The present invention is directed to novel optical sensors and novel methods for sensing optical radiation that can be used to control the operation of automatic faucets and flushers. The novel sensors and flow controllers require only small amounts of electrical power for sensing users of bathroom facilities, enabling battery operation for many years. An electronic system for controlling fluid flow may include an electromagnetic actuator, a controller and an optical sensor. Preferred embodiments include a control circuit constructed to sample periodically the detector based on the amount of light detected; a control circuit constructed to adjust a sample period based on the detected amount of light after determining whether a facility is in use; a detector optically coupled to the input port using an optical fiber; the input port may be located in an aerator of the electronic faucet; the system includes batteries for powering the electronic faucet. These embodiments may also include a variety of other features. A passive optical sensor includes a light detector sensitive to ambient (room) light for controlling the operation of automatic faucets or automatic bathroom flushers. An active optical sensor includes a light emitter and a light detector. The detected signals may be processed using novel algorithms