An electric humidifier includes a water level detector needle set in the water cistern. When the water level is normal the needle detector applies a positive bias to the base of a first transistor in a bias supply loop circuit, and the transistor conducts to supply bias current to an ultrasonic oscillation...http://www.google.co.uk/patents/US4820453?utm_source=gb-gplus-sharePatent US4820453 - Water level detector and circuit for an electric humidifier
