Electronic survey target

Claim 1

An electronic target for detecting scanning light reference beams comprising: a. First, second, third and fourth light detectors arranged in a fixed row; b. four timers each connected to a respective one of said detectors; c. four light-emitting devices each connected to the output of a respective one of said timers; d. logic means connected to the output of said timers to provide an output signal responsive to either reference beam acquisition by all four of said detectors or only the second and third of said detectors; e. a sonic indicator connected to said logic means and driven thereby to provide said output signal audibly, repeating at the scan cycle rate of the reference beam, said light-emitting devices serving to indicate the direction of misalignment in the absence of said output signal; and, f. a housing containing all said light-emitting devices, light detectors, timers, logic means and sonic indicator, said light detectors lying behind a first aperture in said housing behind an optical bandpass filter and all said light-emitting devices lying behind a second aperture in said housing, said second aperture lying in a plane perpendicular to said first aperture.