New RF 433Mhz receiver

Following my post about the receiver for La Crosse sensors, I started to doubt about the quality of the reception.

On my left: XY-MK-5V

As I said before, I found this one on ebay for 1 € (inc. postage)!

But it is not specially noted for its quality of reception and output signal is very noisy.

On my right: RXB6

I decided to break the piggy (hum!) and ordered a RXB6 module which has a far better reputation. Cost from China is $5 (inc. pp). Obviously it took 3 weeks to arrive but finally, I was able to try it on the Arduino.

Surprise! It didn't work at all. Nothing, no signal on the PIN 13.

After a while, and really thinking I got a duff one, I hooked it on the Digispark and... miracle!

Performances

But if I had some readings, it occured to me that something must be wrong as TX sensors are quite chatty (a repeted signal every minute or so) and I had only 20 values for a whole evening.

Something was definitely amiss. OK pulseIn is not necessarily the best method ever but since the microcontroller is not doing anything else, this should not be a problem.

While decoding, I ignore the LOW part of the signal (the 1000μS gap) but still...

After a lot of trial and error, I finally understood where the lock was: A tiny little bug!

The Bug

The first loop validate the preamble by checking the value is 10d (00001010b) but the variable was defined by int so depending on the preceding bits it could have held a different (higher) value despite a correct preamble.

To correct this behaviour I could either do a AND 255 with the value but I simply defined by variable by 8-bit byte type.

I also changed the boundary timings to be less restrictive.

Pin 13

Reading about different receivers, it seems that the RXB familly (at least) doesn't cope well with pull-ups. Since the pin 13 of the Arduino has a LED, this might be the reason why it didn't work when I first tried it. It works fine on any other pin...

Additional items

Adding an external antenna to the RXB6 doesn't seem to make much difference but when hooked on the digispark, a decoupling capacitor (100nF) seems mandatory for the RF receiver!

And the winner is...

RXB6 module is definitely better, especially with the outdoor sensor, but it also uses twice as much power (6.7mA vs 3.8mA). And the XY-MK-5V has an unbeatable value for money.

Lacrosse TX2/TX3 Sensors and Digispark

Digispark

The Digispark is a Atmel Attiny85 based microcontroller development board similar to the Arduino line but cheaper and smaller. They had an extremely successful Kickstarter campaign 2 years ago and clones are available on ebay a few euros.

They seem to now have their own ecosystem and can emulate a USB device. The USB is also used to programme the microcontroller, the same way it is done on Arduino.

Wiki pages quickref and basics have some information about pins, differences from the Arduino and limitations.

For reference, here is the schematic of digispark.

Pins

Here I need only two pins for the project : 1 for the RF receiver and the other one for the serial connection to to Raspberry Pi.

I decided to go serial instead of I²C mainly because of the asynchronous nature of the signal and to avoid polling from the I²C master (Raspberry Pi in my case). Here the digispark will simply send its data as soon as it decodes something from the RF receiver.

So PIN 0 will be the RF receiver IN while PIN 2 (default for Serial Debug) will be used for the Serial Communication. 38400 bit/s seems to be the recommended value. Fair enough.

Code is available on github :https://github.com/guillier/la_crosse_sensors

Raspberry Pi

Serial communication

In order to allow serial communication, the serial port has to be freed from any usage as a console. Once it is done (and the Raspberry rebooted), minicom can be used to check if everything is alright.

minicom -b 38400 -o -D /dev/ttyAMA0

Note that the port is /dev/ttyAMA0 while the one for the Z-Wave static controller is /dev/ttyACM0.

Storing values

Same as the values from the airpi sensors, these ones can be stored in RRD databases:

rrdtool create temperature_device33.rrd --step 300 \
DS:temperature:GAUGE:600:0:U \
RRA:AVERAGE:0.9:1:24 \
RRA:AVERAGE:0.9:6:35064 \
RRA:MIN:0.9:288:1826 \
RRA:AVERAGE:0.9:288:1826 \
RRA:MAX:0.9:288:1826

Lacrosse TX2/TX3 Sensors

La Crosse WS7014

I have owned a La Crosse Weather Station WS7014 for quite a long time (I would say it is around 15 years old or so) and if one of the sensor (TX2) can't be used outdoor because it is no longer waterproof, the base station is still going strong. The newer (but at least 7 years old) sensor is a TX3-P.

Receiver

Found on ebay a 433Mhz RF transmitter and receiver kit for 1 € (inc. pp)! It turned out to be a FS1000A + XY-MK-5V.

I'm not too much bothered about the transmitter but if the receiver can do the trick (at this price), why not?

Note that some projects seem to rely heavily on Aurel's modules which are in the 30€ range.

Decoding

All these sensors communicate the 433Mhz band and it would be nice to be able to intercept the signal. Fortunately some people have already decoded it.

Based on this page http://www.f6fbb.org/domo/sensors/tx3_th.php and this one http://www.f6fbb.org/domo/sensors/tx_signals.php for starters and on this arduino forum as well the decoding should have been easy.

But after several hours, nothing was showing up on the arduino serial console.

Using the code posted on one of the messages it turned out that all the timings were off by 25-30%. Ajusting the ranges helped and some values from the nearest sensor appeared within minutes.

If the code in the PDF document (for Funksensor TFA 30.3120.90) is handly, I really think that Mr ROUBELAT is right when he considers a short pulse as bit 1 and a long one as bit 0 and not the other way around.

Another observation is that as far as I can tell the parity bit is always correct from the TX3 but not so from the TX2. So I decided not to rely on it but on the CRC and on the redundancy instead.

At night

During the day I can't receive anything from the outdoor sensor but in the evening, it appears as well. Maybe the dead cheap receiver it not completely up to task to cope with the distance and several concrete walls.

Replacing the Arduino

Due to the asynchronous nature of the transmission and on the timings (a few hundred µS), using the Raspberry Pi directly won't do. Using a full size Arduino for this is certainly a waste. Maybe a ATtiny would do but I have no experience in programming them so I went for something in the middle: A digispark.

To be continued...