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I am specifically interested in the sensors used on the B777 and B787.

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They both use a tire pressure monitoring system (TPMS) from Crane. Crane's marketing materials on the system are available on its site, and this newsletter from Boeing confirms it:

The software group helped with testing on a new tire pressure and brake temperature monitoring system on the 777 for Boeing supplier Crane Aerospace & Electronics.

(Since Boeing confirms that they work with Crane at all, I that doubt Crane would lie about its involvement in the 787 -- that seems like a good way to tick off what I'm sure is a major customer!)

Crane doesn't describe the system in great detail on its site, but from the name ("SmartStem") and description ("wireless tire pressure sensor with integrated fill valve") it appears to be a Direct TPMS sensor installed in the tire's valve stem. From Wikipedia:

In most current designs of direct TPMS, a small electronic assembly which is rugged enough to be mounted inside a tire, measures the pressure using a microelectromechanical system (MEMS) ...

In short: there is a sensor on the valve stem that translates the air pressure to a mechanical process, probably using a piezoelectric sensor or similar. This information is then wirelessly transmitted to the rest of the plane's systems.

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On a Boeing 777, a tire pressure sensor/fill valve is installed on each of the 12 main gear wheels and two nose gear wheels. There is an in-axle transformer in each of the axles.

The tire pressure sensor/fill valve is made of an air fill valve and three independent pressure sensing channels which provide accurate tire pressure.

The tire pressure sensor/fill valve is installed in the wheel.

A relay transformer hubcap assembly connects to the tire pressure sensor/fill valve. The left 28V DC MAIN bus supplies the electrical power to operate the tire pressure indication system.

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(Image source: 777 AMM)

You can see the sensor in these images:

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(Source: airliners.net)

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(Source)

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