A remote alarm circuit provides visible and audible signals to indicate that there is little unused space left on magnetic and optical tracks on disks used to record voice signals in a group of three multichannel voice recorders. In the particular application for which the remote alarm circuit was built, the voice recorders are required to operate without interruption, but the technicians responsible for the continuous operation of the voice recorders perform most of their duties on a different floor of the building in which the voice recorders are located. The remote alarm circuit gives sufficient advance warning to enable the technicians to go to the voice recorders and change recording disks in time to ensure continuity of recording.

ImageThe circuit (see figure) includes a sensor in each voice recorder that closes a switch when the unused space on the recording disk falls below a preset minimum. A sensor switch closure indicates a fault condition in which an audible alarm activates together with a light emitting diode (LED) for the corresponding fault. Three current steering diodes make the voltage across, and current through, the audible alarm independent from the number of simultaneous faults. This keeps the alarm tone consistent despite the number of alarms.

An Audible Alarm and LED Indicators signal when remote voice recorders are low on available recording media space. A pilot LED and push-to-test buttons enable periodic verification of the alarm panel without interfering with alarm functions.

For verifying the alarm panel, normally-open push-button switches are wired in parallel with the remote sensor switches. This arrangement decouples the test circuitry from the alarm circuitry, which (1) allows each alarm to be tested without the presence of a fault condition on a voice recorder, and (2) prevents any failure in the test circuitry itself from disabling an alarm indication when an actual fault condition is present on a voice recorder. Pressing a push-to-test button causes the audible alarm to signal and the corresponding voice recorder LED to light. A green LED is used as a pilot light.

This work was done by Harold Minuskin of NASA’s Jet Propulsion Laboratory and by John Pastor of XteQ, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Electronics/Computers category. NPO-40150