Wednesday, March 04, 2020

Decoding a Digitech Wireless LCD Thermometer

Just picked up a AU$20 indoor/outdoor wireless thermometer from Jaycar. It mentions on the back that it uses 433.9MHz so I connected an RTL-SDR and ran the omni decoding rtl_433 command line tool. Simply installed using apt on Ubuntu.

The remote FT-004-b temperature sensor sends a reading once every 60 seconds. There's a little LED that blinks just before it sends. Here's the setup.

rtl_433 can output decoded data in a variety of formats: kv|json|csv|mqtt|influx|syslog,  so this could be handy for logging temperatures. rtl_433 can decode about 150 different devices and there is documentation on how to add more.

It supports the following modulation types:

OOK_MC_ZEROBIT :  Manchester Code with fixed leading zero bit
OOK_PCM :         Pulse Code Modulation (RZ or NRZ)
OOK_PPM :         Pulse Position Modulation
OOK_PWM :         Pulse Width Modulation
OOK_DMC :         Differential Manchester Code
OOK_PIWM_RAW :    Raw Pulse Interval and Width Modulation
OOK_PIWM_DC :     Differential Pulse Interval and Width Modulation
OOK_MC_OSV1 :     Manchester Code for OSv1 devices
FSK_PCM :         FSK Pulse Code Modulation
FSK_PWM :         FSK Pulse Width Modulation
FSK_MC_ZEROBIT :  Manchester Code with fixed leading zero bit

Thanks to John, VK2ASU, for the nifty 433MHz SMA antenna.

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