Sail switch with 5-inch wide sail activates electronic air cleaner, humidifier, or other equipment in response to airflow from system fan. S688A is mounted in return air duct where sail will be in direct path of unrestricted air stream. Simplified installation with multi-speed fans, inaccessible air handlers, fan motors with voltage or phase different from controlled equipment; eliminates wiring to system fan. Polyester film sail mounted on micro-switch snap switch. Removable spring counter-balances sail to allow mounting in either vertical (up or down) or horizontal air flow. Top and bottom conduit knockouts for wiring convenience. Low air velocity switch operation makes at 250 feet per minute (FPM) and breaks at 75 FPM. For use in ambient temperatures normally prevailing in spaces made for occupation, which usually are not higher than 77F (25C) degrees but occasionally may be as high as 104F (40C) for brief periods.