An arrangement for detecting toilet flushing by utilizing an acoustically-based motion detector in a simple amplifier circuit. The detector is in contact with the toilet bowl, which acts as a resonant system. Upon detection of such flushing, a subsequent event, such as a cleaning or disinfecting a toilet, ventilating the surrounding area or releasing deodorant into the area, may be triggered. The mechanically resonant system with the provided circuit is capable of separating frequencies from the frequency of the flushing water in a specific bowl design and provides a high degree of immunity to unwanted ambient frequencies. In addition, the system consumes low power and is suitable for battery power operation.