHz away.
The very narrow continuous unmodulated signal at (2) 433.063 MHz seems to be coming either from my cable modem or cable line, but does not seem to be an issue I can easily fix, so just ignore that.
Within the Pi-Star dashboard, I selected screen type as "Nextion", port as "Modem" and Nextion Layout as "ON7LDS L3" since that is what I use with my 4.3 Inch Nextion display that runs a modified version of one of the nice layouts found in the file section of the Nextion Ham Radio Screens Facebook Group
Connecting the HC-12 module to the Nextion display was a little more tricky to receive signals though. I connected the RXD and GND from another HC-12 to the Nextion display and ran +5V to power up the display.
I needed an isolated power supply with another 1N4007 diode to power up the HC-12. Everything worked on the second try! The first try was when I accidentally forgot to use a diode to drop the voltage and "smoked" an HC-12, but that is why I ordered a few spare just in case!
On the second try I thought I had a different issue, but as it turns out the default setting of the HC-12 is pretty high transmit power and I recall the manual saying to separate modules by a few feet.
Once I moved the contraption across the room, my display was updating with users chatting away on TG 91 worldwide DMR talk group!
An Additional Future Experiment
Here is where I hope to get some people interested and assist with an extra added feature/benefit. What about decoding the data packets that the HC-12 transmits by using the SDR?
Its possible to decode weather stations, door bells and other 433 MHz range devices with the rtl_433 application when used with an SDR dongle, so maybe someone can find a way to translate the pulses and view those on a computer or smartphone connected to an SDR?
There are likely many protocols that an HC-12 can send and receive, so why not figure out a way to add another to read what the Nextion display expects to see and render that in some sort of application.
 |
| It is easy to record and playback "spectrum" with SDR#. To play back the file related to this article, instead of selecting your own SDR device in the menu drop down, select "IQ File (*.wav) and navigate to where the base-band recording downloaded here was saved to. Then press "Play" and be amazed! |
To get anyone (and everyone) started, I did a base-band recording that can be easily played back in SDR#. You can play back the file as if you were in real time listening to the signals sent by the Nextion. Its even possible to use DSD+ to decode the active DMR audio signal too!
Caution: Using DSD+ can be daunting, so look elsewhere on getting that to work.This way you can know who is speaking to match it against the pulses sent by the HC-12. The pulses will show call sign, talk group and DMR ID to whom ever can decode it so you know what sort of information to look for in the packets.
Just 37 seconds of 2 MHz wide of spectrum took up 225MB, so I have it hosted on Google Drive.
The First HVDN Contest
If you can make some progress, share your tips in the comments below or send a private email instead via one of the HVDN contact forms.
First maker, hacker, ham, etc to make MAJOR documented progress, Hudson Valley Digital Network will reward your efforts with an HC-12 and an RTL-SDR device, so ready.....set..... 01000111 01001111 00100001
