An electrical unit, such as an ambient condition detector, incorporates pattern discrimination circuitry. A received pattern is analyzed and, if it is in accordance with a predetermined pattern, the unit outputs an audible and/or visible indicium synchronized with the incoming pattern for the duration...http://www.google.de/patents/US6472980?utm_source=gb-gplus-sharePatent US6472980 - Device with built-in signal discrimination and output synchronization circuits
