We have added an Tire Pressure Monitoring System (TPMS) to Molly. This is partly for safety, partly for convenience and mainly for fun. We wanted a system that could be connected to our computer system, this way we can see the current tire pressure, but we can also monitor the tire pressure over an extended time period.
Sometimes when we go off-road, and when we do we air-down the tires, and it would be nice to have multiple configurations so the system keeps correctly monitoring the tire pressure, even when in these air-downed configurations.
We have seen too many youtube videos where “Overlanders” drove on bad roads not knowing that one of their tires was in trouble, resulting in a completely shredded tire and in one case wheel damage. The hope is not to experience these type of issues, and the tire pressure system will alarm where the tire pressure is too low.
The system we settled on was the Gateway Lite, also known as the LINK LT, from Pressure Pro. This is a commercial grade product intended for the trucking industry, and acts as a gateway to send tire pressures to fleet management systems. As such, it does not have any display unit. This was perfect for us, as we didn’t really want another thing to clutter up the cab. When we want, we can display the tire pressures and temperatures on our Garmin Overlander, our phones or the laptop.
The computer will constantly monitor the pressure and temperature signals from the TPMS, and immediately raise the alarm should it detect a leak and other tire pressure or temperature related problem. Having the computer always checking the tires will hopefully allowing us to take care of any issue before it becomes a bigger problem. Slow leaks is the most likely thing to go wrong, and we have had a number of slow leaks from nails and sharp rocks in the past (on our Land Rover). The tire pressure readings from the sensor will also be easier to make sure the tires are evenly inflated, including when we air-down and air back up.
The tire pressure sensors screw directly onto the valve stem, reporting both tire pressure and temperature. The sensors are sealed and submersible and the internal battery lasts about 5 years.
Operating Range: 8 to 215 PSI
Accuracy: ±2-3 PSI (13kPa)
Resolution 1 PSI (6kPa)
Operating range: -20°C to 70°C
The Gateway Lite is mounted in the electrical cabinet with a connection to an external antenna mounted under Molly. The Gateway Lite is connected to 12 volts for power and to the house computer via an RS232 interface. The RS232 signal levels are converted to a more computer friendly USB connection with a standard FDTI based cable.
The Pressure Pro communication protocol is proprietary, but they were kind enough to share the protocol details with us. This allows us to locally read the status and alert messages for pressure and temperature, log them and ultimately display them.
Writing the Software
The only complication of the system we choose is that we have to write some software to monitor the system and raise the alarm. Turns out this software is actually quite simple.
The communications protocol from the Gateway Lite is relatively simple, it only took a few hours to configure node-red to talk its language. Because it is a proprietary protocol, I will not go into the details of the actual protocol, but it is simple and easy to decode. I can however explain the structure of the software.
Incoming messages are checked for integrity by making sure the message checksum is valid. We configured the gateway to broadcast the tire pressure status messages every minute, and on receipt, these are decoded and the key parameters like current tire pressure, tire temperature and target tire pressure are extracted. Alerts indicating issues with tire pressure are also broadcast every minute, and decoded to determine if an alert is active.
We included a communication watchdog to make sure at least 1 valid message is received every 10 minutes, and if not, then an alert is also triggered indicating the system is malfunctioning.
Each sensor reports new readings every 4 to 5 minutes. But if there is a fault condition, like low pressure or high temperature, then they alert immediately. And if a fault condition is detected, then this is alerted immediately to the “Buzzer”. The buzzer is our alarm. Otherwise the monitoring software records metrics every minute. for long term trending.
Installing the Equipment
The Gateway Lite is installed into our electrical cabinet, providing easy access to power and the communication connection to the computer. This is a better location than under Molly, where there is more dust and rocks.
The antenna is located under Molly and is cabled back to the Gateway Lite. The sensors are installed onto the 4 tires and also the spare tire. The Gateway Lite can report the radio signal strength from each sensor, this allowed us to confirm that the antenna was located in a good position was getting a good signal.
The sensor themselves are merely screwed onto the valve stem of the tire.
As there is no tire pressure display in our front cab, we need a different way to alert us of tire related issues. Actually we need a way to alert us to any type of issue. And this is where the “buzzer” comes into play. Should the TPMS detect an issue, or should the buzzer lose communications with the TPMS, then our buzzer will alert us to the problem. If it is a tire pressure related problem, then it will re-alert every 60 seconds until we fix the issue. The buzzer is located in the front cab, behind the dash, and sounds a lot like a smoke alarm. It is loud, in fact it is loud enough to wake us up in the middle of the night.
It is quite important that this little buzzer thing works. So, there are a couple of safety measure built-in. Firstly, when the engine start is detected, the TPMS software sends a short signal to the buzzer and performs a sound check, which lasts half a second. This is so we know the system as a whole is working. The controller for the buzzer also has a heartbeat or watchdog timer set to 10 minutes, if the last message received from the TPMS software is more than 10 minutes old, then something has clearly gone wrong with the wifi communications or the TPMS gateway. This triggers the watchdog and causes the buzzer to go off for a quarter of a seconds, repeating every 10 minutes until a message is received.
The buzzer is controlled by a Wemos D1 mini (ESP8266) with a relay shield and software we wrote. The buzzer is wired to the normally open contact of the relay. During normal operation, the relay is off. If the controller gets a message that the tire pressure is in alarm, then it will close the relay for 500 milliseconds, just enough time to get our attention. The controller will continue to alert every 60 seconds until a tire pressure ok message is received. As mentioned above, should the controller not get any messages, well then, it will also alert. Working out what the problem is best done by looking at the Grafana graphs, which have all the details for each tire.