limit switch is thrown. Pop-up bread toasters allow the time to be set. Non-electronic gas ovens will regulate the temperature with a thermostat controlling the flow of gas to the gas burner. These may feature a sensor bulb sited within the main chamber of the oven. In addition, there may be a safety cut-off flame supervision device: after ignition, the burner's control knob must be held for a short time in order for a sensor to become hot, and permit the flow of gas to the burner. If the safety sensor becomes cold, this may indicate the flame on the burner has become extinguished, and to prevent a continuous leak of gas the flow is stopped. Electric ovens use a temperature sensor and will turn on heating elements when the temperature is too low. More advanced ovens will actuate fans in response to temperature sensors, to distribute heat or to cool. A common toilet refills the water tank until a float closes the valve. The float is acting as a water level sensor. Automotive Modern automobiles have complex instrumentation. In addition to displays of engine rotational speed and vehicle linear speed, there are also displays of battery voltage and current, fluid levels, fluid temperatures, distance traveled and feedbacks of various controls (turn signals, parking brake, headlights, transmission position). Cautions may be displayed for special problems (fuel low, check engine, tire pressure low, door ajar, seat belt unfastened). Problems are recorded so they can be reported to diagnostic equipment. Navigation systems can provide voice commands to reach a destination. Automotive instrumentation must be cheap and reliable over long periods in harsh environments. There may be independent airbag systems which contain sensors, logic and actuators. Anti-skid braking systems use sensors to control the brakes, while cruise control affects throttle position. A wide variety of services can be provided via communication links as the OnStar system. Autonomous cars (with exotic instrumentation) have been demonstrated. Aircraft Early aircraft had a few sensors. "Steam gauges" converted air pressures into needle deflections that could be interpreted as altitude and airspeed. A magnetic compass provided a sense of direction. The displays to the pilot were as critical as the measurements. A modern aircraft has a far more sophisticated suite of sensors and displays, which are embedded into avionics systems. The aircraft may contain inertial navigation systems, global positioning systems, weather radar, autopilots, and aircraft stabilization systems. Redundant