A contact temperature sensor has been chosen for this module. One or two of these sensors are attached to the warm and/or cold water pipe(s) to detect water flow by measuring changes in pipe temperature. Other sensors, such as a piezoelectric vibration sensor and a capacitor microphone have been unable to provide usable data. While ultrasonic flow measurement would be the most accurate method of measuring water flow, it has not been chosen due to the high price, low availability and possibly requirements in terms of power usage and supply voltage that the module would not be able to fulfill.

The TMP75B temperature sensor provides a resolution 0f 0.0625°C/LSB and accepts supply voltages between 1.4V and 3.6V, making it ideal for operation in the test setup in a 3.3V environment while being able to connect to the STM3xx module without the need of level shifting. The TMP75B offers an alert signal with a configurable threshold, this feature cannot be used on one of the WAKE pins of the STM3xx module due to the power supply for periphery shutting down when the module is in sleep mode.

In a test setup, the temperature sensor was attached to a pipe connected to a toilet water tank and an ATMega328 was used to collect readings once per second over 1600 seconds. The readings clearly show a sharp drop in temperature every time the toilet was flushed. Over time, the pipe temperature returns to approximately room temperature. A simple threshold can be used to detect water flow and maxima in the differential can be used to pinpoint times when the water starts flowing.