ExpressLRS is gaining the popularity rapidly. There is a number of well known brands that are making the ExpressLRS hardware. However, several years ago, at the start of ExpressLRS project, it was impossible to purchase any ExpressLRS transmitter module or any ExpressLRS receiver as nobody was manufacturing them. It started as Do-It-Yourself project. The only way to get the ExpressLRS hardware was to build it yourself. Even the slogan says it is “The best RC link you can build yourself“. So I decided to try to build it by myself.
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DIY Nano TX module build uses modular design it means it uses pre build modules. Only a few additional components are used (one resistor and one capacitor). Almost anyone could build this TX module with little soldering experience.
Nano sized DIY TX module can be installed in the full and nano size JR module case. It can be even installed inside the radio as it is really small.
Also you will need 10uF SMD capacitor (3528 or type B size) and the 1k SMD resistor (0402 size).
Other things you will need: 5 pin 2.54mm header socket, silicone wires and of course a soldering iron set.
Soldered the ESP32-WROOM module and 10k resistor and 10uF capacitor. Soldering the resistor is the trickiest part as resistor is tiny (0402 size).
After the ESP32 module is soldered, you need to tempararily solder the RX, TX, 3.3V and GND wires. Connect these wires to the corresponding pins on the FTDI or any other USB to serial adapter. You can even use Arduino board. Don’t forget to short the BOOT pads for flashing the firmware by UART.
At this stage I would recommend to flash the ExpressLRS firmware to the ESP32-WROOM-32 module. For
Press [Build & Flash]. Wait several minutes for it to compile.
And flashing successful!
Now you can unsolder the FTDI (USB to Serial) adapter.
Next step solder the JR bay connector.
Prepare the power (voltage regulator) module. Short the 5V and 12V pads on the back side of the DC-DC regulator module. This will set the regulator to output the 3.45V, which is slightly more than 3.3V, but within the 5% tolerance of the modules.
Turn the variable resistor to the clockwise end position (more about it below).
And solder the module to the EN, VBAT, G and 3V3 with the help of the pin headers.
I also would like to suggest to connect the ExpressLRS module to the JR bay, power the radio, enable the External module in the model setup and measure the voltage between the GND and VO+ pads. It should be slightly more than 3.3V, the recommended voltage is 3.45V. In my case I have measured the voltage 3.58V, which is in the range of the absolute maximum voltage ratings (+10%) for the ESP32 and E28 modules, but its not recommended to operate the modules at this voltage, so I have turned slightly the dial on the variable resistor to set the 3.45V, which is 5% more than standard power voltage for modules. In theory E28 module should be happy to have a slight voltage overhead.
I need to notice, that at this point the ESP32 and E28 modules are not powered as the Reg En jumper pads are not connected.
Once the power voltage is set and verified, you can short the Reg En solder pads and solder the E28-2GM27S module.
Another tricky part is to switch the zero Ohm resistor from the PCB antenna to the external (ipex) antenna position. I ended up just making the solder blob between the two pads as the resistor is zero ohms anyway.
Completely assembled with all the antennas connected. Notice that 3.3V power line Reg En jumper pads are soldered near the top right corner of the PCB board.
“Super slim” PCB variant of the ExpressLRS TX module fits easily in the JR case. Should also fit into the JR Nano case or even inside the radio as internal module.
Power output tests
It is curious what RF power output does DIY ExpressLRS module gives. I have connected the ImmersionRC power meter to the antenna output of the DIY ExpressLRS module. The ExpressLRS module RF power output was set to 10mW, Dynamic power set to OFF. I’ve measured the exactly the 10mW of the power output.
At the 100mW output setting I’ve measured the 115mW output.
And at the 250mW setting my RF power meter measured 255mW of the RF output.
The 250mW output is stable, E28-2GM27S RF module does not heat up too much so no fan is needed.
DIY ExpressLRS Nano Receiver
DIY ExpressLRS Nano receiver uses 0402 size components. They are small as 1.0 x 0.5mm (0.040″ x 0.020″). It is almost impossible or at least it is very hard to solder these components without hot air gun. You will need proper equipment and soldering skills for soldering this small RX.
2.4GHz low pass filter (ballun) – 2450FM07D0034T or 2450FM07A0029T. These are harder to get. Look for it on Mouser or DigiKey.
You will definitely need the RX antenna also.
The PCB and the components are so tiny, that you need definitely need the magnifying glass, hot air soldering gun, precision tweezers and SMD soldering paste. You will have to apply the soldering paste, place the SMD components on the back side of the receiver first, solder them with a help of hot air gun and then repeat the same on the top side of the receiver.
Photos of the soldered DIY ELRS Receiver.
After the ESP-01F module is soldered, you need to solder the RX, TX, 3.3V and GND wires. Connect these wires to the corresponding pins on the FTDI or any other USB to serial adapter. You can also use Arduino Uno board for USB to serial adapter. Don’t forget to short the BOOT pads on the RX board for flashing the firmware by UART.