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Oct 22, 2019, 04:37 AM
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ExpressLRS, DIY LoRa based, race-optimized RC link system


I want to present to the community ExpressLRS, a pet project I've been working on for awhile.

ExpressLRS is an open source RC link for RC aircraft. It is based on SX127x hardware combined with an ESP8285 and ESP32 for RX and TX respectively. It can be built with various hardware or customized to suit. The standard build fits in a JR module and the standard RX can be mounted in a 20x20mm stack.

You may be aware of some similar projects such as QCZEK LRS, QuadMeUp_Crossbow and RavenLRS, looking at these I felt that they don't really meet my needs for an RC system for my race quads given the low frame rates their authors have chosen.

Compared to these, ExpressLRS uses a very compact (8bytes) packet size, It transmits the first 4 channels at full resolution and 4 channels as 7 state switches (although not at the same time). It can run at 200 Hz, 100 Hz or 50 Hz depending on if you prefer range or low latency. At 200 Hz it is possibly the fastest RC link on the market with a measured Stick -> OpenTX > RF -> RX -> Serial Packet Latency of ~10ms.

Due to the optimized packet structure only basic telemetry that gives uplink/downlink information is currently supported. This system does FHSS over 20 channels combined with LoRa modulation.

TX and RX modules communicate via the standard CRSF serial protocol for easy use with Betaflight and openTX.
TX module can be built for $30 and recievers for $15-20.

PCB files are not released yet because some errors needs to be corrected and tested before they are public.
I run custom hardware, but this software should support using ESP32/LoRa dev boards found on AliExpress and the like.

This project is still WIP but works well enough that I am migrating all my quads over.

https://github.com/AlessandroAU/ExpressLRS

Discord Chat: https://discord.gg/dS6ReFY
Last edited by AlessandroAU; Oct 22, 2019 at 05:02 AM. Reason: Edit: Fixed Link, edit text typo
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Oct 22, 2019, 04:39 AM
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Oct 22, 2019, 05:17 AM
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Flew a few test packs today at 200Hz and it happily went beyond 300m with a structure between the quad and Tx.

Bring on the next race meet!!!

Oct 22, 2019, 05:43 AM
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What a great project and nicely executed AlessandroAU
Oct 22, 2019, 09:02 AM
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Interesting! And thanks for sharing!

One question still: LoRa is afaik intended to work without channel hopping, why implement FHSS? Doesn't that interfere?
I have only read a bit about it, so please explain.
Or are you using Lora modules the way normal RF modules are, so essentially any 868MHz would do in the same manner? (915 is illegal here, but it's technically similar.)
Oct 22, 2019, 09:43 AM
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Originally Posted by learningarduino
Interesting! And thanks for sharing!

One question still: LoRa is afaik intended to work without channel hopping, why implement FHSS? Doesn't that interfere?
I have only read a bit about it, so please explain.
Or are you using Lora modules the way normal RF modules are, so essentially any 868MHz would do in the same manner? (915 is illegal here, but it's technically similar.)
On the contrary, LoRa is designed and intended to be used with FHSS.
You can even FHSS during the middle of a packet, (See SX127x datasheet section 4.1.1.8)
This is not running a LoRa stack, that would be silly. This is using LoRa modulation as a means of transferring data.
Oct 22, 2019, 10:19 AM
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Quote:
Originally Posted by AlessandroAU
This is not running a LoRa stack, that would be silly. This is using LoRa modulation as a means of transferring data.
Ah, that's the secret. Thanks!
Oct 23, 2019, 08:25 AM
KD2PBU - Fly No Evil
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How does this compare to a Dragon link?
Oct 23, 2019, 09:28 AM
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Originally Posted by davidbitton
How does this compare to a Dragon link?
I don't consider them to be comparable, my system is for medium range, low latency control. Dragon link is for range above all else and has the output power and price to boot. The ExpressLRS TX fits in a JR module and can sip power all day, the RX module is 20x20mm and weights 3g, they are for different applications.
Oct 24, 2019, 12:32 AM
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Damn it right after I get all the stuff for qczeklrs, haha. I might have to build this if the latency is too much with qczek for a micro quad and just leave qczek for my extra long range planes. I'll be following this thread closely.
Oct 24, 2019, 07:32 AM
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Looks like an awesome project, how well does it play with Crossfire users in a race environment?
Oct 24, 2019, 02:27 PM
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Originally Posted by Benaresq
Looks like an awesome project, how well does it play with Crossfire users in a race environment?
I don't know yet, but crossfire on 150hz mode uses fsk which should play nice with my system using Lora.
Oct 25, 2019, 09:57 AM
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Today we tested out to 320m with two ExpressLRS systems running at the same time, this was running at 50mW.
ExpressLRS field test out to 320m and back (1 min 42 sec)
Nov 07, 2019, 08:18 AM
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ExpressLRS now supports using TTGO LoRa board as TX units!

IE:
https://www.aliexpress.com/item/4000059700341.html
https://www.aliexpress.com/item/32840238513.html
Nov 08, 2019, 01:08 AM
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Flew the TTGO LoRa based module all day, with the stock crappy antenna, and it didnt miss a beat. 1 lap below with debug info on screen showing RC Smoothing set to 5ms, aka 200Hz packet rate.

ExpressLRS Race Testing (0 min 19 sec)


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