A drop detector having a piezoelectric membrane mounted to a substrate is presented. When a drop strikes the piezoelectric membrane, the membrane vibrates at a selected resonant frequency. The resonant frequency is selected to enhance the sensitivity of the drop detector and the signal-to-noise ratio...http://www.google.com.au/patents/US4835435?utm_source=gb-gplus-sharePatent US4835435 - Simple, sensitive, frequency-tuned drop detector
