ExpressLRS - 868/915 Mhz equipment

Hi Tim,

New topic, as requested.

Hope the title is OK.

David

ExpressLRS is the newest of the current commercial crop of 868/915mhz equipment.

the first release was sent out in january of 2021, so it is only a couple years old, but in those 2.5 years, it has now had its third major software update.

whether it was originally intended to reallocate Frsky hardware (which i feel is not the first use) or if it was basically a ground up rewrite of OpenLRS to support 2 things, it currently offers more features than any of the others. Depending on how you use it.

It offers "OTA" updates for both the TX modules and receivers, in that "true" ExpressLRS hardware (not repurposed frsky hardware) all offers wifi connectivity. you can literally boot the hardware in "wifi" mode, connect to it as an access point and see what version of the firmware it is, change some settings, and update the firmware by uploading .bin files.

The main purpose of ExpressLRS was to have a very low latency, very fast refresh performance LORA based r/c link. using the 2.4ghz version, refresh rates as high as 1000 a second are possible, and latency as low as 6.5ms.

the designers "borrowed" the CRSF protocol from TBS Crossfire. Since it is a high speed serial connection, that helps in reducing the latency. There is no other protocol for communication between transmitter and module. the same serial based CRSF protocol is used to communicate with the receiver and either a flight controller or a PWM adapter.

ExpressLRS therefore requires a transmitter with both a high speed serial port, and a firmware that is either OpenTX/EdgeTX, or something similar (Ethos or ErSkyTX) the module currently does not support PPM, but a few people that fly airplanes feel it would benefit the community if that option were provided.

the later two versions (v2 and v3) have offered "advanced" features to support "wing" devices, which benefits r/c submarine use. v2 supports up to 12 channels, with 4 being full proportional, and 1 mandatory on/off channel, and the remaining 7 can be configured with varying settings from 2 position on/off to 7 bit proportional (128 positions) which is enough for dive planes, especially if you have a pitch controller.

v3 of the protocol supports up to 16 channels to the receiver, but the data is multiplexed, and that mode is only available at certain refresh rates.

ExpressLRS is the only protocol that is open source, and therefore hardware is made by several 3rd party manufacturers. I have bought equipment from 5 different manufacturers, and it all works together as long as the hardware is using the same major protocol version.

for servo output, there is one manufacturer of PWM adapters that makes a series of boards that allow anywhere from 6 to 12 servos to be used.

for those that are more DIY oriented, a stm32 or esp32 or rp2040 board has the horsepower to decode the CRSF protocol without any additional hardware, and a simple program can be written to convert output to pwm.


so, the ExpressLRS 868/915mhz equipment is the least expensive, and is available from multiple sources.

it has the highest refresh rate, and the lowest latency of the three. features that really aren't that important to subs.

it also has the highest hardware requirements, needing newer computer radios with firmware that can "speak" the crsf protocol.

like TBS crossfire, it has dynamic power allocation, so it will only use the power it needs to keep the signal good. you can set the max power level.

due to the opensource nature, different manufacturers offer different hardware, but the majority use the same chipset, which maxes out at 250mw, and some limit that to 100mw. Different country regulations may also limit max output power.

ExpressLRS is also the only protocol that offers both 2.4ghz and 900mhz equipment, so you definitely need to pay attention when purchasing, making sure you do not buy 2.4ghz equipment.

Both TBS and ExpressLRS have the same attitude about telemetry.


There is telemetry sent back by the receiver (link), and telemetry sent back by the Flight Controller (Data)

link telemetry is stuff like signal strength and quality of signal

data telemetry is stuff like gps and compass and battery voltage.

the only data telemetry a receiver will return is rxbatt voltage. several of the matek crsf->pwm servo adapters will include both batt voltage and batt current with little to no extra effort.

the telemetry that the FC returns is entirely dependent on the hardware and firmware available on the FC.

ExpressLRS is unique in that you can configure how often telemetry packets are sent to the transmitter.

This is expressed as a fraction, and is basically how you define the rate. rates ranging from 1:2 to 1:128 are the choices, meaning for every 128 packets sent from the transmitter, 1 is sent back to the transmitter. there is a giant chart on the expresslrs wiki that gives basic time equivalents for these ratios at the different base frame rates.

One of the biggest advantages that ExpressLRS has over the other two 900mhz systems available, is that it is Open Source, and due to that, available from multiple different manufacturers.

a short list of manufacturers:

BetaFPV
Foxeer
HappyModel
HGLRC
IFlight
Jumper


of the manufacturers listed, i have purchased and tested equipement from all but IFlight.

One important thing to note.  Almost all the receivers made for ExpressLRS are meant to be used with drones, so they will not drive a servo directly (one exception to that rule).  this means that there will be soldering involved to make something we can use, by either adding the receiver to a PWM (servo) adapter, or a flight controller.  Based on other topics on this website, i don't see that as a problem.

I have JR style and Nano/Lite style transmitter modules from BetaFPV
I have JR style transmitter modules from HappyModel

I have classic nano style receivers that all look basically identical from BetaFPV, Foxeer, and HGLRC.  There size and layout are identical, i am assuming they are designed from a generic pcb layout.  They also appear to work identically in the limited testing i have been able to do on these.

I have two different receivers from HappyModel, a "classic" model similar to the BetaFPV/Foxeer/HGLRC, but a different form factor.  it is about half the physical size of those 3.  I don't like it as much because the pinouts are not labelled, which is a pain when soldering these receivers to their servo output components.

I also purchased as many as 6 of HappyModel's PWM servo receivers, which were a receiver made specifically for "wing" style systems that used servos.  this receiver has 6 servo outputs, and a more elaborate wifi configuration page that allows for setting failsafe values.  I originally liked these a lot, due to them being an "all-in-one" setup, no need to get a PWM adapter or wing style flight controller for servos.   The issue is that over time, these receivers have started to fail.   three of them have been retired, having hardware failures that put them into bootloader mode.   two were never put into service, when originally wired up and tested, they had issues running servos on all six outputs.

current setup is the BetaFPV ELRS Nano 900MHz receiver with a matek PWM adapter.  generally the 6 servo pwm adapter, but i have tested an 8 servo and a 10 servo adapter.   The only issue i have with this setup is the limited configurability of the matek pwm adapters.

as geofrancis has pointed out in other posts in other threads, there are very good advantages to use a flight controller with these receivers.

this might be usefull for you, its a crsf pwm decoder that runs on a stm31f1
https://github.com/CapnBry/CRServoF

geofrancis wrote:this might be usefull for you, its a crsf pwm decoder that runs on a stm31f1
https://github.com/CapnBry/CRServoF

yeah, i have seen that, and am interested, it would be interesting to make one just to see how well it works, and how configurable it can be made to be.

maybe see if i can find a physically smaller board than a blue pill.

I had a look through the source code for that adapter, from what I can tell the only thing tying it to the stm32 is the servo output. if you wanted to run it on another chip like esp it would just need the servo output library changed.

Okay, so i added another piece of ExpressLRS equipment to my stable.

A Jumper T20 transmitter.

it is a compact size stick style radio with one feature that makes it different from all my other radios. It has an ExpressLRS 915mhz rf deck built into it.

out of the box, it came with the Australian frequency firmware, version 2.5.1, so i immediately updated that to version 3.3.0 of the USA frequency firmware. version 3.3.0 is the only official version available for this hardware, so that basically means i get to upgrade all my other rf modules and receivers to match.

Yet another purchase in the pipeline:

Betafpv has basically stuck their diversity receiver into a "Flyer" style form factor.

https://betafpv.com/collections/expresslrs-series-accessories/products/superp-14ch-diversity-receiver

this will drive up to 10 servos without any modification, and with a tiny amount of time spent wiring up two harnesses, up to 14 servos.

I have one on order for testing. It should arrive sometime in the next two weeks. i wil post more when i get time to play with it.

Radiomaster, a great maker of newer EdgeTX based radios (TX16s, TX12, Boxer, Pocket, Zorro, MT12) has started making transmitter modules for different protocols... and they just announced their "Bandit" line of transmitter modules and receivers.

The reason i am posting about them here is because their Bandit line is all 915mhz equipment. a standard and diversity receiver, as well as a standard JR style module and a nano/lite style module have been announced, with availability sometime in the next two weeks.

I will be picking up a nano tx module and a standard receiver as soon as i can for testing, and will post here when the testing is done.

I just got a radiomaster pocket for my daughter, my old 9xr pro was too big. I have been impressed with it so far, it feels like a big xbox controller.

geofrancis wrote:I just got a radiomaster pocket for my daughter, my old 9xr pro was too big. I have been impressed with it so far, it feels like a big xbox controller.

I have the Pocket, the Zorro, the TX12, and will have a MT12 as soon as i can get one.

There is nothing out there for the price of the Pocket that comes close to its capabilities.

Having said that, i like the Zorro more than the Pocket.

tsenecal wrote:

geofrancis wrote:I just got a radiomaster pocket for my daughter, my old 9xr pro was too big. I have been impressed with it so far, it feels like a big xbox controller.

I have the Pocket, the Zorro, the TX12, and will have a MT12 as soon as i can get one.

There is nothing out there for the price of the Pocket that comes close to its capabilities.

Having said that, i like the Zorro more than the Pocket.

The only thing that annoys me is that it doesnt charge the batteries with usb.

geofrancis wrote:

tsenecal wrote:

geofrancis wrote:I just got a radiomaster pocket for my daughter, my old 9xr pro was too big. I have been impressed with it so far, it feels like a big xbox controller.

I have the Pocket, the Zorro, the TX12, and will have a MT12 as soon as i can get one.

There is nothing out there for the price of the Pocket that comes close to its capabilities.

Having said that, i like the Zorro more than the Pocket.

The only thing that annoys me is that it doesnt charge the batteries with usb.

just double checking that you are plugging the usb-c cable into the "bottom" port, next to the microsd card slot? mine charges fine that way.

with the power to the radio off, the power switch should glow red.

one more thing, make sure you have the battery voltage properly calibrated. (hardware settings)

tsenecal wrote:

geofrancis wrote:

tsenecal wrote:

geofrancis wrote:I just got a radiomaster pocket for my daughter, my old 9xr pro was too big. I have been impressed with it so far, it feels like a big xbox controller.

I have the Pocket, the Zorro, the TX12, and will have a MT12 as soon as i can get one.

There is nothing out there for the price of the Pocket that comes close to its capabilities.

Having said that, i like the Zorro more than the Pocket.

The only thing that annoys me is that it doesnt charge the batteries with usb.

just double checking that you are plugging the usb-c cable into the "bottom" port, next to the microsd card slot?  mine charges fine that way.

with the power to the radio off, the power switch should glow red.

one more thing, make sure you have the battery voltage properly calibrated. (hardware settings)

haha I didnt see the usb port at the bottom, i was trying to use the port at the top.

Not quite sure how this didn't get posted, or maybe i posted it in a different area, but Radiomaster now makes its own line of 915mhz transmitter modules and receivers, under their "Bandit" model name.

they make 3 different transmitter modules, nano, micro, and standalone, and both a single and dual antenna serial receiver. (need a crsf to pwm converter to use servos)

the big advantage to all the Radiomaster transmitter modules is that all 3 of them offer what appear to be micro oled displays, as well as a mini 5 axis joystick, so you can change settings and bind to a receiver without using anything on the transmitter (no need for any lua scripts)

I did get the nano tx module, and one of the single antenna receivers to test.

Another unique advantage for sub guys is the Radiomaster BR3 receiver which has telemetry output of up to 500mW ! Most 9xx MHz receivers have telemetry output of 100mW or less since this is most often ok since the aircraft receiver has a high vantage point being up in the air but for sub use obviously this is not the case. I plan on doing depth/distance testing of both the BR1 and BR3 ... but it will probably be a month or three till I get to it. Khim

you can get the bayck for 15 at the moment https://www.aliexpress.com/item/1005004841827716.html

I have been looking at a lot of the elrs stuff recently as mLRS is getting ported over to esp32 chips so it's compatible with ELRS hardware..

geofrancis wrote:you can get the bayck for 15 at the moment https://www.aliexpress.com/item/1005004841827716.html

I have been looking at a lot of the elrs stuff recently as mLRS is getting ported over to esp32 chips so it's compatible with ELRS hardware..

thanks, I was not aware of that one, yes I'm in the ELRS camp due to the ability to write my own control hubs and data acq peripherals using a serial port and crsf protocol

if your familiar with flight controllers, something like inav would give you all the sensors you need.

geofrancis wrote:if your familiar with flight controllers, something like inav would give you all the sensors you need.

well I'm more familiar with betaflight than inav but I tend to go with other generic cpu boards because I'm using multiple types of pressure and current sensors etc. that are more sub specific for command control, i.e. send a command from the radio to "dive at angle X to depth Y and level off" and have the on board software handle the details but thx for your response

not sure how much of a benefit the 500mw telemetry power output is.

in my testing of all the different devices back in september, i was using 250mw dynamic power setting, and was able to drive the subs anywhere in the casting pond...  and had telemetry the entire time as well.  my understanding (and this could be 100% wrong) is that receiver telemetry is 10% of transmitter power.

a LOT of people seem to think that pushing out as much power as possible is "best".  i have seen a lot of people also complaining that when they have telemetry turned on (at max power), their servos jitter.

of course, another common complaint is that when they are using 2000mw output, the battery in the radio won't last more than an hour.


in all of my testing of OpenLRSng, ChipLRS, and ExpressLRS, i have always lived by the motto of using the lowest power setting needed to get the job done.

This summer I am finishing a new sub which is very modular in design which will permit quickly changing components and electronics for evaluation (they are in small waterproof modules instead of inside the larger enclosure), so part of the evaluation will be testing 27Mhz, 50Mhz, and 900Mhz for telemetry at various power levels. I am particularly keen on 50Mhz telemetry (amateur). The lower frequencies will use an SDR for the telemetry receiver and the high frequency will use ExpressLRS telemetry. By the fall I should have a good number of measurements and will document and share with everyone interested so I will hold off sweeping assumptions until then but attenuation calculations lead me to believe that sufficient RF power will be required for deep operation. I may also test 440Mhz but since the US does not have an ISM band at this frequency it is not of wide interest here .. but I can test as I have an amateur radio license. I have RF lab equipment and will do spectrum/power measures of above. I will maintain 75Mhz and 900Mhz for sub control.

As is obvious I am most interested in telemetry as I like to run my subs deep when conditions permit (9 ft + in diving pools or clear water) so I want the software to continuously monitor on board conditions for faults to allow surfacing.

I look forward to sharing the testing at a future time, thx for your comments always like to discuss sub electronics, Khim

What I like about modern receivers is that you can select the power output, I usually run them at 50mw. But it's nice having the option for more even if I never use it.

When OpenLRSng equipment was available in the 433mhz range in the US, i was using that. (2014-ish)  local HOA here where i live has a swimming pool with an 8 foot "deep end"...  and the 433mhz equipment was able to reach that depth at 100mw.  While testing to see what it was capable of, i was using as little as 10mw, and still getting down to 4+ feet.

DavidF and company here still use the OpenLRSng equipment, but in UK it is at 458mhz.   If you are looking for 433mhz equipment, Adafruit still carries a large selection of different boards that a DIY OpenLRSng tx/rx setup could be made from.   If you have the certs for 50mhz HAM, you have the certs for 433...