MMM-updateFromStdOut

A Magic Mirror module to receive input via stdout from a program running in the background and then update the mirror with this information. Initially used for displaying temperature readings from a USB DVB-T stick acting as a 433Mhz SDR, receiving input from a cheap wireless weather sensor (GT-WT-01).

This is published more as a template to use output received via stdout from any arbitrary program running in the background. And also so I don't forget the stuff I learned in the course of implementing this ;)

If you want to use it as I did

Hardware

The sensor is part of a wireless weather station set sold by Hofer (i.e. Aldi Süd in Austria). The sensor model is also sold seperately; you can find it on e.g. ebay with its model number GT-WT-01.

First I tried receiving its data via 433Mhz receiver modules directly attached to the Raspberry's GPIO pins, but was totally unsuccessful with this approach.

Then I bought a USB DVB-T stick based on the RTL2832U chipset, available for ~15€ on Amazon (example), and that just works like a charm. Hardware based on this chipset can be used as a software defined radio receiver!

Software

I used this blog entry as a starting point, which will lead you to installing rtl_433. After you tweaked the parameters for rtl_433 right to receive and parse your weather stations's output (note: my sensor is recognized as model GT-WT-02, but works perfectly), customize node_helper.js to use these command line parameters.

Stuff I learned

1. In this case, use spawn, not exec.

   You use the spawn method with the detached parameter set to true to keep the spawned program running in the background

2. Socket communication needs to be initialized by the module code

   If node_helper.js wants to communicate with the module code (in my case MMM-updateFromStdOut.js) via sockets , then the module code needs to initialize communication (i.e. send the first request via the socket). Only then can node_helper.js send stuff via the socket connection.