Homebrew Tire Pressure Monitoring System

When [upir] saw that you could buy tire valve stem caps that read pressure electronically, he decided to roll his own Tire Pressure Monitoring System (TPMS) like the one found on modern cars. An ESP32 and an OLED display read the pressure values. He didn’t have a car tire on his workbench though, so he had to improvise there.

Of course, a real TPMS sensor goes inside the tire, but screwing them on the valve stem is much easier to deal with. The sensors use Bluetooth Low Energy and take tiny batteries. In theory, you’re supposed to connect to them to your phone, although two different apps failed to find the sensors. Even a BLE scanner app wouldn’t pick them up. Turns out — and this makes sense — the sensors don’t send data if there’s no pressure on them, so as not to run down the batteries. Putting pressure on them made them pop up on the scanner.

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SDR Listens In To Your Tires

[Ross] has a 2008 Toyota Tacoma. Like many late model cars, each tire contains a direct tire pressure monitoring sensor or TPMS that wirelessly sends data about the tire status to the car. However, unlike some cars, the system has exactly one notification to the driver: one of your tires is low. It doesn’t tell you which one. Sure, you can check each tire, but [Ross] had a different problem. One sensor was bad and he had no way to know which one it was. He didn’t have any equipment to test the sensor, but he did have an RTL-SDR dongle and some know-how to figure out how to listen in on the sensors.

The key was to use some software called RTL-433 that is made to pick up these kinds of signals. It is available for Linux, Windows, or Mac, and supports hundreds of wireless sensors ranging from X10 RF to KlikAanKlikUit wireless switches.

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