Abstract: A chamber filled with an oscillation damping liquid has a non-magnetic plumb bob suspended from an upper portion of the chamber and a radiated beam path traverses the chamber below the suspension point of the freely suspended plumb bob. The beam path and plumb bob are arranged in a desired relationship whereby in order for the suspending member of the plumb bob to completely block the beam path the chamber, or the body on which it is mounted, must be in a desired predetermined position, such as a true horizontal or a true vertical position. An electric power source and circuitry is provided to operate in response to blocking of the beam path to provide a desired electrical output indicating the desired predetermined position has been achieved and is being maintained. The electrical output may be used to operate visual or aural indication emitting device to notify the user when the desired position is achieved.

Abstract: An angular orientation detector and indicator has a plumb bob suspended in a vertically extending chamber of a housing. A beam of radiation is emitted across the chamber from a beam emitter to a beam receiver and the plumb bob is swingable through a beam path extending between the emitter and receiver. When the housing of the detector is positioned in a preselected angular orientation the plumb bob comes to rest blocking the beam path to substantially reduce or prevent the radiated beam from being received by the receiver. An electrically actuated signal, such as an audible or visual signal, or both, is thereby actuated to indicate that the detector is at a preselected angular orientation. Such detectors may be mounted on builder's levels to indicate when the level is in a preselected angular orientation, such as a horizontal or vertical orientation. Other embodiments and useful applications of such detectors are also disclosed.