How do wireless tyre pressure/temperature sensing systems work?

Thread Starter

teliocide

Joined Sep 26, 2013
113
After looking at different systems it appears most operate at a Transmission Frequency of 433.92Mhz.
My experience of "things "operating on this frequency is limited to ON/OFF systems like gate and door opening remote controls.
So my question specifically focuses on how the temperature and pressure values are encoded into the 433.92 Mhz transmission and how this is decoded and displayed at the receiver.?
 

Reloadron

Joined Jan 15, 2015
6,963
Plenty of analog signals are easily sent over WiFi. That includes 2.4 GHz and 5.0 GHz bands. There is also Bluetooth operating in the 2.45 GHz bands. There are also 433 MHZ, 862 MHZ and 870 MHz transmitters which will transmit analog signals. You can digitize and analog level and send it. Temperature, pressure and about anything you want. Pressure just as an example. I use a pressure transducer to convert my units of pressure to a current or voltage proportional to my units of pressure. I then run my units of voltage into an A/D converter and transmit it. This is a very basic overlay.

Ron
 

WBahn

Joined Mar 31, 2012
27,392
After looking at different systems it appears most operate at a Transmission Frequency of 433.92Mhz.
My experience of "things "operating on this frequency is limited to ON/OFF systems like gate and door opening remote controls.
So my question specifically focuses on how the temperature and pressure values are encoded into the 433.92 Mhz transmission and how this is decoded and displayed at the receiver.?
I don't think there is any real standard (yet) since that only becomes an issue when you want to ensure interoperability between devices from different manufacturers. I don't think we are there yet (though, if we aren't, we're probably not too far away from it).

When transmitting non-critical analog data (non-critical in the sense that you can tolerate quite a bit of error in the result), one common way to do it is through FM where the receiver just measures the dominant frequency (within the band where it is expecting it) and then uses that to compute the value. A problem with this approach for this application is that there are going to be a lot of vehicles in close proximity that are all transmitting their data, so how is the receiver supposed to know that it is getting the correct data and not someone else's. There's ways to deal with this, but in the digital age it is so much easier to use a digital solution. My guess is that no TPMS system transmits data in analog form.

I would expect that most, if not all, of the systems transform the analog data to digital form in the transmitter and then transmit the data after encoding it to minimize the chance of the receiver accepting an wrong value due to corruption of the waveform. This also allows you to transmit identifying information so that the receiver can ignore data from other sources as well as know which tire the data is coming from. There is probably also a preamble of some kind to allow the receiver to lock onto the start of a packet, since they are sent very infrequently (once a minute or so), and probably a post-amble to ensure that the receiver didn't experience a framing error. The error-detection is most likely a checksum of some kind.
 
Top