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 thereof. The unit also includes circuitry for emitting a corresponding output signal pattern to at least one other detector. A system which incorporates a plurality of electrical units, at least one of which has the pattern discrimination circuitry produces at least one synchronized audible or visible output indicative of a predetermined condition throughout the system. Patterns can include predetermined tonal alarm indicating output patterns as well as predetermined visible alarm indicating output patterns.