Detecing LoRaWan traffic with SDR-RTL

RTL-SDR is a Software Defined Radio using a cheap DVB-T receiver with a chip from Realtek (RTL).

It can tune to 868 MHz and show activity, which is really handy when you are debugging a LoRa node and need to figure out if it is actually transmitting anything.

I bought the receiver from van Dijken Electronica at their stand at the VERON fleamarket for about 25 euro. It plugs into USB or a raspberry pi and the software is free (as in beer). I use SDR# from Airspy.

This is a couple of minutes traffic from a LoRa node building meetup.
The top graph is the current spectrum around 868 MHz, the lower graph is the history. The fuzzy red rectangles are spread spectrum LoRa transmissions.


PoE Pi : Powering a Raspberry Pi with Power over Ethernet

Power over Ethernet (PoE) is a standard for powering devices over an ethernet cable. Aside from the IEEE 802.3af standard (description) there are other ways of powering devices over ethernet cables.

Shopping on internet shows two different PoE solutions, the cheap (and slightly sloppy and maybe dangerous) way is to use unused pairs in the Cat 5 cable and use a passive injector and splitter. The problem with that solution is the dissipation and loss in the network cable at higher powers. The nicer and safer solution is the official IEEE 802.3af solution that I wanted, and I wanted this to work with thin 2-pair Cat 5 cable. Regular Cat5 cable has 4 pairs. I’ll only be using the IEEE 802.3af standard here.

I bought a (second hand and very loud) Cisco Home Office PoE switch to try this all out.


2 Pair Cat-5


I bought a PoE splitter from Aliexpress

IMG_20160515_105601The PoE splitter was fairly cheap (about 7 euro) and there were some doubts if this was actually an active splitter. The sticker on the back states IEEE802.3af compatibility.

IMG_20160515_105620The insides seems legit,an actual PoE controller chip, isolating transformer and Ethernet transformer. The parts used look like the application note with one difference : the application note describes a 10/100/100 MB/s solution with 4 wire pairs, this splitter only supports 2 pairs.


The track clearance is not great though. And there is something going on with the connections between two cable pairs but this configuration seems to be used in other setups as well ( figure 4, figure 1.1 ).


These PoE splitters deliver 12 V. The raspberry Pi needs 5 V on the micro USB port. I’ll need to convert it to 5 V.

Aliexpress and other sites sell step down converters. I bought these, trimmed it to 5 V. and fixated the pot with nail polish.


I sacrificed a USB cable, added a barrel jack and hotsnotted it al in place.



Success ! The setup works with RTL-SDR on a Raspberry Pi.


Philips changes support for 3rd party lamps from Hue, twice.

It seems a firmware update for the Hue bridge and Hue bridge 2 has removed compatibility to 3rd party lamps, that version of the firmware will only work with “Friends of Hue” lamps and no longer support generic Zigbee Light Link lamps.

There was a lot of backlash on this as it can make existing, working, setups useless and raised questions about Philips still effectively being a  member of the  TCLA and if Hue bridges are still Zigbee complaint ?

So, Philips have announced they will (or have) reverse(d) the update on twitter and on meethue.

One sore point seems to remain : Apple HomeKit certification apparently not only applies to the Hue bridge but also to the lamps connected to the Hue Bridge :

Before the 1.11 software update, a bug in the Philips Hue system allowed some non-Apple HomeKit certified lights to work with Apple HomeKit. Our 1.11 software update removed the bug with the result that non-Apple HomeKit certified lights no longer work with Apple HomeKit. This remains the case.

(from meethue). Welcome to the long arm of Apple‘s Walled Garden.



The discovery and confirmation of the update that removed support :
iPhone-Ticker :
Meethue on Twitter :

The announcement of the reversal :
Meethue :
Twitter :

Having cheap PCB’s made part 2.

A while ago i made a popular post about having cheap PCB’s made. Some things have changed so it’s time for a new post.

BatchPCB is now OSH Park.

EuroCircuits have broadened their range of services to better cater to prototype runs and hobbyists.

Seeedstudio have a PCB prototype service.

Where there are lots of suppliers and customers shopping for the best price, sooner or later someone is going to set up a price comparison site like : PCBShopper.

The most interesting manufacturer for me is now DirtyPCBs (from the people behind Dangerous Prototypes who made the Bus Pirate), my last 3 batches were ordered there.

The first batch was a new run of the LivingColors Arduino shield.

New batch of LivingColors Arduino shields.
New batch of LivingColors Arduino shields.

The second batch was a breakout board for the NiceRF SX1276 LoRa module. Here i experimented a little with contour routing.

SX1276 module breakout board.
SX1276 module breakout board.

The third batch was an adapter board to use no-name CC2500 modules in boards designed for the Quasar QFM-TRX1-24G. The boards are small (about 20 mm x 25 mm) and the minimal size for dirtypcbs PCB’s is 100 mm x 100 mm. Here I experimented with breakout panels, putting 4 PCB’s in one design.

Adapter boards
Adapter boards

The end result worked out nicely, two things I’d do better now are :

  • Position the “mouse bites” differently so the rough edges won’t protrude, see this post.
  • Copy and fixate the silkscreen text to a new layer before copying and rotating the sub-PCB’s. If you look closely at the PCB’s you’ll see that the text didn’t rotate with the rotated copies of the traces. That’s what you want for a big PCB but not for a breakout PCB. There is a ULP for that and a nice tutorial.