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Mar 17, 2015, 01:27 PM
Registered User

μLRS - a tiny OpenLRSng dedicated hardware

Hi all,

I'd like to present you the product I'm lately working on. I've identified a gap in the market for a tiny and light LRS receiver for 250-class quad. So I started with looking at the schema of Flytron RX V2... then I browsed OpenLRSng code and finally decided to make a board hardware compatible with OpenLRSng board nr 8 by kha (

At this moment I'm after initial review and comments at local polish forum and am just before ordering the first prototype PCBs. So I thought it's a great moment to share the project with you guys, as it's a last chance to modify it :-)

Let me start with some details first:
- PCB size is 0.79" x 0.7" (ca 20mm x 18mm)
- transceiver module will be RFM22b (S2 version)
- Atmega 328P-MU (QFN version aka MLF)
- U.FL antenna connector
- has reverse-polarity protection
- can handle max 15-16V input voltage
- the board (RX) is primarily dedicated to 250-class fpv quads, outputting PPM signal to any FC supporting it, like CC3D etc
- hardware can be either TX or RX
- it's definietly not DIY board, high soldering skills are required
- UPDATE! Onboard LFCN-490 filter

I/O pads are:
- gnd and input voltage at the top
- ppm, 3.3v and gnd at the bottom
- rssi, tx, rx, dtr, scl and sda at the right

The board layout is also divided into following sections:
- main voltage stuff (regulators, filtering capacitors) at the top
- radio section: power supply and antenna connector to the left
- digital i/oto the right and at the bottom

The board has also AVRISP pads easily accessible. RESET pad is on the board, and MISO, MOSI and SCK are directly connected to RFM22b module on the bottom of the board. So in case of bootloader failure, it won't be a huge pain to flash it even for users with a bit less soldering skills.

All I/O pads (except one gnd) have an additional via to make the copper stronger, hoping it won't pull off the board while soldering by end user... (thanks for the tip kha!)

Below is the screenshot of current sheme:

and the top layer with ground filled:

Last but not least is the 3D visualisation - keep in my mind that although the visualisation is quite huge, the board is really tiny

I'll start with making a really small batch of units for testing, assembling the boards manually. I'm not saying NO to make the product widely available to the public (actually I'd love it to be), it all depends how hard will it be for me to assemble it. And I'm sure it will :-) And of course... it all depends if you'll like it.
Last edited by acc007; Mar 22, 2015 at 02:19 PM.
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Mar 18, 2015, 02:19 AM
openLRSng haxor
Nice board, I will have a quick look on schematics etc. This should be fully supported by openLRSng as it follows reference schematic.

Notably you do not have the lowpass filter between rfm and antenna output (minicircuits LFCN490). This may be issue when a vtx is nearby.
Mar 18, 2015, 05:48 AM
Registered User
As I observed there are more 250-class fliers using 5.8Ghz video link than 1.2Ghz, so lack of this filter is somehow by purpose (yes, I've seen it in your board anyway).
Mar 18, 2015, 06:08 AM
openLRSng haxor
It is quite easy to make the filter optional as it can be substituted with 1206 0R resistor.

There is benefit from the filter even with higher freq vtxs. It seems 2g4 is quite popular nowadays too.
Mar 18, 2015, 06:14 AM
Registered User
It's rather a matter of space required for the filter And as I looked in datasheet it seems to need some empty gnd around it. So more space needed. But I'll see what can be done...
Mar 18, 2015, 01:28 PM
20x18mm? wow that's small.
I'd definitely perfer to have the filter, even if it doubles the size
Mar 18, 2015, 01:45 PM
openLRSng haxor
Originally Posted by cmavr8
20x18mm? wow that's small.
I'd definitely perfer to have the filter, even if it doubles the size
The microRX available here has filter, SMA possibility, and thru hole headers, while being only about 22x23mm. It also uses TQFP version of atmega which is easier to manage for diy.
Mar 18, 2015, 02:28 PM
Registered User
Indeed, that's all true. It also has an inverter for sbus which my board is missing as well.

My idea was to make a board as small as possible, even if some components might be missing. A kind of miniaturization exercise for me. Designed for weight maniacs for whom every 0.1g matters Of course while still being manually solderable...
Mar 18, 2015, 03:16 PM
Oi oi! You lucky people!
seebuyfly's Avatar
Would also be interested if it was filtered
Mar 18, 2015, 04:47 PM
Registered User
OK guys, I made it... LFCN-490 added and board size remained the same - 20x18mm.

Updated first post with current pictures. Filter placement is not perfect, but not the worst as well. I added some VIAs to meet mini circuits' recommendations. Anyway it should be much better than without the filter. Actually I think it's better than in openLRSminiRX (a lot of gnd vias are missing there, at least for github version)
Mar 18, 2015, 05:41 PM
Registered User
Good job i like!

I'll follow the thread!
Mar 19, 2015, 12:16 AM
openLRSng haxor
Good work!
Mar 19, 2015, 07:01 AM
Registered User
j0bro's Avatar
Nice work, subbed!
Call us when you're ready to ship the first batch :P
Mar 19, 2015, 10:35 AM
Registered User
JulianGoesPro's Avatar
Originally Posted by acc007
I'll start with making a really small batch of units for testing.
Nice, do you have an estimation for pricing?

How do you like the idea of putting this on a CC3D size board and add a minimOSD (or something like that) to the "empty space" so we can screw it right onto our FCs
Mar 19, 2015, 10:50 AM
Registered User
As for the price, Chineese will always beat me... especially for the first batch when I'll do everything on my own. Well, I'll swear during soldering, but QA will be 100% working :-) I don't know yet what should be the target price, what do you think would be an acceptable price for such a micro board ?

I've just added the filter, so the price is automatically about 5$ higher. But it is very easy to offer two boards versions - one with filter, one without it, 5$ cheaper

As for minimOSD - it is some idea to put two boards onto one pcb (especially when there is already micro minim osd available), but I doubt I'll go for it.

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