Pressure and strain gauges are used in a variety of IoT deployments, from smart cities monitoring infrastructure to industrial manufacturing. These are typically used to measure fluid and gas pressures. The heart of the sensor is a piezoelectric circuit. A diaphragm will be placed above or below a cavity on the piezoelectric substrate. The substrate is flexible, and allows the piezo crystals to change shape. This change in shape results in a directly correlated resistance change in the material:  

This type of sensor, as well as others listed in this chapter based on an excitation current, relies on a Wheatstone bridge to measure changes. Wheatstone bridges may come in two, four, or six-wire combinations. The change in voltage is measured across the bridge as the piezoelectric substrate flexes and changes resistance: